prevent_escape_chapter_6.8

6.8. ‘ESCAPE THROUGH SPAWNING’: SOLUTIONS TO REDUCE THE ESCAPES OF VIABLE FISH EGGS FROM SEA-CAGES

Fish farming in sea-cages is increasing worldwide; both the volumes produced and the numbers of fish species cultured are increasing. During the last decade, culture of species that may reproduce within sea-cages has become more common. Examples of such species within European aquaculture are Atlantic cod (Gadus morhua; Jørstad et al. 2008) and seabream (Sparus aurata; Dimitriou et al. 2007). Knowledge of the extent and ecological effects of reproduction of farmed fish within sea-cages is, however, sparse.

In the culture of Atlantic cod, some fish mature during the first year of culture, while a majority of farmed cod are believed to mature during the second culture year. This means that almost the entire culture stock in any particular farm has the potential to spawn in the sea-cages before they are slaughtered. Results from the Prevent Escape project (Prevent Escape Compendium Chapter 5.3, Uglem et al. 2012) indicate that spawning in sea-cages is likely to be widespread in the industry and that eggs and larvae (Figure 6.8.1) are likely to survive and spread throughout coastal waters. Therefore, there is considerable potential for larvae from escaped cod eggs to experience favourable conditions for survival and recruitment to coastal cod stocks. This may cause significant ecological and genetic effects in wild populations in the future.

INTRODUCTION

Authors: Tim Dempster1, Ingebrigt Uglem2, Pablo Sanchez-Jerez3, Stelios Somarakis4

1 SINTEF Fisheries & Aquaculture, Norway2 Norwegian Institute of Nature Research, Norway3 University of Alicante, Spain4 Hellenic Centre of Marine Research, Greece

Cite this article as: Dempster T, Uglem I, Sanchez-Jerez P, Somarakis S (2013) ‘Escape through spawning’: solutions to reduce the escapes of viable fish eggs from sea-cages. In: PREVENT ESCAPE Project Compendium. Chapter 6.8. Commission of the European Communities, 7th Research Framework Program. www.preventescape.eu

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In the Mediterranean region, information about spawning by fish kept in sea-cages prior to the Prevent Escape project was sparse. Results from the Prevent Escape project (Prevent Escape Compendium Chapter 5.2) indicate that spawning in sea-cages is likely to be widespread. While eggs and larvae produced by farmed seabream were of poor quality compared to wild fish, large numbers may still survive and flow out from farms into natural environments. They may therefore recruit to wild seabream populations. As for cod, this may cause significant ecological and genetic effects in wild populations in the future.

The emerging species of interest to sea-cage aquaculture in the Mediterranean, the meagre (Argyrosomus regius) did not mature in sea-cages and does not appear to mature (Prevent Escape Compendium Chapter 5.4). It appears unlikely that egg escapes of this species could occur, unless production sizes alter dramatically, or the size at first maturity changes.

From the information generated in the Prevent Escape project and existing literature on the production of other species, it is possible to derive a general principle: pelagic spawning species, where both sexes may be present in the same cage at the same time are at risk of escape through spawning if the weight at maturity is smaller than the harvest weight (Figure 6.8.2).

For new species entering culture, this principle will give the capacity to predict whether they will be susceptible to escape through spawning. Data on the average size or weight at maturity from wild populations cannot be substituted for data on size or weight of maturity in cultured populations. This is evident for both cod and seabream, where the weight or size at maturity is lower than for wild populations. Some capacity to predict whether cultured individuals of a species will have a different size at maturity than wild populations may be available from wild populations if there is evidence for spatial or temporal differences in size at maturity which would indicate that this reproductive trait was relatively plastic and thus able to change.

OVERARCHING GENERAL PRINCIPLE TO AVOID EGG ESCAPE

Figure 6.8.1. Atlantic Cod eggs ready to hatch and a newly hatched larva. Photo:

Tora Bardal, IBI, NTNU.

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Recent developments within the cod farming industry have involved increased growth rates in the sea-cage phase. Slaughter of farmed fish before spawning during the second season in the sea is now common. Hence, a simple, realistic and profitable action to drastically reduce the risk of ecological effects as a result of egg escape is to make slaughter before the second spawning season mandatory.

Technical solutions aimed at preventing spawned eggs to enter the sea have also been suggested. Such solutions could, for instance, involve the use of closed sea cages and mechanical filters to remove eggs. However, closed cages are still not an economically viable solution due to high technological and operational costs. Furthermore, development of technology for closed cage systems is still in an early stage.

Photoperiod manipulation will most likely reduce significantly possible effects of egg escape during the first season through reduced survival of eggs and larvae in the wild. In addition, the use of photoperiod manipulation to culture cod in submerged cages is promising (Kørsoen et al. unpublished data). Photoperiod manipulation may be more effective in submerged cages as the typically high natural surface light dissipates with depth and allows the artificial lights to provide more constant light conditions. Recent results indicate that while this technique does not stop maturation altogether, it may delay maturation such that when spawning occurs it does so outside of the main spawning season. Presumably, this will mean that eggs and larvae will then be subject to environmental conditions that are less suited to their survival.

Other options to reduce the amounts of eggs escaping are hybridization, sterilization and polyploidy. Recent research has shown that production of triploid Atlantic cod may practically eliminate the risk of egg escape, as gamete production by triploid females is delayed and dramatically lowered compared to diploid females and that any larvae produced by triploid

SPECIFIC RECOMMENDATIONS FOR ATLANTIC COD

Fig. 6.8.2. Weight at maturity verses harvest weight for pelagic broadcast spawning species where weight at maturity has been documented in aquaculture settings. Boxes indicate the ranges for different species. Boxes that fall below the black dashed line are at risk of creating ‘escape through spawning’.

fish were non-viable (e.g. Feindel et al. 2010, 2011). This is promising as it indicates that triploid females will not mature before harvest, and growth rates will increase through reduced investment in gonad production. However, problems such as initially higher mortality, greater fingerling costs, maturation of triploid males and consumer acceptance need to be solved before production of triploid fish is taken up by industry (Triantafyllidis et al. 2007; Feindel et al. 2010).

Make slaughter before the second spawning season in the sea mandatory

Introduce photoperiod manipulation

Investigate the combination of culture in submerged cages and photoperiod manipulation to enhance the suppression of sexual maturation

Investigate triploidy as a viable option to reduce the ability of farmed cod to spawn and reduce the viability of any spawned eggs and larvae

There are two specific cases in which the probability of producing more eggs, or that larvae may recruit to the wild populations, are increased for seabream:

(a) the sex ratio in cages is balanced (close to 1:1)

(b) farms occur where seabream are able to close their life cycle, e.g. close to lagoons.

The use of a device such as a curtain-like egg collector as constructed as used in the Prevent Escape project could prevent the dispersion of eggs away from the cages. However, there are significant technical challenges in using such devices and their effectiveness remains to be tested at full-scale. Further, restricting water flow through the upper part of a cage can compromise oxygen levels for the cultured fish and compromise their welfare (Oppedal et al. 2011). Future studies that look into filtration as a method to remove spawned eggs must therefore also take the welfare of cultured fish into account.

KEY RECOMMENDATIONS

SPECIFIC RECOMMENDATIONS FOR GILTHEAD SEABREAM

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Other options to reduce the amounts of eggs escaping would be the use of hybridization, sterilization and polyploidy. However, problems such as initially higher mortality, greater fingerling costs, poorer growth and consumer acceptance need to be solved first.

Available information on seabream ecology is sparse, but indicates that it is probably an estuary-dependent species. Thus, a precautionary mitigation measure to reduce the likelihood of negative ecological interactions arising from escape through spawning would be to restrict the culture of large seabream (of sizes beyond that of sex reversal) in areas close to known nursery grounds of the species, such as lagoons.

Restrict the culture of large seabream (of sizes beyond that of sex reversal) in areas close to known important nursery grounds of the species, such as coastal lagoons

KEY RECOMMENDATION

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Benetti DD, O’Hanlon B, Rivera JA, Welch AW, Maxey C Orhun MR (2010) Growth rates of cobia (Rachycentron canadum) cultured in open ocean submerged cages in the Caribbean. Aquaculture 302: 195-201

Feindel NJ, Benfey TJ, Trippel EA (2010) Competitive spawning success and fertility of triploid male Atlantic cod Gadus morhua. Aquacult Environ Interact 1:47-55

Feindel N, Benfey T, Trippel E (2011) Gonadal development of triploid Atlantic Cod (Gadus morhua). J Fish Biol 78: 1900-1912

Jørstad KE, van der Meeren T, Paulsen OI, Thomsen T, Thorsen A, Svåsand T (2008) "Escapes" of eggs from farmed cod spawning in net pens: recruitment to wild stocks. Rev Fish Sci 16: 1-11

Oppedal F, Dempster T, Stien L (2011) Environmental drivers of Atlantic salmon behaviour in sea-cages: a review. Aquaculture 311: 1-18

Uglem I, Knutsen Ø, Kjesbu OS, Hansen ØJ, Mork J, Bjørn PA, Varne R, Nilsen R, Ellingsen I, Dempster T (2012) Extent and ecological importance of escape through spawning in sea-cages for Atlantic cod (Gadus morhua L.) Aquacult Environ Interact (in press)

REFERENCES CITED

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