Body condition as a shared response to environment in a commercially important demersal fish assemblage

Philina English, Sean C. Anderson , Robyn E. Forrest

Measures of an organism's weight at a given length are often considered reliable indicators of energy reserves or `condition', which can be related to fecundity and risk of mortality. Understanding the impact of environmental change on fish condition may therefore be critical for sustainable management of human activities in marine ecosystems.
We investigated how changes in Canadian Pacific waters may be influencing average condition of 35 commercially, culturally, or ecologically important demersal fish species. Because condition of mature male and female, and immature individuals have different implications for population dynamics, ecological drivers, and measurement, we separated individual fish and overall catches into these components, then estimated density distributions, calculated Le Cren's relative body condition deviations, modelled spatiotemporal change in these deviations, and generated density-weighted annual indices of body condition. We then used Bayesian Dynamic Factor Analysis to identify common trends across species and tested for correlations with environmental variables. For most species, warmer sea surface temperature and lagged North Pacific Gyre Oscillation appeared neutrally or positively correlated with condition. Only immature condition was also strongly correlated with primary production, but this effect was equally likely to be negative (e.g., Pacific Spiny Dogfish, Lingcod, Sablefish) as positive (e.g., Quillback Rockfish, Southern Rock Sole, Spotted Ratfish). Our approach propagates uncertainty from condition estimation through to environmental correlations and provides both an ecosystem perspective as well as species-specific inference. Robust estimates of relationships between condition and environmental variables can inform ecosystem approaches to fisheries management including short-term forecasts of weight-at-age or recruitment.