Why is haddock overtaking cod? Comparing the effects of temperature and habitat size on both species recruitment dynamics across the N Atlantic

No abstracts are to be cited without prior reference to the author.

Demersal fish stocks in the N Atlantic have experienced serious declines and depletions due to overfishing during the last decades. For some species, such as haddock (Melanogrammus aeglefinus), the situation seems to change and certain stocks are starting to recover. Other species, like cod (Gadus morhua), however, still remain at low levels despite restrictive management measures or even fishing closures. The main aim of our study is to investigate whether the differing responses of these species are related to differing sensitivities of their recruitment dynamics to environmental forcing, and especially temperature. Thus, we developed stock-recruit (SR) models, in a hierarchical framework, combining data across their sympatric populations. By allowing the SR parameters to depend on temperature and habitat 2 size, it is possible to (i) determine the patterns of productivity and carrying capacity in relation to the these factors among and within species and (ii) to borrow strength and provide estimates of increased precision. Temperature during the spawning season was shown to have significant effects on the productivities of both species. Haddock, however, was found to be more resilient to warming conditions, and thus it had a higher reproductive rate than cod at increased temperature. We also found that, for both species, density-dependent regulation is determined by the available habitat size but the relationship is stronger for haddock. Synthesizing these patterns can improve our understanding of environmental impacts on key population parameter and lead to more reliable forecasts under different exploitation and temperature scenarios.