Habitat Preferences of Istiophorid Billfishes in the Western North Atlantic: Applicability of Archival Tag Data to Habitat-Based Stock Assessment Methodologies

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 (billfish abundance may not be as low as it is currently estimated to be); (2) deep set longline hooks may fish at shallower depths than estimated; (3) the majority of billfish may be taken as deep set longlines are deployed and retrieved (a longer time for the bait to be moving through the water column); or (4) billfish may preferentially feed at depth.

In this paper we provide detailed time-at-depth information for two blue marlin released alive from pelagic longline gear with pop-up archival satellite tags (PSATs) programmed to release from the animals after 30 days. Both blue marlin made several dives each day. The fish moved quickly to depth, and the times at depth were greater than those spent in descent or ascent. The depth of dives was greater during daylight hours. These data are suggestive of feeding excursions to deeper waters, movements for which these fish are well adapted with large eyes and a brain (eye) heating organ. If billfish undertake specific feeding movements to depth, then one would expect increased catch rates for deeply set longline gear. Researchers have recently attempted to standardize catch rates of billfish on pelagic longline for the time fish spend at depth (habitat-based models). However, if feeding motivation is not the same for an animal at all depths, then such standardizations will significantly bias catch rates and lead to errors in assessment.