The role of size-structured food webs in the changing structure of the Scotian Shelf fish community

Background/Question/Methods

The collapse of the cod fishery on the Scotian Shelf in the early 1990s led to a dramatic and persistent decline in the abundance and body size of predator fish over the last two decades. Hypotheses for the inability of these large predators to recover include predator-prey role reversal, wherein numerically dominant fish at lower trophic levels (forage fish) prey upon the early life stages of large predator fish, and intensified competition between these forage fish and small-bodied predators . Though there is correlative evidence for these hypotheses, here we explore their role more explicitly through the use of mechanistic, size-structured partial differential equation models. These models account for intraspecific variability in body size and diet by using species traits and individual physiology to track the flow of biomass through time and along a size spectrum (i.e. from small prey to larger predators). In order to evaluate the role of the above mechanisms in preventing the recovery of large predators, we develop a hierarchical Bayesian framework for such a size-structured food web model and apply it to 33 years of abundance-at-length estimates from 13 species from the Scotian Shelf. 

Results/Conclusions

The size-structured food web model is successful in capturing the dynamics and structure of the Scotian Shelf fish community following the collapse of the cod fishery, though it generally performs better for large size classes than small. Estimates of food web coupling strengths in an aggregate tri-trophic model (containing resource, forage, and predator trophospecies) suggest that forage species exert greater predation pressure on predator species than predators on forage species, and that forage species are out-competing predators for access to resources. This supports the hypotheses that predator-prey role reversal and increased competition between small size classes of predator and forage species may be inhibiting the recovery of large predator species. These results offer mechanistic insight into the processes structuring the Scotian Shelf fish community and have the potential to inform management policies that facilitate recovery and prevent such dramatic collapses from occurring elsewhere. In addition, this work provides the foundation for future work that explores more broadly the trait diversity and food web mechanisms required to effectively model the dynamics of real marine communities.