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 Growth Recruitment Reproduction (Nursery Area)

Fisheries Models Population Biomass Similarly, population biomass depends upon growth, reproduction, natural mortality, but also includes the implications of fishing mortality Reproduction Growth Population Biomass Models! Fishing mortality Natural mortality

* Equilibrium – point at which processes balance one another Constructing Fisheries Models Initial goal to to determine maximum sustainable yield (MSY) 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 Complex calculations based upon several life history parameters, including: population density population biomass population growth rate * Equilibrium – point at which processes balance one another *

Logistic population growth Oh, I forgot to er, carry the one “I first observed this technology at the airport gift shop” – Professor John Frink Logistic population growth Bmax 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 MSY Bmax Bmax

Applying Fisheries Models Since 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

A = optimal age at which to catch fish Evaluating Fisheries Models The 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 A 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

Fisheries Models in Action If fishing mortality rates are set too high, too many individuals will be taken before they have had a chance to grow – growth overfishing If fishing mortality is too low, although individuals will be large when captured, the total yield will be low Y/R B/R Fishing mortality - F Yield per recruit (Y/R) and population biomass per recruit (B/R) for a single cohort of fish, for various potential fishing mortalities, F Overfishing!

Fisheries Management Fisheries 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

Multiplicity Current 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