Habitat preferences of istiophorid billfishes in the western North Atlantic: Applicability of archival tag data to habitat-based stock assessment methodologies

• Main Authors: John E. Graves, David W. Kerstetter, Brian E. Luckhurst, Eric D. Prince
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 2003
• Subjects: North Atlantic
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.576.7826; http://www.iccat.int/Documents/CVSP/CV055_2003/no_2/cv055020594.pdf

The Japanese pelagic longline fishery, which has a broad temporal and spatial coverage in the Atlantic Ocean, provides an important time series used in assessments of istiophorid billfishes. Past assessments of Atlantic blue marlin and white marlin by the ICCAT SCRS indicate a pronounced decrease in stock abundance of both species over the past fifty years. The current biomass of the white marlin is estimated to be 15 % of that necessary for maximum sustainable yield (BMSY), while blue marlin are estimated to be at 0.4 BMSY. Over the past fifty years Japanese pelagic longline fishing operations have moved from shallow longline sets targeting yellowfin tuna to deeper longline sets targeting swordfish and bigeye tuna. As billfish are known to spend the majority of their time in surface waters, it was expected that billfish catch rates would be reduced for deeper set longlines. Coupled with the historical decrease in abundance of the stocks, the reduction of catch rates of deeper sets in recent years should be more pronounced. However, in some instances, billfish catch rates of deeper set longlines in recent years have exceeded those of shallow set longlines during times of higher billfish abundance. Such findings are enigmatic, and there are several possible explanations for the observed results (none of which are mutually exclusive): (1) the assessments may not be correct