Sustainable use of flatfish resources: Addressing the credibility crisis in mixed fisheries management

57 (2007) 114–125www.elsevier.com/locate/seares

Journal of Sea Research

Sustainable use of flatfish resources: Addressing the credibility crisisin mixed fisheries management

A.D. Rijnsdorp ⁎, N. Daan, W. Dekker, J.J. Poos, W.L.T. Van Densen

Wageningen IMARES, Institute for Marine Resources and Ecosystem Studies, PO Box 68, 1970 AB IJmuiden, The Netherlands

Received 6 January 2006; accepted 25 September 2006Available online 25 October 2006

Abstract

Many flatfish species are caught in mixed demersal trawl fisheries and managed by Total Allowable Catch (TAC). Despitedecades of fisheries management, several major stocks are severely depleted. Using the Common Fisheries Policy (CFP) as anexample, the failure of mixed-fisheries management is analysed by focussing on: the management system; the role of science; therole of managers and politicians; the response of fisheries to management. Failure of the CFP management could be ascribed to:incorrect management advice owing to bias in stock assessments; the tendency of politicians to set the TAC well above therecommended level; and non-compliance of the fisheries with the management regulations. We conclude that TAC management,although apparently successful in some single-species fisheries, inevitably leads to unsustainable exploitation of stocks caught inmixed demersal fisheries as it promotes discarding of over-quota catch and misreporting of catches, thereby corrupting the basis ofthe scientific advice and increasing the risk of stock collapse. This failure in mixed demersal fisheries has resulted in the loss ofcredibility of both scientists and managers, and has undermined the support of fishermen for management regulations. An approachis developed to convert the TAC system into a system that controls the total allowable effort (TAE). The approach takes account ofthe differences in catch efficiency between fleets as well as seasonal changes in the distribution of the target species and can also beapplied in the recovery plans for rebuilding specific components of the demersal fish community, such as plaice, cod and hake.© 2006 Elsevier B.V. All rights reserved.

Keywords: Catchability; CFP; Effort management; Flatfish; Fisheries management; Mixed fisheries; TAC

1. Introduction

Flatfish are generally exploited by demersal trawlfisheries targeting a mixed bag of species (Millner et al.,2005; Munroe, 2005; Wilderbuer et al., 2005). Althoughthe management systems used are diverse, most systemsbuild upon total allowable catches (TAC) for individualspecies accompanied by technical measures such as gearrestrictions, minimum mesh sizes, closed areas and

⁎ Corresponding author.E-mail address: [email protected] (A.D. Rijnsdorp).

1385-1101/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.seares.2006.09.003

seasons (Rice et al., 2005). Examples of effort manage-ment that attempts to control fishing mortality on flatfishprimarily by effort restrictions are scarce. Most effortmanagement systems are based on some kind ofcapacity control or limiting the number of entries byissuing licenses, but this may only slow down theprocess leading to overexploitation rather than restrictfishing mortality (OECD, 1997). In line with the statusof most commercially exploited fish stocks worldwide(FAO, 1995), many flatfish stocks (38 out of 65) havebeen overfished at least during some period during theirexploitation history (Rice and Cooper, 2003). The

mailto:[email protected]

http://dx.doi.org/10.1016/j.seares.2006.09.003

115A.D. Rijnsdorp et al. / Journal of Sea Research 57 (2007) 114–125

global decline in fish stocks has raised public concernand the causes of the failure of fisheries management areintensively debated (Pauly et al., 2002; Caddy and Seij,2005; Garcia, 2005; Smith and Link, 2005). As the threemain actors involved (fishing industry, managementauthority, fisheries science) all play a distinct role in themanagement process, failure may be related to imper-fections in the system at any of these levels, although forall the flaws in science advice, fisheries are more likelyto be sustainable if managers follow scientific advicethan if they do not (Rice and Cooper, 2003). Majorissues are the scope for enforcement (Nielsen andMathiesen, 2003), the management policy adopted andthe quality of the scientific advice (EC, 2002). Whateverthe underlying reason, the scientific advisors appear tohave lost their credibility, exemplified by the accusa-tions by fishers that assessments do not correspond totheir daily experience, by the responsible authorities thatthe annual advice is inconsistent, and by academics orNGO's that fisheries scientists are too closely related tothe management bodies or fishing industry to be able togive impartial advice (Finlayson, 1994; Hutchings et al.,1997; see also Garcia, 2005; Smith and Link, 2005).

We review the situation in respect of success orfailure of fisheries management as exemplified by thesituation in Europe. We argue that sustainable exploita-tion in mixed fisheries cannot be achieved by single-species TAC because fishers may continue to fish afterthe TAC of one of the species is taken. As a potentialalternative, we develop a method that allows TACmanagement to be converted into a system of effortmanagement that may contribute to the rebuilding ofdepleted demersal stocks and of the credibility of theentire management system, including the role of fish-eries science.

2. The common fisheries policy

The legal basis of fisheries management in theEuropean Union is laid down in the Common FisheriesPolicy (CFP), agreed upon in 1982 (Holden, 1994) andpursuing the ultimate objective of sustainable exploita-tion of renewable marine resources taking account of theintegrity of the marine ecosystem as well as social andeconomic conditions (EC, 2002). The main instrumentscomprise stock-specific TACs agreed upon annually bythe Council of Ministers, various technical measures(e.g., mesh sizes, gear and bycatch restrictions, closedareas and seasons), and since 1993 a five-year multi-annual guidance program directed at reducing fleetcapacity of individual countries. The CFP was agreed tobe reviewed at 10-year intervals, and adapted if member

countries should agree to do so. It was not until the thirdcycle (2003–2012) that the CFP was modified toincorporate the possibility to regulate fishing effort bysetting limits to the days-at-sea for specific fleets on anannual basis (EC, 2002). However, the use of effortregulation is restricted to stocks under recovery plansand the TAC still remains the main managementinstrument. The main reason why the EU originallyopted for a TAC system with fixed shares betweenmember states based on historic catches was that such asystem was envisaged to ensure ‘relative stability’ of thenational fishing industries (i.e., all nations would sufferor profit equally from changes in TACs; Holden, 1994).This original objective is strongly adhered to by themember states, even though other economic develop-ments have resulted in marked deviations owing toquota hopping and re-flagging of vessels (Hatcher et al.,2002).

The scientific basis for the TAC negotiations is laidby the stock assessments and catch forecasts providedannually by the International Council for the Explora-tion of the Sea (Rozwadowski, 2002; ICES, 2004). TheSTECF (Scientific, Technical and Economic Committeeon Fisheries Management), a scientific advisory com-mittee of the European Commission (EC), reviews theICES advice and provides recommendations to the EU.Stock status is evaluated with reference to precautionaryreference points for spawning stock biomass and fishingmortality derived from the empiric relationship betweenrecruitment and spawning stock. This type of manage-ment advice relies heavily on reliable catch statistics,age compositions of the landings and recruitmentestimates based on research vessel surveys. Dependingon the status, the ICES advice comprises a range ofoptions that are consistent with sustainable exploitationwithin a single-species context.

The EC generally follows the advice and selects aparticular option within the ‘advised’ range. Because ofeconomic or societal concerns, the EC generallyproposes the option that is on the borderline of beingestimated as ‘sustainable’ by scientists. These proposedTACs are not necessarily followed up by the Council ofMinisters (the ‘politicians’), leading in some years to asubstantial discrepancy between the ‘proposed’ TACand the ‘agreed’ TAC (Daan, 1997).

3. Quality of catch statistics

The quality of stock assessment is directly linked tothe quality of the catch statistics and negatively affectedby illegal or misreported landings as well as bydiscarding of under-sized or over-quota fish. In mixed

Fig. 1. Sole and plaice landings (top), effort allocation over 4 fishinggrounds with various abundance of sole and plaice (middle), and theproportion of the catch that is high graded by month (bottom) of theDutch beam trawl fleet fishing under various levels of ITQ for plaice.The abundance of sole and plaice varied seasonally over the 4 fishinggrounds. At the highest level the ITQ for plaice is not restrictive andthe fleet mainly fishes in the east where both sole and plaice areabundant. At lower plaice ITQ, the fleet targets areas where plaice isless abundant (south and west) and increasingly discard the plaicecatch that exceeds the ITQ. At the lowest ITQ, vessels stay in harbourfor 40% of the time (Poos et al., 2006).

116 A.D. Rijnsdorp et al. / Journal of Sea Research 57 (2007) 114–125

fisheries, discarding of under-sized fish can be sub-stantial owing to the mismatch among the selectivitycharacteristics of the various species. In the beam-trawlfishery targeting sole, discard rates of plaice may be ashigh as 50% in numbers (Van Beek, 1998; Pastoors etal., 2000), while their survival chance is less than 5%(Van Beek et al., 1990). Obtaining reliable stock-wideand fleet-wide estimates of discards is often prohibitedby the high costs of sampling at sea. While during onemorning in the fish market two technicians may samplethe total landings of several vessels after their weeklytrip, they usually have to stay on board a single vesselfor a whole week to sample its discards at one particularlocation. Combining two data sets with completelydifferent error sources, landings statistics being purpo-sefully biased by fishermen's actions and discardsstatistics being affected by sampling limitations, doesnot necessarily improve the advice!

Discarding of over-quota catches is entirely legalunder the CFP (Daan, 1997; Nielsen and Mathiesen,2003; Hatcher, 2005) and can be expected under a TACsystem (Anderson, 1994; Gillis et al., 1995), but isdifficult to quantify empirically. Using a dynamic statevariable model of effort allocation and high gradingin the Dutch flatfish fishery under a TAC system, Pooset al. (2006) showed that a reduction in the individualquotum for plaice (ITQ), the less valuable of the twotarget species, was compensated for by re-allocation ofeffort from an area with a high abundance of plaice and alow abundance of sole, towards fishing grounds with ahigher abundance of sole and a lower abundance ofplaice (Fig. 1). When the ITQ for plaice was decreasedfurther, the fleet could only continue fishing bydiscarding an increasing part of the plaice catch untilthe fishery had to stop completely because fishing wasno longer profitable. The model results are qualitativelysupported by information from the industry suggestingthat high-grading may occur at the beginning of the yearwhen catch rates of plaice are high and comprise of lessvaluable spent fish, and at the end of the year when catchrates increase owing to the recruitment of a new yearclass or quota become exhausted (Fig. 1 bottom).Although the extent of high-grading remains unknown,it is likely to affect age groups differentially. As aconsequence, the impact on the quality of stockassessment may be severe, even when the amountdiscarded represents only a relatively small proportionof the annual catch. Owing to non-compliance, catchstatistics may also be distorted by estimated illegal(unallocated in ICES terminology; whether unreportedor misreported) landings (Nielsen and Mathiesen, 2003;Hatcher, 2005). Unallocated landings were substantial

in the 1980s in both sole and plaice (Daan, 1997) andwidespread underreporting has been reported recentlyfrom the fisheries for cod and haddock (ICES, 2004).

Landings and effort statistics are recorded under theresponsibility of national governments. Although theircumulative accuracy is of crucial importance, scientistshave little insight in this matter. The basis is contained inlogbooks to be filled in by skippers on a daily basis,which are collected when ships enter the harbour to landtheir catch. Ships may be visited at sea by inspectionservices or their landings may be compared to theirlogbook data. If discrepancies are found, the logbookdata are corrected. However, the essential piece of

Fig. 2. Difference in the estimated mean fishing mortality (F) in theyear of the year of the assessment (□) and the most recent assessment(♦) for (a) North Sea cod, (b) plaice, and (c) sole.


information required to evaluate the quality of catchstatistics is the sampling intensity for inspectionpurposes as well as the average discrepancy betweenlogbook catch and actual landings, because this wouldallow at least some estimate of the total over-quotalandings. Although this information is not madepublicly available, informal contacts with the fishingindustry often suggest major discrepancies betweenreported and actual landings. For some fleets, it has beenpossible to estimate the rate of underreporting ormisreporting based on confidential information fromthe industry, and the ‘unallocated landings’ category hasbeen corrected accordingly. However, the confidentialnature of such information prohibits detailed descrip-tions of the raising procedure used and the use of‘unallocated landings’ has not contributed to thetransparency of the assessment and the formulation ofadvice.

4. Quality of the scientific advice

The analysis of the historic performance of stockassessments reveals both the uncertainty of the estimatesof stock size and fishing mortality in the most recentyear, as well as the bias that may occur during certainperiods (Fig. 2). Uncertainty may be due to bias andmeasurement error in the input data, as well as tovariability in fleet dynamics (e.g. catchability) and stockdynamics (e.g. growth rates) (Shepherd, 1988; Hilbornand Walters, 1992; Patterson et al., 2001). Bias mayfurther be due to the analytical technique used (Mohn,1999). Although models that assume the catch-at-agematrix to be uncertain (Deriso et al., 1985; Kimura,1986) are considered superior to models such asExtended Survivor Analysis (Shepherd, 1999) thattake the catch matrix as exact, the latter are still usedfor flatfish stocks in Europe. Problems may also arisefrom non-stationarity of data (Mohn, 1999), or from anincorrect model specification, for instance becausesubstantial stock components migrate between differentareas (Kell et al., 2004), because the assumption ofconstant natural mortality is violated, or becausebiological realism is lacking (Kell and Bromley, 2004).

The calibration of any assessment model requires areliable and unbiased indicator of temporal populationtrends. The use of commercial catch rates is problematicin this respect owing to potential changes in spatialdistribution of the resource (Paloheimo and Dickie,1964), in the spatial distribution of the fleet relative tothe resource (Walters, 2003), in the technical efficiencyof the fleet (Marchal et al., 2001, 2003; Rijnsdorp et al.,2006), in interactions among vessels (Gillis and Peter-

man, 1998), as well as owing to effects of managementmeasures on catchability (Marchal et al., 2002).Research vessel surveys have the advantage thatsampling can be standardized, but the number of towswill inevitably be small compared to the use ofcommercial fleet data, leading to wide confidencelimits. And also for research vessel gears the assumptionof constant catch efficiency may be violated becausecatch rates may be affected by changing fishing patternsof the commercial fleet through disturbance of fish orinteractions among vessels (Gillis and Peterman, 1998;Rijnsdorp et al., 2000a,b; Gillis, 2003; Poos andRijnsdorp, in press). On a longer time scale, fisheries-induced evolutionary changes in fish behaviour towardsfishing gear may reduce catch efficiency (Heino andGodø, 2002).

Retrospective errors in stock assessment are impor-tant in the context of the credibility of managementadvice and their effects should be clearly communicated


to the customers. Assessment results are commonlypresented as the singular, best possible representation ofthe current status of a stock (Corkett, 2002; Finlayson,1994), while errors in the past are ignored as being non-informative. However, even if those errors can be clearlyexplained, we argue that the uncertainty should bepresented as integral part of the assessment to convey itslimitations (Pastoors, 2005).

There is generally one year between the lastpopulation estimate of the assessment and the forecastyear for which TAC advice is requested. Uncertaintyabout the total catch in the ongoing year may have far-reaching consequences for the TAC advice. If manage-ment aims for status quo F, the random error in theestimate of stock size will be largely balanced by anopposite error in the estimate of F. However, ifmanagement aims for a reduction in F – as is oftenthe case given the depleted state of many stocks— or if

Fig. 3. Summary of the trends in fishing mortality F (left) and spawning stockmortality (F) estimated in the most recent stock assessment (drawn line)recommended TAC and FTAC (▪) corresponding to the TAC that was agreed alimit reference levels for F and SSB, the horizontal grey lines indicate the p

there are systematic trends in biological parameters suchas growth and maturity or in the spatial dynamics of thefishery, the uncertainty and bias will propagate in theshort-term forecast (Cook et al., 1991; Gascuel et al.,1998; Van Beek and Pastoors, 1999; Pastoors, 2005).This may result in a TAC advice that is either toorestrictive or too loose for several years to come. Ifstocks are already in danger, a systematic bias of thissort may be particularly dangerous and increase the riskof stock collapse.

5. Management of mixed fisheries

The failure of mixed-fisheries management under theCFP may be illustrated by the historic trends inspawning stock biomass (SSB) and fishing mortalityrate (F) of three major demersal stocks in the North Sea(Fig. 3). SSB of cod has declined to below the limit

biomass SSB (right) of cod (a, b), plaice (c, d) and sole (e, f). Fishingis compared to the fishing mortality Frec (□) corresponding to themong the Council of Ministers. The horizontal black lines indicate therecautionary level of F.


reference point for sustainable exploitation, whereas theSSB of plaice and sole have declined to just above thelimit. In sole, SSB has temporarily increased followingthe recruitment of two extremely abundant year classesin 1987 and 1991. F for cod has remained above thelimit reference point, whereas F for plaice has beenbelow the limit reference point throughout the timeseries. For sole, a limit reference point has not beendefined.

Fig. 2 also provides information on the F correspond-ing to theTAC advice (Frec) and the agreed TAC (FTAC ),which may be compared to the F subsequently realised.At the start of the CFP, the Frec was only slightly belowthe actual F for cod. In 1986, Frec was reduced to ∼0.6until 2001 when the advice was to stop fishing (Frec=0).With a time lag of two years, the Council of Ministersclosely followed the scientific advice and FTAC. was onlyjust above Frec. Since the zero-catch advice in 2001, aTAC of about 50 000 t has been agreed, correspondingto a FTAC=∼0.5. Although this FTAC has been well belowthe current level since 1988, it did not achieve theintended reduction in F (Fig. 3a). In plaice, the scientificadvice was based on maximum sustainable yield (MSY)considerations until 1985 (FMAX ). Subsequently, theadvice was relaxed, first to reduce F (1986–1989) andlater to status quo F (1990–1994). Since 1995, whenSSB showed clear signs of declining to unsustainablelevels, a strong reduction in the TAC was recommendedto rebuild SSB. The FTAC , however, was generally wellabove Frec, and between 1985–1994 even above therealised F (Fig. 3c). Since 1995, the agreed TAC hasbecome restrictive and realised F appears to havedecreased slightly although still well above the FTAC. Insole, a similar relaxation of the scientific advice can beseen between the mid 1980s (reduction in F) and theperiod 1993–1996 (status quo F). Since then, areduction in F has been recommended correspondingto the precautionary reference point for forecasts (0.4).Despite the nominally restrictive TACs set since 1995,there is no evidence for a reduction in realised F, exceptperhaps in the last year (Fig. 3e). As part of the recoveryplan for cod, restrictive TAC in the mixed fisheries havebeen accompanied by additional measures such as aclosed area in 2001, and since 2003, a restriction of thenumber of fishing days to 10–23 days per monthdepending on fishing gear.

The relaxation in the F advice in the late 1980s andearly 1990s was related to a shift in philosophy withinICES to move from the indeterminate FMAX objective tokeeping the stock above a minimum biologicallyacceptable level (MBAL) on a year-to-year basis. Inpractice, medium-term considerations were ignored and

a warning was issued only if SSB should fall belowMBAL in the TAC year. In the mid 1990s, theimplementation of the precautionary approach, whichaccounts explicitly for uncertainty from a medium-termperspective, led to a substantial reduction in Frec.

The deviance between realised F and FTAC (Fig. 3)can be partly explained by a bias in stock assessment,which has caused underestimation of F in recent years(Fig. 2). Between 1995 and 1999, the average differencein apparent F between the original assessment and theconverged assessment later has amounted to 50%, 32%and 18% for cod, plaice and sole, respectively, whereasthe average difference between realised F and FTAC was76%, 36% and 28%, respectively. For cod, the actualcatch is suspected to have exceeded the official catchbecause of misreporting in those years (ICES, 2005a).For plaice and sole, anecdotal information from thefishing industry suggests that in some years part of thecatch may have been subject to high-grading.

The main conclusion here must be that the CFP hasnot achieved the envisaged reduction in F in the threedemersal stocks, even though the agreed TAC formallyshould have been restrictive, at least in most years. Thecauses are less clear, but bias in stock assessment,unreliable landings statistics and agreed TAC wellabove recommended catch options may have played arole. The apparent decline in the productivity ofdemersal species in the 1990s when recruitment ofplaice and various roundfish species (Hislop, 1996;Pope and Macer, 1996; Kell and Bromley, 2004) andgrowth rates of plaice and sole (Rijnsdorp et al., 2004;Van Keeken et al., 2007-this issue) declined made theconsequences of the TAC management for the fisheryworse.

The failure of management for mixed demersalstocks is not restricted to the examples shown here butalso applies to many other demersal stocks in Europeanwaters (ICES, 2005b). Their management historycontrasts markedly with the herring stocks, which aremainly taken in directed, single-species fisheries. In the1960s, the North Sea herring stock was severelydepleted (Simmonds, 2005). After a four-year morator-ium between 1978 and 1982, the stock showed signs ofrecovery and since then the fishery has been managedeffectively by TAC, allowing the stock to rebuild to themanagement targets (ICES, 2005b; Simmonds, 2005).

6. The credibility crisis in mixed fisheriesmanagement

Although many sources of uncertainty exist inassessments and catch forecasts, the prime problem is


the catch statistics, because the total catch serves as adirect raising factor for the estimated stock sizes. A biasin the catch data will not necessarily show up inretrospective analyses, but will remain present in allfuture assessments and thus bias our perception of thehistoric performance as well as any reference pointsderived from these.

The more restrictive the TAC, the larger the incentiveto land over-quota fish illegally or high-grade. Becauseremovals are underestimated, current and forecast stocksizes are also underestimated. Thus, the TAC manage-ment system has a feed-back component, which resultsin deteriorating catch statistics and ever more unreliableassessments. Although this warning has been statedclearly in almost every ACFM report, the fact that theTAC system itself is ultimately responsible for deterior-ating catch statistics does not seem to have comethrough. Can this problem be resolved by betterenforcement? The answer for mixed fisheries must beno, because it remains perfectly legal to high-grade anddiscard over-quota catches and the effect on theassessment is the same. Moreover, a managementsystem that is aimed at controlling exploitation rates,but only affects the proportion landed cannot beexpected to result in sustainable fisheries!

Although ICES is requested to provide TAC advice,strictly the advice is formulated in terms of F: “ICESrecommends that fishing mortality be less than Fpa=X,corresponding to landings of less than Y in year Z”. Ifcatch predictions are translated into a TAC, the inherentassumptions are that landings statistics are correct andthat discards represent a constant proportion, which areboth patently incorrect.

After 25 years of experience with the CFP, one mightrightly wonder why we are still left with a TAC systemthat obviously does not work in mixed fisheries. Canthere be any expectation that suddenly everything willwork as it should? Does it help the credibility of thescientific advice, if the VPA-machine is kept going toget all forecasts out in time for the EU to decide on TACthat will not control exploitation rates?

7. Solving the credibility crisis

In the public perception, the ultimate condition forregaining credibility would be if managers, guided byadvice, should succeed in rebuilding the now depletedstocks to a sustainable level as well as preventing otherstocks reaching depleted conditions. For the mixedfisheries, the current system suffers from the intrinsicimperfection that fishing continues after the TAC of oneof the target species has been taken, because discarding

is not forbidden and fishing operations are notcontrolled by independent observers. Hence, a majorreform of the management system seems needed.Transforming the present output control system intoan input control system might solve several of theexisting problems (Daan, 1997; Shepherd, 2003).Enforcement of restrictions of number of days fishing,and allowing the marketable catch to be landed, wouldseem a lot easier than controlling landings. Mostimportantly, however, the scientific assessment of thestock is not directly affected by whether fishers fool theinspection services, high grade or increase their catchrates. The basis for evaluating the status of the stocksshould remain correct, and adaptive rather thanprescriptive management should solve any problemsencountered.

Of course, a major change in the CFP will be difficultto achieve, as the institutional framework is anchored innational and international law and has evolved in a slowand complicated process (Scheffer et al., 2005).However, since the second review in 2002, the CFPincludes the possibility of direct effort management onan annual basis as part of the recovery plans for depletedspecies (EC, 2002). Even if it is not now an alternativefor TACs, this modification represents a crucial stepforward by providing a legal basis for effort manage-ment. The question remains how the present TACsystem might be turned into a Total Allowable Effort(TAE) system, without compromising the starting pointof ‘relative stability’, and how the management andadvisory system might again gain credibility.

8. Converting a TAC into a TAE system

To convert a TAC system into a TAE system, anindicator of effective effort (Shepherd, 2003) by fleet (f)is required, which may be estimated from the relation-ship between fishing effort (Ef) and the fishing mortalityrealised by that fleet (Ff): Ff =q Ef (Rijnsdorp et al.,2006). The TAC system assumes that national catchescontribute proportionally to total F. Therefore, the FTACmay be split in shares in terms of allowed partial F byfleet according to the existing percent quota shares(FTAC,f). The catchability coefficient q gives the fishingmortality imposed per fishing day. With this q and thefishing mortality rate corresponding to the share of theagreed TAC (FTAC,f), the total allowable number offishing days (TADf) can be calculated as TADf=FTAC,f

q−1. For a single vessel, the individual effort quotumwill be IEQ=TADf / n, where n is the number of vesselsin the fleet. In a mixed-fisheries system, where severalfishing fleets are targeting a mixture of fish species

Fig. 4. Seasonal changes in the conversion rate of a unit of fishingeffort in four different fishing areas in the North Sea (South, West, Eastand Central) for plaice (a) and sole (b). The conversion rate curvesreflect the differences in the availability of the resource betweenfishing areas and month (adapted from Rijnsdorp et al., 2006).

Table 2Seasonal differences in the price that a fishing vessel has to pay forfishing one day in four different fishing areas in the North Sea

month Plaice Sole

South West East Central South West East Central

1 1.7 1.2 1.6 1.0 1.1 1.0 0.7 0.02 1.3 1.0 1.5 1.2 1.1 0.9 0.7 0.03 0.6 0.8 1.0 1.5 1.2 0.9 0.8 0.04 0.5 0.8 0.7 1.4 1.1 0.7 0.8 0.05 0.6 1.1 0.7 1.3 0.9 0.6 0.8 0.06 0.6 1.0 0.8 1.3 0.9 0.7 0.7 0.07 0.4 0.9 0.6 1.4 1.1 0.7 0.9 0.08 0.5 0.9 0.6 1.3 1.3 0.7 1.2 0.09 0.7 1.0 0.7 1.3 1.5 0.8 1.5 0.010 0.9 0.9 1.1 1.3 1.5 0.9 1.3 0.011 1.0 0.9 1.2 1.2 1.5 1.1 1.0 0.012 1.3 1.1 1.3 1.0 1.3 1.1 0.9 0.0

If a vessel decides to fish in August in area South for 10 days, its IEQwill be reduced by 5 units for plaice and 13 units for sole.


using a variety of fishing gears, the IEQ's will be bothspecies and fleet specific.

Given enough data, an effort management systemmight become really sophisticated. We describe oneexample based on real data. Because of seasonalmigrations and recruitment to the exploitable stock,the availability of fish will vary between areas andseasons. A fishing day, therefore, will generate a higherfishing mortality when a species is temporarily

Table 1Hypothetical calculation of the total allowable effort for the Dutchbeam trawl fleet fishing for sole and plaice based on the partial fishingmortality rate generated per fishing day by a typical 2000 hp trawler

Sole Plaice

Agreed F 0.4 0.25NL share of F 0.3 0.125q 1.0E−05 6.0E−06TAD (days at sea) 3.0E+04 2.1E+04#vessels 150 150IEQ (#days/vessel) 200 139

concentrated in for instance a spawning area thanwhen the species is dispersed over the feeding grounds.An effort management system may take account ofpredictable variations in q by charging a certain price foreach combination of fishing area and season (Fig. 4).The total allowable number of fishing days (IEQ) can beviewed as a currency that can be translated into actualfishing days given a conversion rate (qij) that varies inspace (i) and time (j).

The calculation is illustrated for the Dutch beamtrawl fleet targeting sole and plaice using estimates ofthe partial fishing mortality generated per fishing day byeach vessel during each trip (Fpue) as a function ofengine power, fishing area and time of the year(Rijnsdorp et al., 2006). With the Fpue estimates for astandard 2000 hp beam trawler and the Dutch share ofthe TAC for sole and plaice, the IEQ can be calculated as200 days for sole and 139 for plaice (Table 1). Becausethe availability of fish varies between areas and seasons(Fig. 3), the conversion rate of IEQ units into actualfishing days will vary accordingly (Table 2).

9. Discussion

A pre-requisite for an effort management system isthat the patterns in q are, if not relatively stable, at leastpredictable. In reality, the q will vary due to changes inthe behaviour of the species (Paloheimo and Dickie,1964; Horwood and Millner, 1998; Poos and Rijnsdorp,in press) and may change due to technological advancesin the fleets (Marchal et al., 2001; Ulrich et al., 2002;Marchal et al., 2003; Rijnsdorp et al., 2006). Theessence of an input control system is to update q at


regular intervals according to the observed trends q,Alternatively, catchabilities can already be increased bya certain percentage annually to take account of theexpected increase in technical efficiency.

As the variability in q can be quantified usinghistoric data, the robustness of management systems canbe explored to cope with the variability in q. Rijnsdorpet al. (2006) decomposed the variance in q in a randomcomponent, representing the variation in q betweenindividual fishing trips (plaice — 44%; sole — 43%), apredictable seasonal pattern for each fishing ground(plaice — 43%; sole — 39%) and a remainingcomponent reflecting the variation generated by weekto week differences (plaice — 13%; sole — 18%).Quantification of the structural but unpredictablevariation in q on different time scales (week, month,year) and fishing grounds will provide the basis toevaluate the scope for an increase in q, and hence inrealised F, above the management target.

Within a TAE system, each vessel would be free tomake her own fish plan, within the constraints set by theavailable IEQ units and the conversion rates for thedifferent species, and to land all fish above the minimumlanding size. In case the national quotum is allocatedinto individual transferable quota (ITQ), such as in theDutch beam trawl fleet, the IEQ would vary betweenvessels in proportion to their ITQs.

Some major advantages of the TAE system proposedare: (1) it offers a solution to a major deficiency of thecurrent TAC system as there will be no incentive formisreporting or high-grading catches; (2) it allows aconversion of the current TAC system within theconstraints of maintaining relative stability by usingnational percentage quota shares to distribute partial Fshares; (3) it controls the target F's of each of the speciestaken in the mixed fisheries; and (4) it allows remedialaction at the national level, whereas under the TACsystem all countries suffer equally from reduced TAC, ifone particular country has exceeded its quota share byillegal landings or high grading.

ATAE system like this, however, will certainly havedeficiencies of its own that need careful evaluation. Theeffectiveness of the approach will depend on thepredictability of the seasonal pattern in q, and thepossibilities for vessels to increase their efficiencywithin the constraints set by TAE management. Onemight argue that non-compliance of effort regulationsmay jeopardize the quality of the stock assessment as abasis for fisheries management, analogous to misreport-ing catches under a TAC system, because fishermenmay try to under-report fishing effort to increase fishingtime and generate a higher revenue. However, because

there is no incentive to misreport catches, F and stocksize should still be correctly estimated. Under-reportingof effort would lead to an apparent increase incatchability, and hence in a reduction in the effortquotum of that fleet after reappraisal at regular intervals,but does not influence the assessment of the stock status.On the other hand, a fisherman may want to increase hiseffort quotum in the future and may start high-grading atarget fish species in a preferred area. Such unwantedbehaviour would lead to an immediate loss of revenueand a potential but uncertain future profit. Furtherstudies on the behaviour of fishermen may be useful toexplore the scope for responses that undermine theeffectiveness of a TAE system.

Enforcement of effort regulations will be a pre-requisite for an effective TAE system, but should beeasier than enforcement of catch regulations, as fishers'activities are already monitored by the vessel monitor-ing system in use within the CFP. However, effortmanagement will need additional regulations on typeand numbers of gear used (size of the trawl, number oflong lines, gill nets, pots or traps), which is moredifficult to control. This will need particular attention asthe type of gear has a large influence on catch efficiency.

In an input control system, skippers will have astrong incentive to increase their fishing efficiency.Therefore, we may expect that the transition from a TACto a TAE system may lead to a stronger increase incatchability than observed in the past. Under the currentTAC regime, catchability of sole and plaice in the beamtrawl fishery has been reduced because vessels havepartly redirected their fishing effort to other species forwhich the TAC is less restrictive (Poos et al., 2001).Therefore, we might expect a sudden increase incatchability of the most valuable species in particular.Such increases may only be estimated in retrospect andtherefore TAE management is unlikely to control fishingmortality precisely at the agreed level (Ftac).

A question that needs careful consideration is theoptimal spatial and temporal scale for catchabilityestimates as conversion units for effort quota. Thisshould be a compromise between the utilitarian wish tohave a rather simple approach and the biological realityof marked variations in space and time. If the resolutionchosen is large, there will remain a structure in q presentthat vessels can utilise to increase their catchabilityabove the historic average. On the other hand, smaller-scale patterns will be increasingly more difficult toquantify reliably.

A TAE system is likely to stimulate investment intechnological innovations, and therefore enhance com-petition, but this would not be a new development. Also


under the TAC system vessels are outcompeted if theirefficiency falls behind that of others. The ITQ systemintroduced in the Netherlands resulted in a concentrationof fishing rights (Davidse, 2001), and even within theEuropean Union, re-flagging of vessels has led to aconcentration of fishing rights that is incompatible withthe concept of relative stability (Hatcher et al., 2002). Ifin the Netherlands, the allocation of fishing days wasdirectly coupled to the existing ITQ's, the monetaryinvestments in the latter might be smoothly transferredto a TAE system.

The sensitivity of the TAE system for various sourcesof uncertainties may be explored quantitatively using anevaluation framework developed to explore the con-tribution of the various sources of uncertainty in thescientific basis for fisheries management (see review inHarwood and Stokes, 2003). This framework comprisesa biological model that captures the relevant processesdetermining the dynamics of the resource and thedynamics of the fishery (Butterworth and Punt, 1999),an observational model that analyses the sampling dataobtained from the biological model, leading to aperception of the state of the stock and a managementmodel that determines the management actions based onstrict harvest control rules. In all model components, therelevant factors contributing to uncertainty and error aretaken into account. By comparing the perceived with thetrue dynamics, the sensitivity of the management systemfor various sources of error and uncertainty in any of thethree models may be explored. This approach is a majorstep towards improving fisheries management adviceand has been explored for North Sea flatfish androundfish by Kell et al. (2005) and Kell and Bromley(2004). The TAE system proposed should be a suitablecandidate for testing.

A TAE approach may also contribute directly to theprotection of North Sea cod. Since 2001, ICES hasadvised a stop to fishing for cod in order to rebuild thestock. However, as this species is taken as a bycatch in alarge number of demersal fisheries, it has provedpolitically unacceptable to stop all cod-related fisheries.Applying our approach, the matrix of q values by areaand season of all cod-related fisheries may be estimatedfor cod as well as the other demersal species managedby single species TAC's taken in the cod-relatedfisheries. This matrix would allow managers to find acompromise in minimising the total F on cod, whilemaximising the allowable F on the other species withinthe agreed Ftac, taking account of the share of thevarious countries (relative stability).

The input control system proposed here should not beconsidered a panacea for the solution of all mixed-

fisheries management problems. Although it addressesthe key problem — the control instrument (TAC)undermines the scientific basis of management (stockassessment) — that needs to be resolved with highpriority, other problems may prevent fisheries manage-ment becoming successful. For instance, the problem ofuncertainty and bias in stock assessments (Mohn, 1999;Harwood and Stokes, 2003), problems related with thecomplexity and non-equilibrium nature of marinesystems (Caddy and Seij, 2005), fisheries dynamicsand their response to the management regulations (Gilliset al., 1995; Dinmore et al., 2003; Gillis, 2003; Salas andGaertner, 2004), enforcement (Payne and Bannister,2003), lack of transparency of the management systemand the governance structure (Gray and Hatchard, 2003;Daw and Gray, 2005), and the overcapacity of fleets needto be resolved. The proposed alternative converts analready complicated TAC management system into anequally complicated TAE system that depends heavilyon fisheries–dependent data. Alternative solutions mightbe found in a more simple management system that isless dependent on precise stock assessments butmanagesexploitation rate based on a number of transparent andsimple indicators of the trends in resource biomass andfishing effort (Degnbol, 2005). However, it seems un-likely that man will ever be able to manage fish stockswithout managing the amount of fishing by individualfleets. In order to achieve the sustainable management ofmixed fisheries we need (1) a carefully designed man-agement system that takes account of the dynamics ofthe resource and the dynamics of the fisheries; (2) agovernance system that acknowledges the separate rolesof the scientist, managers and fishermen and accepts theimpartiality of scientist, and (3) an efficient andtransparent enforcement system. The impartiality andquality of the fisheries science might be enhanced if theevaluation of the management system and exploration ofalternative systems were to become part of the routineresearch agenda.

Acknowledgements

This paper partly results from the NWO-priorityprogram on the ‘Sustainable Use of Marine LivingResources’ and is contribution MPS-06052 to theNetwork of Excellence MARBEF. The critical com-ments of the reviewers are gratefully acknowledged.

References

Anderson, L.G., 1994. Highgrading in ITQ fisheries. Mar. Res. Econ.9, 209–226.


Butterworth, D.S., Punt, A.E., 1999. Experiences in the evaluation andimplementation of management procedures. ICES J. Mar. Sci. 56,985–998.

Caddy, J.F., Seij, J.C., 2005. This is more difficult than we thought!The responsibility of scientists, managers and stakeholders tomitigate the unsustainability of marine fisheries. Philos. Trans. - R.Soc., Biol. Sci. 360, 59–75.

Cook, R.M., Kunzlik, P.A., Fryer, R.J., 1991. On the quality of NorthSea cod stock forecasts. ICES J. Mar. Sci. 48, 1–13.

Corkett, C.J., 2002. Fish stock assessment as a non-falsifiable science:replacing an inductive and instrumental view with a criticalrational one. Fish. Res. 56, 117–123.

Daan, N., 1997. TAC management in North Sea flatfish fisheries.J. Sea Res. 37, 321–341.

Davidse, W.P., 2001. The effects of transferable property rights on thefleet capacity and ownership of harvesting rights in the Dutchdemersal North Sea fisheries. In: Shotton, R. (Ed.), Case Studieson the Effects of Transferable Fishing Rights on Fleet Capacity andConcentration of Quota Ownership. . FAO Fisheries TechnicalPaper, vol. 412. FAO, Rome, pp. 17–25.

Daw, T., Gray, T., 2005. Fisheries science and sustainability ininternational policy: a study of failure in the European Union'sCommon Fisheries Policy. Mar. Policy 29, 189–197.

Degnbol, P., 2005. Indicators as a means of communicating knowl-edge. ICES J. Mar. Sci. 62, 606–611.

Deriso, R.B., Quinn, T.J., Neal, P.R., 1985. Catch-age analysis withauxiliary information. Can. J. Fish. Aquat. Sci. 42, 815–824.

Dinmore, T.A., Duplisea, D.E., Rackham, B.D., Maxwell, D.L.,Jennings, S., 2003. Impact of a large-scale area closure on patternsof fishing disturbance and the consequences for benthic commu-nities. ICES J. Mar. Sci. 60, 371–380.

EC, 2002. On the conservation and sustainable exploitation of fisheriesresources under the Common Fisheries Policy. Council Regulation(EC) No 2371/2002. Official Journal of the European Commu-nities L, vol. 358/59. 22 pp.

FAO, 1995. The state of world fisheries and aquaculture. SpecialPublication for the 21st Session of the Committee on Fisheries,March 1995. FAO, Rome.

Finlayson, A.C., 1994. Fishing for truth; a sociological analysis ofNorthern cod stock assessments from 1977–1990. Meml. Univ.Newfoundl., Inst. Soc. Econ. Res. Soc. Econ. Stud. 52.

Garcia, S.M., 2005. Fishery science and decision-making: Direstraights to sustainability. Bull. Mar. Sci. 76, 171–196.

Gascuel, D., Fontenelle, G., Maguer, C., Biseau, A., 1998. Consistencyof stock assessment and evaluation of fisheries management forEuropean fish stocks, 1983–1995. Alaska Sea Grant ProgramAK-SG-98-01. In: Funk, F., et al. (Ed.), Fishery Stock Assess-ment Models. Alaska Sea Grant Publication, pp. 917–931.

Gillis, D.M., 2003. Ideal free distributions in fleet dynamics: abehavioral perspective on vessel movement in fisheries analysis.Can. J. Zool. 81, 177–187.

Gillis, D.M., Peterman, R.M., 1998. Implications of interferenceamong fishing vessels and the ideal free distribution to theinterpretation of CPUE. Can. J. Fish. Aquat. Sci. 55, 37–46.

Gillis, D.M., Pikitch, E.K., Peterman, R.M., 1995. Dynamic discardingdecisions: foraging theory for high-grading in a trawl fishery.Behav. Ecol. 6, 146–154.

Gray, T., Hatchard, J., 2003. The 2002 reform of the CommonFisheries Policy's system of governance—rhetoric or reality? Mar.Policy 27, 545–554.

Harwood, J., Stokes, K., 2003. Coping with uncertainty in ecologicaladvice: lessons from fisheries. Trends Ecol. Evol. 18, 617–622.

Hatcher, A., 2005. Non-compliance and the quota price in an ITQfishery. J. Environ. Econ. Manage. 49, 427–436.

Hatcher, A., Frere, J., Pascoe, S., Robinson, K., 2002. Quota-hoppingand the foreign ownership of UK vessels. Mar. Policy 26, 1–11.

Heino, M., Godø, O.R., 2002. Fisheries-induced selection pressures inthe context of sustainable fisheries. Bull. Mar. Sci. 70, 639–656.

Hilborn, R., Walters, C.J., 1992. Quantitative Fisheries StockAssessment: Choice, Dynamics and Uncertainty. Chapman andHall, New York. 570 pp.

Hislop, J.R.G., 1996. Changes in North Sea gadoid stocks. ICES J.Mar. Sci. 53, 1146–1156.

Holden, M., 1994. The Common Fisheries Policy. Origin, Evaluationand Future. The Buckland Foundation, London.

Horwood, J.W., Millner, R.S., 1998. Cold induced abnormal catches ofSole. J. Mar. Biol. Assoc. U.K. 78, 345–347.

Hutchings, J.A., Walters, C., Haedrich, R.L., 1997. Is scientific inquiryincompatible with government information control? Can. J. Fish.Aquat. Sci. 54, 1198–1210.

ICES, 2004. Report of the ICES Advisory Committee on FisheryManagement and Advisory Committee on Ecosystems, 2004.ICES Advice, vol. 1, p. 1544.

ICES, 2005a. Report of the Working Group on the Assessment ofDemersal Stocks in the North Sea and Skagerak. Bergen, 7–16September 2004. ICES. C.M. 2005 / ACFM: 07.

ICES, 2005b. Report of the ICES Advisory Committee on FisheryManagement, Advisory Committee on the Marine Environmentand Advisory Committee on Ecosystems, 2005. ICES Advice, vol.1–11. 1418 pp.

Kell, L.T., Bromley, P.J., 2004. Implications for current managementadvice for North Sea plaice (Pleuronectes platessa L.): Part II.Increased biological realism in recruitment, growth, densitydependent sexual maturation and the impact of sexual dimorphismand fishery discards. J. Sea Res. 51, 301–312.

Kell, L.T., Scott, R., Hunter, E., 2004. Implications for currentmanagement advice for North Sea plaice: Part I. Migrationbetween the North Sea and English Channel. J. Sea Res. 51,287–299.

Kell, L.T., Pilling, G.M., Kirkwood, G.P., Pastoors, M.A., Mesnil, B.,Korsbrekke, K., Abaunza, P., Aps, R., Biseau, A., Kunzlik, P.A.,Needle, C., Roel, B.A., Ulrich-Reschan, C.M., 2005. An evalua-tion of the implicit management procedure used for some ICESroundfish stocks. ICES J. Mar. Sci. 62, 750–759.

Kimura, D.K., 1986. Variability, tuning and simulation for theDoubleday-Deriso Catch-at-Age model. Can. J. Fish. Aquat. Sci.46, 941–949.

Marchal, P., Nielsen, J.R., Hovgard, H., Lassen, H., 2001. Timechanges in fishing power in the Danish cod fisheries of the BalticSea. ICES J. Mar. Sci. 58, 298–310.

Marchal, P., Ulrich, C., Pastoors, M., 2002. Area-based managementand fishing efficiency. Aquat. Living Resour. 15, 73–85.

Marchal, P., Ulrich, C., Korsbrekke, K., Pastoors, M., Rackham, B.,2003. Annual trends in catchability and fish stock assessments.Sci. Mar. 67, 63–73.

Millner, R.S., Walsh, S.J., Diaz de Astarloa, J.M., 2005. Atlanticflatfish fisheries. In: Gibson, R.N. (Ed.), Flatfishes. Biology andExploitation. Blackwell Science Ltd., Oxford, UK, pp. 240–271.

Mohn, R., 1999. The retrospective problem in sequential populationanalysis: an investigation using cod fishery and simulated data.ICES J. Mar. Sci 56, 473–488.

Munroe, T.A., 2005. Tropical flatfish fisheries. In: Gibson, R.N. (Ed.),Flatfishes. Biology and Exploitation. Blackwell Science Ltd.,Oxford, UK, pp. 292–318.


Nielsen, J.R., Mathiesen, C., 2003. Important factors influencing rulecompliance in fisheries lessons from Denmark. Mar. Policy 27,409–416.

OECD, 1997. Towards Sustainable Fisheries. Economic Aspects of theManagement of Living Marine Resources. Organisation forEconomic Co-operation and Development, Paris. 268 pp.

Paloheimo, J.E., Dickie, L.M., 1964. Abundance and fishing success.Rapp. P.-V. Reun. - Cons. Int. Explor. Mer 155, 152–163.

Pastoors, M.A., 2005. Evaluating Fisheries Management Advice forSome North Sea Stocks: Is Bias Inversely Related to Stock Size?ICES. CM 200 /V:20, 14 pp.

Pastoors, M.A., Rijnsdorp, A.D., Van Beek, F.A., 2000. Evaluation ofthe effects of a closed area in the North Sea (‘Plaice Box’) on thestock development of plaice (Pleuronectes platessa L.). ICES J.Mar. Sci. 57, 1014–1022.

Patterson, K.R., Cook, R.M., Darby, C.D., Gavaris, S., Kell, L.T.,Lewy, P., Mesnil, B., Punt, A.E., Restrepo, V.R., Skagen, D.,Stefansson, G., 2001. Estimating uncertainty in fish stockassessment and forecasting. Fish Fish. 2, 125–157.

Pauly, D., Christensen, V., Guénette, S., Pitcher, T.J., Rashid Sumaila,U., Walters, C.J., Watson, R., Zeller, D., 2002. Towards sustain-ability in world fisheries. Nature 418, 689–695.

Payne, A.I.L., Bannister, R.C.A., 2003. Science and fisheries man-agement in southern Africa and Europe. Afr. J. Mar. Sci. 25, 1–23.

Poos, J.J., Rijnsdorp, A.D., in press. An “experiment” on effortallocation of fishing vessels: the role of interference competitionand area specialization. Can. J. Fish. Aquat. Sci.

Poos, J.J., Rijnsdorp, A.D., in press. The dynamics of small-scalepatchiness of plaice and sole as reflected in the catch rates of theDutch beam trawl fleet and its implications for the fleet dynamics.J. Sea Res.

Poos, J.J., Pastoors, M.A., Rijnsdorp, A.D., 2001. Quota Regulationand Efficiency in the Dutch Beam Trawl Fleet. ICES. CM 2001/N14, 16 pp.

Poos, J.J., Rijnsdorp, A.D., Borges, L.M., Dickey-Collas, M., 2006.Analyses of the effect of fisheries management on the individualbehaviour of fishing vessels with respect to spatial fishing effortdistribution and discarding. RIVO Report CO06/00.

Pope, J.G., Macer, C.T., 1996. An evaluation of the stock structure ofNorth Sea cod, haddock, and whiting since 1920, together with aconsideration of the impacts of fisheries and predation effects ontheir biomass and recruitment. ICES J. Mar. Sci. 53, 1157–1169.

Rice, J., Cooper, J.A., 2003. Management of flatfish fisheries - whatfactors matter? J. Sea Res. 50, 227–243.

Rice, J., Cadrin, S.X., Clark, W.G., 2005. Assessment and managementof flatfish stocks. In: Gibson, R.N. (Ed.), Flatfishes. Biology andExploitation. Blackwell Science Ltd., Oxford, UK, pp. 319–346.

Rijnsdorp, A.D., Dol, W., Hoyer, M., Pastoors, M.A., 2000a. Effects offishing power and competitive interactions among vessels on theeffort allocation on the trip level of the Dutch beam trawl fleet.ICES J. Mar. Sci. 57, 927–937.

Rijnsdorp, A.D., Van Maurik Broekman, P.L., Visser, E.G., 2000b.Competitive interactions among beam trawlers exploiting local

patches of flatfish in the North Sea. ICES J. Mar. Sci. 57,894–902 38.

Rijnsdorp, A.D., Van Keeken, O.A., Bolle, L.J., 2004. Changes inProductivity of the Southeastern North Sea as Reflected in theGrowth of Plaice and Sole. ICES. CM2004/K:13.

Rijnsdorp, A.D., Dekker, W., Daan, N., 2006. Partial fishing mortalityper fishing trip: a useful indicator of effective fishing effort inmanagement of mixed demersal fisheries. ICES J. Mar. Sci. 63,556–566.

Rozwadowski, H.M., 2002. The Sea Knows No Boundaries: ACentury of Marine Sciences Under ICES. University of Washing-ton Press, Seattle.

Salas, S., Gaertner, D., 2004. The behavioural dynamics of fishers:management implications. Fish Fish. 5, 153–167.

Scheffer, M., Westley, F., Brock, W., 2005. Slow response of societiesto new problems: causes and costs. Ecosystems 6, 493–502.

Shepherd, J.G., 1988. Fish stock assessment and their data require-ment. In: Gulland, J.A. (Ed.), Fish Population Dynamics. Wiley,Chichester, pp. 35–62.

Shepherd, J.G., 1999. Extended survivors analysis: an improvedmethod for the analysis of catch at age data and abundance indices.ICES J. Mar. Sci. 56, 584–591.

Shepherd, J.G., 2003. Fishing effort control: could it work under thecommon fisheries policy? Fish. Res. 63, 149–153.

Simmonds, E.J., 2005. Comparison of Two Periods of the North SeaHerring Stock Management; Success, Failure and the Value ofManagement. ICES. CM 2005/W:10, 14 pp.

Smith, T.D., Link, J.S., 2005. Autopsy your dead … and living: aproposal for fisheries science, fisheries management and fisheries.Fish Fish. 6, 73–87.

Ulrich, C., Pascoe, S., Sparre, P.J., De Wilde, J.W., Marchal, P., 2002.Influence of trends in fishing power on bioeconomics in the NorthSea flatfish fishery regulated by catches or by effort quotas. Can. J.Fish. Aquat. Sci. 59, 829–843.

Van Beek, F.A., 1998. Discarding in the Dutch Beam Trawl Fishery.ICES, p. 28. CM 1998/BB:5.

Van Beek, F.A., Pastoors, M.A., 1999. Evaluating ICES CatchForecasts: The Relationships Between Implied and RealizedFishing Mortality. ICES. CM 1999/R:4.

Van Beek, F.A., Rijnsdorp, A.D., Van Leeuwen, P.I., 1990. On thesurvival of plaice and sole discards in the otter-trawl and beam-trawl fisheries in the North Sea. Neth. J. Sea Res. 26, 151–160.

Van Keeken, O.A., Van Hoppe, M., Grift, R.E., Rijnsdorp, A.D., 2007.The implications of changes in the spatial distribution of juvenilesfor the management of North Sea plaice (Pleuronectes platessa).J. Sea Res. 57, 187–197 (this issue).

Walters, C.J., 2003. Folly and fantasy in the analysis of spatial catchrate data. Can. J. Fish. Aquat. Sci. 60, 1433–1436.

Wilderbuer, T., Leaman, B., Zhang, C.I., Fargo, J., Paul, L., 2005.Pacific flatfish fisheries. In: Gibson, R.N. (Ed.), Flatfishes.Biology and Exploitation. Blackwell Science Ltd., Oxford, UK,pp. 272–291.

Sustainable use of flatfish resources: Addressing the credibility crisis in mixed fisheries management

Documents