Age- versus size-structured populations: The ecological distinction and its importance for fisheries
Decades of intensive fisheries have brought us to the point where marine predator population collapses are a ubiquitous phenomenon. Fisheries management, which in many cases has been developed alongside the intensification of fisheries, has not been able to prevent population collapses. Throughout this period, fisheries management has relied on population models that are structured according to age-cohorts. Furthermore, predation mortality is merely taken into account as a loss-term for prey. I present results from two size-structured models that contrast with age-structured predictions. The first model scenario is of competition for herring, reflecting competition between seal and cod, while these predators have different prey-size preferences. The second model is a more trophically complex system, in which we study the potential competition between seal and fishermen, when both target the same cod population.
Mortality, as for example induced by predation or fisheries, can lead to an increase rather than an intuitively predicted decrease in prey biomass (i.e. overcompensation). In the cod-seal competition model, this (stage-specific) biomass overcompensation results in facilitation of cod through predation imposed on herring by seal. In addition, overcompensation may lead to facilitation among marine mammals and fisheries, as is shown in the second model, accounting for the predation imposed on cod by seal, which improves the harvestable cod stocks (preferred size-classes) for fisheries. These results only become evident when population models account for population size- rather than age-structure, and are based on a consistent individual energy budget that incorporates food- or prey-dependent growth and reproduction.