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California Sheephead Populations Dwindling

Researchers Claim Fishing Disrupted Species’ Intricate Gender-Change Process


Scientists have observed a sharp decline in the numbers and sizes of California sheephead inhabiting waters near the Channel Islands, which historically have been home to abundant populations of the fish.

Dr. Scott Hamilton, a postdoctoral research biologist at UCSB, explained that for years sport fishing has selectively targeted big, strong male sheephead, whose absence upsets the social organization of the sheephead community and causes a dramatic reduction in its size. Hamilton and Dr. Jenn Caselle, a research scientist at UCSB, are currently studying the size structure, life history, and reproductive function of this curious fish, along with collaborators Dr. Chris Lowe, Dr, Kelly Young, and Kerri Loke at Cal State University Long Beach.

Scientists have long known that every sheephead (Semicossyphus pulcher) as it’s known scientifically, is born female. When she becomes large enough and old enough to secure and protect a territory, a female sheephead experiences the necessary hormonal changes to change sex and become male. Scientists term this process “protogynous hermaphrodism.” A female sheephead will not switch her sex, however, if she feels threatened by an aggressive alpha male that’s calling the shots in a given territory. “These large terminal-phase males will control and patrol a particular area,” Hamilton said. “There will be a harem of females they’ll mate with exclusively and they’ll exclude other males from their territories.”

The natural process is such that when an alpha male dies, the largest and most capable female in the area will switch her sex to take his place. But the sudden absence of a dominant male caused by sport fishing will still signal a female to switch sex, even if she’s too young and too small to effectively patrol the territory that her male predecessor did. “Fishing will take a large male out of the system, and all of a sudden there’s no male around,” Hamilton said. “That will stimulate a female to change sex into a male, so in places like Catalina where fishing is removing a lot of the big males, you’re seeing a lot of really small, tiny males, a lot of dwarf males which you didn’t see historically.”

The result, then, of sport fishing in the Channel Islands is an overabundance of tiny, underdeveloped males and a shortage of egg-producing females. Premature sex-change also causes a deficiency of older, bigger females, which are capable of producing a greater number of eggs.

Essentially, you see that these fish aren’t spending as many years producing eggs as a female, because they’re changing sex at a smaller size and a younger age,” Hamilton said.

Hamilton cited historical collections figures from the 1970s and 80s which reflect a substantial population of large sheephead in the Channel Islands. More recent collections, though, reveal that the fish is shrinking in physical size, and that its population is beginning to dwindle. Such changes, Hamilton said, are a direct result of sport fishing and the commercial fishing boom of the 1990s.

Hamilton cited San Nicolas as a glimmer of hope - it’s an example of a case in which sheephead populations were able to make a comeback after experiencing severe declines caused by periods of unregulated fishing.

In the nineties, sheephead at San Nicolas were changing sex seven or eight years earlier than they should have been, and they were dwindling in size, as was the island’s entire sheephead population. After 9/11, though, the U.S. Navy tightened restrictions on the waters surrounding San Nicolas, which significantly reduced instances of commercial fishing in the area.

Restriction of access to San Nicolas coincided with a spike in gas prices, which discouraged boats from reaching the remotest of the Channel Islands. A further obstacle between fishermen and San Nic came in the form of new fishing regulations drafted by the Department of Fish and Game, including catch and size limits.

Hamilton said that a return visit to San Nicolas in 2008 confirmed a recovery in the sheephead population. In fact, the sheephead are “growing faster on San Nic than anywhere else in Southern California,” he said. He also said that the male-to-female sex ratio appears to be much more even and that the sheephead have increased dramatically in size. Monitoring of fish populations in the Channel Islands has proven that sheephead inhabiting marine reserves are generally larger and more abundant.

To that end, Hamilton and Caselle plan to amass data to present to the Department of Fish and Game in order that it can devise a means of reviving the California sheephead population, particularly in the waters surrounding Catalina Island.

Hamilton suggested that implementing slot limits, or setting thresholds for the minimum and maximum sizes a fisherman can legally hook, will begin to alleviate the sheephead problem. Slot limits, Hamilton said, will allow large alpha males to escape fishing pressure, which will likely restore some sense of balance to the life-cycle of the California sheephead.

Rachel Reeves is an Independent intern.



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