Scientists on NOAA Research Cruise are Studying the Physiology of Pelagic Fishes and Ways to Reduce Impacts of Longline Fishery Bycatch

November 2, 2006
Live sharks and other fishes studied aboard the NOAA Ship Oscar Elton Sette are lifted aboard carefully and transferred to a sling for handling.

Scientists on the NOAA Ship Oscar Elton Sette are conducting an array of important biological experiments off the Kona coast of the island of Hawaii ("Big Island") and nearby Cross Seamount. Information collected in the experiments will help NOAA find ways to reduce the impacts of bycatch in longline fisheries and improve knowledge of the biological factors that affect the ocean distribution of tunas and other fish species that support valuable commercial and recreational fisheries and play key roles in the pelagic ecosystem.

The Sette expedition involves a diverse team of scientists from the NOAA Pacific Islands Fisheries Science Center and the Joint Institute for Marine and Atmospheric Research (JIMAR), University of Hawaii. The field party also includes a large contingent of collaborating scientists from other NOAA offices, universities in the U.S. and United Kingdom, and other organizations. Chief Scientist for the cruise is Michael Musyl, a JIMAR scientist working at the PIFSC in Honolulu.

Some tunas hauled aboard are returned to the water after being outfitted with satellite archival tags with embedded depth and temperature sensors. Sensor data collected by the free-swimming fish are later transmitted from the tags to onshore computers via satellite.

The scientists on the Sette will conduct a wide range of physiological studies with fish captured on baited longline hooks and other fishing gear. In one set of experiments, researchers will measure cardiac properties and capacities of tuna and mahimahi. The heart studies will shed light on the varying abilities of different fish species to function in various depth and temperature conditions in the ocean. For example, yellowfin tuna mahimahi typically occupy warm, near-surface water, whereas bigeye tuna and bluefin tuna may be found in water near the sea surface but are also able to dive into deep (up to 1000 m), cold water. The studies will involve electrocardiograms (EKGs) of isolated whole fish hearts, detailed investigation of individual heart cells, and more. One of the hypotheses being examined is that bigeye tuna hearts are more tolerant than those of yellowfin tuna to the very low temperatures and low oxygen conditions found in deeper water. If so, this would help explain observed differences in the vertical distribution of these two important tuna species.

Hearing experiments will also be conducted on the Sette to determine whether tuna and mahimahi might be able to detect the sounds produced by fish aggregating devices (FADs) and their associated schools of prey species and thereby locate such prey more readily than by random foraging.

Other physiological studies will be conducted to determine whether blue sharks, a common bycatch species in the longline fishery, have the ability to generate heat to keep their muscles and other organs warm when they descend into cold waters. Blue sharks have a diving behavior similar to that of several other pelagic fish species with known capacities for thermogenesis – species like bigeye tuna, swordfish, mako shark, and bigeye thresher shark. To see if blue sharks have such ability, scientists on the Sette will measure the internal temperatures of red muscles in moribund blue sharks and live blue sharks caught and restrained aboard the ship. A pelagic fish's ability to maintain muscle temperatures above ambient water temperature while descending into colder water increases its metabolic efficiency.

Scientists think that turtles and fish taken as "bycatch" and released back into the sea will have a greater chance of surviving if they are caught on a barbless hook (left), instead of a standard barbed hook (right).

Another important objective of the Sette expedition is to learn more about how to reduce the impacts of bycatch, the capture of unwanted species on longline gear. A major goal of NOAA Fisheries is to reduce the number of fatalities to sea turtles, non-commercial sharks, and other bycatch inadvertently caused by fishermen when they set baited longline hooks to catch tunas and billfish. One way to reduce such fatalities would be to shorten the time required for fishermen to unhook and free the bycatch animal while it is swimming alongside the boat after retrieval. NOAA scientists on the Sette will conduct a study to compare the release times for animals caught with normal barbed hooks versus those caught using barbless hooks. A quicker release is expected with the barbless hooks. However, there is more to the story: the barbless hook solution can be viable only if it is also sufficiently effective in catching the target fish species. Thus the research will also investigate the ability of the barbless hook to retain the bait and hold the target fish after it is hooked. In addition to studying barbless hooks, researchers will explore new ways of releasing bycatch animals using various "de-hooking" devices. The bycatch studies are aimed at finding new ways to catch tuna and billfish with longline gear that reduce harm to sea turtles and other bycatch species while maintaining the profitability of commercial fishing operations.

While at Cross Seamount, the Sette will retrieve, service, and replace a High-frequency Autonomous Acoustic Recording Package (HARP). PIFSC has deployed the oceanographic recording instrument to collect broadband acoustic data including vocalizations from marine mammals in the vicinity of the seamount. Scientists on the Sette will retrieve the HARP, download accumulated acoustic data, perform routine maintenance of the instrument, and redeploy the package to the seafloor. Later, the acoustic data will be analyzed to identify the presence of various marine mammals in the area, including species like false killer whales that interact with longline fishing gear. The purpose of the HARP research is to monitor the presence of cetaceans near the seamount and ultimately to develop ways to census the marine mammal populations using acoustic data.