Investigating variability in catch rates of halibut (Hippoglossus stenolepis) in the Pribilof Islands: Is temperature important?

From 2002 to 2004, a study was conducted to test the hypothesis that commercial catch per unit effort (CPUE) of Pacific halibut (Hippoglossus stenolepis) in the Pribilof Islands local fishery varies on intra-annual (weeks–months) and inter-annual (among fishing seasons) time scales due to (a) variation in seasonal migration timing, (b) temperature-dependent habitat preference, or (c) average local abundance governed by late spring hydrographic conditions. Changes in CPUE over the progression of three fishing seasons were examined for evidence of seasonal migration signals. Relationships between catch and water temperature were examined by deploying temperature recorders from local commercial vessels and correlating the observations with vessel-standardized CPUE intra- and inter-annually. Temperature data were obtained from 735-longline sets; simultaneous depth data were collected from a subset of 412 of those deployments. Annual temperature data clearly demonstrated seasonal warming trends, and temperature was negatively correlated with depth. Warmest conditions were observed in 2003: maximum temperatures of ∼6 °C were observed in early June and increased to ∼10 °C in early September. Conditions in 2002 and 2004 were similar to one another and on average about 1 °C cooler than in 2003, warming from ∼5 °C in June to ∼9.5 °C in September. No intra-annual trend in CPUE was apparent. A relationship was not detected between temperature and vessel-standardized CPUE within years, whether CPUE was compared to absolute temperature or the difference between observed temperature and daily-predicted maximum. These results suggest that halibut do not respond strongly to temperatures within the observed range, nor was there evidence that the fishery was influenced by seasonal movement patterns. These conclusions are supported by concurrent archival tagging that suggests the Pribilof fishery may be too short to capture seasonal migration periods; summer local abundance is likely established prior to commencement of the fishery. Temperatures observed by the fleet were also within the known thermal tolerances of Pacific halibut. Inter-annually, cumulative fleet-wide CPUE and total catch were associated with coolest annual mean temperature. A relationship might exist between annual CPUE and mean temperature, but the available time series is too short to draw conclusions in this regard. While there was no trend in CPUE from June through August or across temperatures ranging from 5 to 10 °C, the results may be valuable in refining hypotheses and focusing future effort. Additional research should seek to examine catch trends earlier and later in the year, as well as at higher and lower temperatures, perhaps at the edges of the species's geographic range or during years characterized by anomalous hydrographic conditions.