Case of the Shrinking Fish: Causes of Declines in Size-at-Age of Pacific Halibut

Pacific halibut (Hippoglossus stenolepis), a culturally and commercially valuable flatfish in the North Pacific, have undergone a 60% decline in size-at-age since the 1980s.  For example, an average age-20 female halibut weighed 120 pounds in 1988 and < 45 pounds in 2013.  Our study aims to identify causes of declines in size-at-age. We hypothesize that size-at-age is influenced by environmental variability, changing ecological conditions including intra- and inter-specific competition (density-dependent growth), changes in bioenergetics, and size-selective fishing. We test these hypotheses using a regression approach, where the mean annual change in length of each cohort is modeled as a function of covariates. Specific potential environmental covariates include the Pacific Decadal Oscillation, which is an index of regional sea surface temperatures, and ocean bottom temperatures recorded at an oceanographic station (GAK1) in the northern Gulf of Alaska near Seward, Alaska.  Specific ecological variables include annual biomass estimates of arrowtooth flounder (Atheresthes stomias) and Pacific halibut to investigate the potential role of inter- and intra-specific competition, respectively. While all factors appear to influence halibut size-at-age, preliminary results suggest a strong negative correlation between arrowtooth flounder biomass and halibut growth. We discuss the implications of our findings on halibut conservation and fishery management.