Interactions of food resource variability with eco-evolutionary harvest impacts

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Matthew P. Schumm and Daniel K. Okamoto, Biology, Florida State University, Tallahassee, FL

Background/Question/Methods
Prediction of sustainability of populations under harvest requires an understanding of how evolutionary effects of harvest interact with non-evolutionary effects, as well as other exogenous variables. We have built a simulation model, currently parameterized for Atlantic cod (Gadus morhua), of harvested populations that we intend to use to evaluate the effects of increased variability in food resources on whether and what changes occur in populations when harvested. The model we are building is a discrete-time superindividual-based model where individuals in discrete genotypes and cohorts consume food, grow, die, and reproduce. This model is an “eco-genetic model” as described by Dunlop, Heino and Deickmann (2009), allowing for both density-dependent effects and frequency- and density-dependent selection to act simultaneously. This model is also a physiologically and bioenergetically structured model based on dynamic energy budget (DEB) theory. The trait in the model that is allowed to evolve is the length-dependent allocation of energy to reversible mass (e.g., fat stores, potential reproduction) over structural mass.
Results/Conclusions
We find that mortality and size selection due to harvest are expected to select for younger age and smaller length at maturity, and that environmental variability can also select for younger and shorter maturers. We also find that food variability has the plastic effect of increasing age at first reproduction, partly or entirely obscuring the evolutionary effects on this trait due to harvest, and that food variability may plastically increase or decrease size at first reproduction depending on presence of genetic variance and evolution. Examining growth and size across fishes’ lifetimes, harvest-induced evolutionary effects are likely to reduce size-at-age more than it is increased by the plastic effects of reduced population density due to harvest, leading to net reduction in size-at-age for evolving harvested populations. Food variability can dramatically reduce size-at-age as well, and can prevent or dampen the phenotypic effects of fisheries-induced evolution on size-at-age, potentially preventing detection of loss of genetic diversity.