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