Quantifying changes in the Distribution of Atlantic Cod and Yellowtail Flounder on Georges Bank

Georges Bank (GB) has historically been one of the world’s most productive fishing grounds. Several formerly abundant groundfish stocks have experienced declines that have resulted in the cessation of directed fisheries. Subsequently, management agencies in both Canada and the United States (U.S.) have implemented various restrictions in an effort to rebuild these stocks; these restrictions include the implementation of closures. Yellowtail Flounder (Limanda ferruginea) and Atlantic Cod (Gadus morhua) are two groundfish stocks that have substantially declined and the rationale for closures on GB has included the need to protect these stocks. In the U.S., these closures are larger and focus more broadly on rebuilding these and other stocks on GB, while in Canada the closures are smaller and directed towards the protection of these two stocks during spawning. This document presents temporally variable species distribution models for these two stocks using a suite of static environmental covariates and presence-absence information from groundfish trawl surveys in Canada and the U.S. These models indicate that there have been both seasonal and long-term shifts in the distribution of both stocks. Significant predictors of the distribution of both stocks throughout the year were the average Sea Surface Temperature (SST; average from 1997-2008) and depth (Dep), while sediment type (Sed) was also a significant predictor for Yellowtail Flounder. The seasonal distribution of the core area of Atlantic Cod is similar in the late winter and spring, while in the fall the distribution shifts to the edge of the bank. For Yellowtail Flounder, the core area is found in a similar location throughout the year. The distribution of Atlantic Cod differed approximately every five years, while the Yellowtail Flounder distribution differed every three years. These shifts in distribution are not random, with the centre of gravity of the core areas for both stocks shifting to the northeast throughout the study period. Much of this shift is due to the loss of core area from southern and western portions of GB. The shifts in the distributions have resulted in an increase in the proportion of both stocks in Canadian waters throughout the year. During Atlantic Cod spawning there has been a substantial decline in the core area inside the two large closures in the U.S. In recent years, Yellowtail Flounder have predominately straddled the Canada-U.S. border; during spawning, the proportion of core area observed in Closed Area II (U.S.) has steadily increased to almost half of the total core area on GB. In Canada, the closures to protect Atlantic Cod and Yellowtail Flounder during spawning both predominately include core area, but due to their limited size these closures make up only a small proportion of the core area on the Canadian portion of GB. Finally, during spawning, the models for both stocks were successful at capturing the spatial dynamics up to three years into the future. The framework methodology applied here provides novel insights into both seasonal and inter-annual variability in the spatial distributions of these stocks and provides a quantitative evaluation of closures relative to these distributions. Incorporation of the information from approaches such as this into the science advice process, can facilitate improved decision-making for fishery management.

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