UW Aquatic & Fishery Sciences Quantitative Seminar
UW-School of Aquatic and Fishery Sciences
Nine decades of changing length distributions in Norwegian coastal cod: climate, competition, and spatial heterogeneity
Changes in size at age have been observed for many fish populations, but disentangling the relative effects of climate, density, and harvest on size distributions requires lengthy, high quality datasets which are rare for marine systems. I will present a 91-year time series of individual juvenile cod (Gadus morhua) lengths from surveys which began in 1919, representing 177 stations along the Norwegian Skagerrak coast. We use these data, combined with environmental observations, to determine the effects of warming temperatures and density on cod lengths and length variability. Using linear mixed effects models, we account for spatial population structure and the nested structure of the survey data to reveal some underlying biological processes shaping both the mean and variance in cod lengths. We find that warm temperatures in the coastal Skagerrak are limiting for juvenile growth in the summers. In contrast with this result, warmer springs result in larger juvenile cod, with less variation in lengths within a cohort, likely due to a temperature-driven contraction in spring spawning period. We also find strong evidence for density-dependent growth at this juvenile stage. These results contrast with earlier studies which ignored spatial population structure and biases arising from sampling, and illustrate the importance of long ecological time series, when combined with good statistical tools, for understanding the potential impacts of climate change on marine populations.