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Study Concludes Gillnet Fishing Has ‘Modest’ Impact On Sharks

Published: September 3, 2013

Commercial gillnet fishing in Southern California’s coastal waters “modestly” affects young great white sharks and the survival rate for sharks accidentally caught in gillnets is high, according to a new research study led by the CSULB Shark Lab in partnership with the Monterey Bay Aquarium.

The paper, “The degree and result of gillnet fishery interactions with juvenile white sharks in Southern California assessed by fishery-independent and -dependent methods,” appears in the Aug. 20 online edition of the journal Fisheries Research.

The lead author is master’s student Kady Lyons, a member of the CSULB Shark Lab directed by Marine Biology’s Christopher Lowe. Lowe is a co-author along with CSULB alumna Erica Jarvis of the California Department of Fish and Wildlife (DFW); Salvador Jorgensen and John O’Sullivan of the Monterey Bay Aquarium; Kevin Weng of the University of Hawai‘i at Manoa; and Chuck Winkler of the Southern California Marine Institute. The researchers are collaborators in Monterey Bay Aquarium’s Project White Shark which funded the study.

Southern California’s coastal waters are nursery grounds for juvenile great white sharks. Assessing the interaction and effect of young white sharks with gillnet fisheries is important for proper management and conservation.

The researchers concluded:

• About 18 percent of DFW fishing block areas where white sharks were detected overlapped with blocks that were also heavily utilized by gillnet fisheries.
• Total gillnet effort tended to peak in the month of July before declining substantially whereas detections of tagged sharks were the most numerous in fall months.
• Nearly 93 percent of sharks retrieved live in gillnets and tagged by researchers survived.

“We concluded that, overall, commercial gillnet fishing only modestly affects young white sharks in the area,” Lyons said.

“We were interested at looking at this because of a recent petition to list white sharks under state and federal Endangered Species Acts,” explained Lyons. “It’s important to provide fisheries managers the best available data to make the most informed decisions. One petition argument for listing white sharks as endangered has to do with the impact of gillnets on the babies.”

Gillnets are large nets that commercial fishing crews deploy in one of three ways—fixed along the seafloor bottom, or drifting along with the current farther or nearer to shore—depending on the kinds of fish they’re trying to catch. Because gillnets caught unintended species such as sea lions, dolphins and sharks, California voters enacted Proposition 132 in 1994. The initiative banned gillnet fishing within three nautical miles of the state coast and within one nautical mile of islands. Subsequently, the number of gillnet fishers significantly decreased. Yet, the number of young white sharks incidentally caught in the nets increased. Researchers say it is difficult to conclude if the rise in white shark-fishery interactions is due to an increase in the number of young white sharks or a shift in fishing location and effort.

Fishing crews that incidentally catch young white sharks must report catches to the DFW. Some crews voluntarily participate in the Monterey Bay Aquarium’s white shark program and bring incidentally-caught live sharks into port where researchers measure the animals, outfit them with an electronic tracking tag, implant a small acoustic transmitter in their body and then release them back to sea. Researchers also specifically target, capture, tag and release other white sharks and track their movements over time.

Gabe Bartalos Work
PHOTO COURTESY OF CHRIS LOWE
Graduate student Kady Lyons tests white shark tissue samples on the gas chromatograph mass spectrometer in CSULB’s Institute for Integrated Research in Materials, Environments and Society.

The researchers examined DFW fishing logs and white shark catch-and-release satellite tagging data from 2006 to 2009 to map the waters that sharks and fishing crews both use, and how much they overlap.

“The most interesting thing we found is that sharks are using larger areas than the fishermen, who concentrate their effort in certain locations outside state waters. The amount of overlap between the two areas is smaller than anyone thought,” Lyons said.

“The sharks spend about half their time inside the three-nautical-mile buffer where gillnet fishing is prohibited,” she continued. “In addition, the peak fishing season occurs earlier than the peak season when young sharks are in Southern California. They’re occupying different places and at different times.”

Young white sharks like to eat fish while adults generally feed on marine mammals. They appear to prefer shallower coastal waters where food along the bottom is more plentiful.

The study documents a 92.9 percent survival rate for sharks caught accidentally in gillnets in Southern California waters. “We see comparably high survival rates for these sharks when compared with sharks that we directly targeted and the handling time was much less. This finding is encouraging,” Lyons added.

Lowe noted that scientists don’t fully understand why white sharks have a higher survival rate when netted than many other shark species. Regardless, he encourages anyone who catches a young great white shark—a protected species in California and Mexico—to release it as soon as possible.

Overall, this research benefits both scientists and the fishing crews, Lowe said. “The fishermen who participated were genuinely interested in what we were doing. We’re hoping that the information we’re giving back to them will further reduce interactions and increase survival rates. That’s good for their fishery and it’s good for the sharks.”

The study is available at http://dx.doi.org/10.1016/j.fishres.2013.07.009.