Applying the ideal free distribution to the movement of a highly mobile gillnet fishery for Pacific salmon

Ecological models are used to understand where fishing vessels operate, improving fisheries management success. The ideal free distribution (IFD) predicts equalization of catch across locations in response to distributions of fish and competing vessels. We applied the IFD as a null model to investigate the movement and catch per vessel (CPUE) of the sockeye salmon drift gillnet fleet in Bristol Bay, AK, from 1980 to 2019. The IFD prediction of equal CPUE across areas was not supported, so we explored violations of two assumptions of the theory. We categorized each vessel according to its mobility, highlighting the "free to move" assumption, and found that when we removed nonmobile vessels, some districts had fewer vessels than predicted from their high CPUE. We examined the "equal competitive ability" assumption by calculating the average CPUE, relative to success of other vessels. The underutilization of profitable districts persisted. Overall, the fleet underused high CPUE areas when considering differences in mobility and relative fishing success, indicating that unmeasured safety concerns, travel costs, knowledge of fishing grounds, or other factors could be preventing an IFD.