Recently published in Fisheries Oceanography,by Nereus Alumnus Andre Boustany (Duke University) and Principal Investigator Patrick Halpin (Duke University), was the study “Tuna and swordfish catch in the U.S. northwest Atlantic longline fishery in relation to mesoscale eddies”. This research looks at the effects of different variables on the catch of tuna and swordfish — including mesoscale eddies, which are a type of ocean current, sea surface temperature and fishing gear used.
Mesoscale eddies are swirling masses of water which last from months to years and transport heat, salt and carbon around the ocean. Eddies are classified as anticyclonic, rotating counterclockwise and upwelling water at the center, or cyclonic, rotating clockwise and downwelling water. The researchers found that bluefin tuna catch was highest in anticyclonic eddies whereas yellowfin and bigeye tuna catches were highest in cyclonic eddies. Swordfish catch was found preferentially in regions outside of eddies.
It was also found that the number of light sticks used was an important variable in estimating catch. Light sticks, like glow sticks with chemiluminescent fluids, are sometimes attached to the fishing line above the hook to attract fish, mainly at night. The increased use of light sticks was found to decrease bluefin tuna catch, while increase swordfish catch.
“We argue that overfishing of bluefin tuna can be alleviated and that swordfish can be targeted more efficiently by avoiding fishing in anticyclonic eddies and in near-shore waters and using more light sticks and fishing at night in our study area, although further studies are needed to propose a solid oceanography-based management plan for catch selection,” write the authors.
Abstract
To analyze the effects of mesoscale eddies, sea surface temperature (SST), and gear configuration on the catch of Atlantic bluefin (Thunnus thynnus), yellowfin (Thunnus albacares), and bigeye tuna (Thunnus obesus) and swordfish (Xiphias gladius) in the U.S. northwest Atlantic longline fishery, we constructed multivariate statistical models relating these variables to the catch of the four species in 62 121 longline hauls made between 1993 and 2005. During the same 13-year period, 103 anticyclonic eddies and 269 cyclonic eddies were detected by our algorithm in the region 30–55°N, 30–80°W. Our results show that tuna and swordfish catches were associated with different eddy structures. Bluefin tuna catch was highest in anticyclonic eddies whereas yellowfin and bigeye tuna catches were highest in cyclonic eddies. Swordfish catch was found preferentially in regions outside of eddies. Our study confirms that the common practice of targeting tuna with day sets and swordfish with night sets is effective. In addition, bluefin tuna and swordfish catches responded to most of the variables we tested in the opposite directions. Bluefin tuna catch was negatively correlated with longitude and the number of light sticks used whereas swordfish catch was positively correlated with these two variables. We argue that overfishing of bluefin tuna can be alleviated and that swordfish can be targeted more efficiently by avoiding fishing in anticyclonic eddies and in near-shore waters and using more light sticks and fishing at night in our study area, although further studies are needed to propose a solid oceanography-based management plan for catch selection.
Nereus researchers found that swordfish catches were highest in regions outside eddies and increased with the use of light sticks. Image: "Hooking a swordfish" by Paul Glavin, CC BY-NC-ND 2.0.
