The Japanese Bluefin Tuna Longline Fishery in the Northeast Atlantic: Report of an Irish Observer

Abstract

This paper describes an observer trip on a Japanese freezer longliner in the international waters of the North East Atlantic in the autumn of 1997. The observation period was 71 days during which 7 species were recorded as catch and bycatch from 57 sets. Although bluefin tuna was the target species, with 18,894kg gilled and gutted weight (GWT) caught, the most frequently retained species was blue shark, Prionace glauca, followed by bluefin tuna. Three shortfin mako shark, Isurus oxyrinchus, one swordfish, Xiphias gladius and one anglerfish, Lophius spp., were also recorded. The most frequently discarded species were lancetfish Alepisaurus ferox, and deal fish Trachipterus arcticus. Bluefin tuna ranged from 139cm to 275cm in fork length (FL) and from 64kg to 347kg in round weight (RWT) with clear modes of 190cm and 136kg. Recent ageing results from the North East Atlantic infer ages of 4 to 17 years old. CPUE was lower than Norwegian sponsored trials in the North East Atlantic in 1998 and similar to those computed by the Japanese longline observer programme in 2000, 2001 and 2002. There was no evidence of trend in bluefin CPUE over the course of the observation period. The modelled length weight relationship predicted higher values than established length weight relationships for bluefin tuna in the East Atlantic: ICCAT modelled RWT for East Atlantic bluefin was 87% of the observed round weight and 90% of the predicted RWT value for Koshin Maru #8 tuna. Over the observation period the condition of bluefin tuna was found to decline and examination of stomachs showed that most were empty or contained low numbers of prey items. Declining condition factors and apparent scarcity of prey are discussed in the context of CPUE. Prey scarcity reflected in declining condition may increase the effectiveness of baited hooks causing abundance estimates derived from CPUE series to over-estimate the population of bluefin tuna in the North East Atlantic. Investigation of condition indices has the potential to estimate stock ratios in longline catches in the North Atlantic. All observed blue shark catch were female with lengths ranging from 140cm to 250cm.