Fisheries Models To produce a good fisheries model, we must account for all contributions to reproduction, growth, and mortality, throughout the life cycle of the fishery resource species Mortality Recruitment Reproduction Growth (Nursery Area)
Jan 03, 2016
Fisheries ModelsTo produce a good fisheries model, we must account for all contributions to reproduction, growth, and mortality, throughout the life cycle of the fishery resource species
MortalityRecruitment
Reproduction
Growth
(Nursery Area)
Fisheries ModelsSimilarly, population biomass depends upon growth, reproduction, natural mortality, but also includes the implications of fishing mortality
Population Biomass
Reproduction Growth
Fishing mortality
Natural mortality
Models!
Constructing Fisheries ModelsInitial goal to to determine maximum sustainable yield (MSY)
Complex calculations based upon several life history parameters, including:
Surplus population models – used to search for the largest fishing mortality rates that can be offset by increased population growth, normally measured in changes in population biomass per unit time
population densitypopulation biomasspopulation growth rate
** Equilibrium – point at which processes balance one another
Oh, I forgot to er, carry the one“I first observed this technology at the airport gift shop” – Professor John Frink
Logistic population growth
Populations grow most quickly at intermediate sizes up to a maximum total biomass Bmax
MSY in biomass occurs at a level of fishing mortality that places the population at an
intermediate size
Bmax
MSY
Bmax Bmax
Applying Fisheries ModelsSince MSY is a small target (an actual number) and is also a moving target (due to temporal changes in productivity), actual catch controls are first gauged by simulations of high and low quotas.
If quota set too high:yield would exceed the surplus population so the population would be driven to extinction
If quota set too low:if the population is larger than BMSY – will stabilize and yield lower than BMSY
if population is smaller than BMSY – will become unstable and either increase to equilibrium at the higher population size or crash
Evaluating Fisheries ModelsThe choice of production quotas is minor compared to the procedure of fitting these models to real data to estimate MSY and the level of fishing effort at which it occurs
Several to choose from:e.g., - delay-difference, virtual population, statistical catch-at-age
Yield-per-recruit models – seek fishing mortality rates that achieve the best tradeoff between the sizes of the individual caught, and the number of individuals available for capture
The logic of yield-per-recruit models is based upon the trade-off between growth
and mortality of individuals
A = optimal age at which to catch fish
A
Fisheries Models in ActionIf fishing mortality rates are set too high, too many individuals will be taken before they have had a chance to grow – growth overfishing
Yield per recruit (Y/R) and population biomass per recruit
(B/R) for a single cohort of fish, for various potential fishing
mortalities, F
Y/R
B/R
Fishing mortality - F
Overfishing!
If fishing mortality is too low, although individuals will be large when captured, the total yield will be low
Fisheries ManagementFisheries are managed because the consequences of uncontrolled fishing are undesirable
e.g., - fishery collapse, economic inefficiency, loss of employment, habitat loss, decreases in abundance of rare species
Primary goal – maintain maximum biologically sustainable yield (MSY or BSY)
Recently a mixture of biological, economic, social, and political objectives
MultiplicityCurrent thinking: - concept of MSY may not be useful in fisheries management since overfishing has caused major alterations in the trophic structure of marine food webs
Individual species do not live in a vacuum – they eat each other and may compete for food and space
Biological interactions – mean that population dynamics of different species are inevitably linked