Harvesting and habitat specific productivity governs the structure of lake trout populations

Background/Question/Methods

The importance of stage or size structure in populations has become increasingly acknowledged in food web and population ecology. Most organisms grow and go through ontogenetic development during their life. Subsequently, populations in general display some kind of stage and/or size structure.

Theoretical work has shown that the most limiting processes in a stage structured population, being either development or reproduction, is crucial for how a population will respond to harvesting and for interactions with other species. When populations are subjected to harvesting, compensatory responses take place through both increased developmental rate and increased fecundity. However, these processes will respond to different extents, resulting in redistribution of biomass between stages and subsequently changes in the stage structure of the population.

It is common that juvenile and adult stages differ in their resource use, then the relative productivity of the juvenile and adult resources will be of importance for the limiting process and the compensatory responses mentioned above. Hence it is important to understand how and if these limiting processes and responses change over variation in habitat specific productivity and other environmental gradients

Results/Conclusions

We analyze changes in lake trout populations across gradients of productivity, lake size and harvesting. Importantly we calculate whole lake production, but also separate the production in the benthic and pelagic habitat, since resource utilization of juvenile and adult lake trout differ. Preliminary results show that biomass distributions shift towards more juveniles with increasing contribution of benthic production relative to pelagic production. This could be related to the fact that large lake trout can acquire resources from the benthic habitat. In contrast, juvenile lake trout typically feed on zooplankton and rely on pelagic production. Harvesting resulted in more juveniles, which indicates that lake trout populations are regulated by reproduction rate rather than development. We relate our results to stage structured theory and recent studies on food web structure and habitat coupling.