Vessel Surveys

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Survey vessel equipment. Image credit Russ Vetter.  Survey buoy. Image credit SWFSC/NOAA.

 

The Southern California Bight (SCB), between Point Conception and the Mexican border, is a known nursery ground for a variety of sharks including pelagic species like the mako shark (Isurus oxyrinchus), blue shark (Prionace glauca), and thresher (Alopias vulpinus). Since 1994, the SWFSC has conducted annual shark abundance surveys to monitor trends in abundance and conduct life-history studies on these and other associated species such as pelagic thresher (Alopias pelagicus), bigeye thresher (Alopias superciliosus), and basking shark (Cetorhinus maximus). Center scientists are studying the sharks’ biology, distribution, movements, stock structure, population status, and potential vulnerability to fishing pressure. Efforts to determine abundance trends from the commercial fishery data have been complicated by changes in fishing methods and areas and regulatory restriction over time, which have resulted in wide swings in both catch and distribution that are difficult to interpret. Surveys are conducted to track trends in abundance of juvenile and sub-adult pelagic sharks and to determine movements and obtain information on shark age and growth. This information is provided to international, national, and regional fisheries conservation and management bodies having stewardship for sharks. 

Juvenile Mako and Blue Shark Survey

Although the blue shark is targeted as food in Mexico, it has little market importance in the U.S. but is a leading bycatch species in the drift gillnet and high-seas longline fisheries.  Catches of adult blue and shortfin mako sharks do occur, however, the commercial and sport catch of these species off Southern California consists largely of juvenile sharks. Offshore longline surveys from relatively large research vessels have proved most effective for sampling and estimating abundance trends of the more oceanic shortfin mako and blue sharks. For mako sharks, the surveys have enabled the SWFSC to obtain a valuable abundance index, which can be linked to a historical time series of logbook and landings data from a former experimental shortfin mako longline fishery in the SCB that occurred during 1988-1991. Researchers perform tagging, age validation, length sampling, DNA sampling, and other opportunistic data collection while out to sea for more than a week. 

Neonate Thresher Shark Survey

Thresher sharks are specifically targeted by sport fishers, especially off Southern California, and the catch of these species off Southern California consists largely of juvenile sharks. Surveys for neonate thresher sharks are conducted using a small commercial longline vessel. Initial studies demonstrated that neonate threshers are rarely encountered in waters deeper than about 90 m. Therefore, surveys are conducted in the shallower nearshore waters between Point Conception, California and the U.S.-Mexico border. The primary purpose of this survey is to produce a relative abundance index for the West Coast population by periodically sampling 0-year pups (neonates) in their nursery grounds off Southern California. Although conducted aboard a smaller vessel, researchers are still able to perform tagging, age validation, length sampling, and DNA sampling. In addition to providing important information on abundance and distributions, the thresher shark pre-recruit survey enhances other ongoing research at SWFSC, including age and growth, feeding, and habitat utilization studies.

Large thresher

Researchers bring a large thresher shark to the side of the vessel using a shark "hammock". The eyes of the shark are covered with a wet cloth
to prevent any sunlight damage and to calm the shark down while free flowing seawater is continuously flushed through the gills. 

Last modified: 9/6/2017