Cooperative Studies of Pelagic Fish Habitat and Shark Bycatch Reduction Underway in the Central Pacific

June 9, 2008
Scientists use a de-hooking device to release a live blue shark
Scientists use a de-hooking device to release a live blue shark

The NOAA Ship Oscar Elton Sette is engaged in a pelagic fisheries research expedition in the central North Pacific to learn more about the habitat of commercially-important species of tuna, test methods to reduce bycatch of sharks on longline fishing gear, and conduct a variety of other studies. The research involves collaboration between scientists of the NOAA Pacific Islands Fisheries Science Center, the University of Hawaii Joint Institute for Marine and Atmospheric Research (JIMAR), University of Hawaii Department of Zoology, University of Massachusetts, and the Scripps Institution of Oceanography. Chief Scientist for the cruise is Michael Musyl of JIMAR.

The researchers will use various methods to catch fish, including longlining (primarily), handlining, and trolling. Near each longline hook they will attach a "hook timer" device to record when fish are caught. They will also attach numerous temperature-depth recorders (TDRs) to the longlines to measure temperature conditions in the habitat fished by the gear. They will deploy a conductivity-temperature-depth (CTD) instrument at designated locations to collect more comprehensive information on physical characteristics of the surrounding ocean habitat from the sea surface down to a depth of 500 meters.

Dots indicate positions of a juvenile sandbar shark in a circular tank recorded at 1-sec intervals 
    using a digital video camera. The filled triangle in the upper panel (A) marks the position of an ordinary lead fishing weight placed 
    in the tank and the triangle in the lower panel (B) the position of an electropositive Cerium mishmetal ingot placed in the tank.  In 
    each experimental trial, the shark swam mostly around the periphery of the tank, but its behavior in the lower panel clearly indicates 
    avoidance of the Cerium mishmetal ingot.
Dots indicate positions of a juvenile sandbar shark in a circular tank recorded at 1-sec intervals using a digital video camera. The filled triangle in the upper panel (A) marks the position of an ordinary lead fishing weight placed in the tank and the triangle in the lower panel (B) the position of an electropositive Cerium mishmetal ingot placed in the tank. In each experimental trial, the shark swam mostly around the periphery of the tank, but its behavior in the lower panel clearly indicates avoidance of the Cerium mishmetal ingot.

Data from the TDRs will enable the scientists to measure the temperature conditions at different depths in the water column where the fishing gear is operating and thereby help describe the response of the fishing gear to physical oceanographic and environmental conditions. In addition, by knowing the time and depth at which each fish was caught (from the hook timer data), scientists will be able to estimate, using the CTD and TDR data, the corresponding water temperature and other oceanographic conditions. The combination of fishing data and environmental data provide insights into the distribution patterns of the fish in relation to the environment and the deployed fishing gear (availability and catchability of the fish). Information provided by TDRs and CTDs can be used to predict the fishes' preferred habitat.

In the shark studies, the field party will attach a piece of electropositive metal near the baited longline hooks. During the fishing experiments, researchers expect that the metal device will reduce the catch of sharks without affecting the catch rates of target species of tuna and billfish. The electromagnetic force created by the metal is thought to disrupt, irritate and/or confuse the shark's electrosensory system, causing the shark to avoid the area near the hooks. The repulsive effect of electropositive metal has been demonstrated in shoreside experiments with captive sharks.

Besides the studies of pelagic fish habitat and shark bycatch reduction, research will be conducted on live and moribund specimens of blue sharks to test the hypothesis of thermogenesis in this species. Blue sharks display dive behavior (determined from data gathered using electronic tags) very similar to that of several pelagic species (including bigeye tuna, swordfish, mako shark and bigeye thresher shark) that have specialized tissues and/or anatomical structures enabling them to maintain their body temperature at levels higher than the surrounding water for extended periods of time (for example, during dives into colder deep water). Using minimally invasive techniques, scientists on the Sette will measure muscle temperatures in restrained live and moribund blue sharks caught on longline gear in deep water (below the thermocline). In addition, some specimens of tunas, marlins, and sharks will be retained for tissue sampling and for physiological and genetic studies.