Spatio-temporal dynamics of summer flounder (Paralichthys dentatus) on the Northeast US shelf

Summer flounder (Paralichthys dentatus) are an economically and ecologically important fish on the Northeast U.S. shelf. There is evidence that their spatial distribution has shifted over time. However, there are conflicting reports on the importance of various potential drivers of the shift. Here, we investigate whether the stock has shifted and the extent to which this can be attributed to changes in abundance, size-structure, environmental variables, and fishing. We do so using a vector-autoregressive spatio-temporal model that incorporates data from two seasonal bottom trawl surveys that together span the nearshore and offshore Northeast US shelf over the past 41 years. We find that the summer flounder distribution has shifted north and east in both the spring and fall. The shift is observed in both recruits and spawners, with recruits shifting northward faster than spawners, suggesting that increased spawner abundance may not be driving the shift in recruits. We find that only a small portion of the variability in distribution can be attributed to changes in abundance, fishing, or environmental covariates. Instead, the shift is most strongly attributed to unidentified factors.