Standardized Catch Per Unit Effort of Albacore Tuna (Thunnus alalunga) for the Taiwanese Distant-water Longline Fishery in the North Atlantic Ocean, 1967-2011

The trends in nominal catch per unit effort (CPUE) can be influenced by many factors in addition to stock abundance, including the choice of fishing location, operating strategy, and target species. Consequently, CPUE data need to be ‘standardized’ to remove the impact of such factors, and then could be used as an index of relative abundance in fisheries stock assessments. The catch and effort data for albacore tuna caught by the Taiwanese distant-water longline fishery in the North Atlantic Ocean from 1967 to 2011 were standardized using alternative approaches, i.e., generalized linear models (GLMs) and the delta approach. Sensitivity was explored to whether targeting effects of other species are included in the analyses. Year and area explained the most of the deviance (78~86%, depending on the model configuration), and both of them were identified consistently among methods. However, the trends in standardized CPUE of albacore tuna were found to be relatively insensitive to the approach used for catch and effort standardization, including whether the targeting effects were included in the analyses. With a drop before 1970, the trend in standardized CPUE of albacore in the North Atlantic Ocean was relatively stable during the 1970s to the late 1980s, but generally decreased thereafter and increased slightly from 2003 until present. The standardized CPUE of albacore from GLM for this fishery could be considered the best available results to reflect the relative abundance of the albacore stock in the North Atlantic Ocean.