Research / Sea Grant News

Study: Sheephead diet linked to body size

Research suggests that fishing pressures on sheephead can impact the larger marine ecosystem. Photo: Kristin Riser

Research suggests that fishing pressures on sheephead can impact the larger marine ecosystem. Photo: Kristin Riser

MOSS LANDING  – The California sheephead is a predatory temperate reef fish that, along with the Southern sea otter, is believed to help control overgrazing of kelp by sea urchins.

A new study, published in the early online edition of the journal Ecology, shows that not all sheephead, however, actually consume the spiny herbivores.

In the kelp forests around San Nicolas Island in the Channel Islands archipelago, only the larger, usually male, sheephead eat the kelp grazers, likely because it takes a larger body size and mouth gape to pry off and break the animal’s hard shell.

The smaller males and females of the species are more likely to consume bivalves, barnacles and shrimp, while fish of all sizes were observed to feast on small crabs, snails and certain kinds of crustaceans.

“We knew that fishing pressure could change the size structure of these fish,” said the study’s lead author Scott Hamilton, a professor at Moss Landing Marine Laboratories, whose research was funded in part by California Sea Grant and the California Ocean Protection Council. “The surprise was to see so clearly in the isotopic data that fishing could also affect the predator’s dietary habits.”

Sequential hermaphrodites, California sheephead are all born female and turn into males when they are large enough to defend a territory and harem. In previous Sea Grant-funded research, Hamilton showed that sport fishing, by removing large “trophy” males, can trigger sex change in females, resulting in smaller males and females, and potentially lower reproductive output (i.e., fewer eggs from the smaller females).

A red sea urchin. Credit: Kirt L. Onthank

Red sea urchins are a dietary staple for large, male sheephead. Credit: Kirt L. Onthank

The new study builds on this earlier work to show that size-selective fishing pressure can also contract the animal’s dietary niche, and conversely that the dietary niche will expand as a population recovers from heavy fishing.

“This study shows that predator-prey relationships can be highly dependent on the sizes of the predators and that fishing activities that change the size structures of these predators may have very complex implications for ecosystems,” Hamilton said. “If you want sheephead to play a role as an urchin predator, you need big sheephead around.”

Seth Newsome with the University of New Mexico and Jennifer Caselle with UC Santa Barbara co-authored the paper “Dietary niche expansion of a kelp forest predator recovering from intense commercial exploitation.”

The Partnership for Interdisciplinary Studies of Coastal Oceans (PISCO), which is funded by the David and Lucile Packard Foundation and the Gordon and Betty Moore Foundation, provided additional support for this project.

Scott Hamilton
Assistant Professor, Moss Landing Marine Laboratories and San Jose State
Moss Landing, CA

Related Works:
Project Profile: Life History of California Sheephead: Historical Comparisons and Fishing Effects
Project Profile: Post-Release Survival in California Sheephead
Project Profile: Catch and Release of California Sheephead: Physiological and Behavioral Stress Effects and Post-Release Survival
Sea Grant News newsletter: Sport Fishing Targeting Large Male Sheephead – Overfishing Triggers Sex Change in Females
2013 Funded Project: Response of Calcified and Fleshy Macroalgae to Warming and Ocean Acidification: From Single Species to Community Interactions
2013 Funded Project: Effects of Ocean Acidification on Olfactory Senses, Swimming Physiology, and Gene Expression in Juvenile Rockfish

Written by Christina S. Johnson,


NOAA’s California Sea Grant College Program is a statewide, multi-university program of marine research, extension services, and education activities administered by Scripps Institution of Oceanography at the University of California, San Diego. It is one of 33 Sea Grant programs and is part of the National Oceanic and Atmospheric Administration (NOAA), U.S. Department of Commerce. Visit our website ( to sign up for email news or follow us on Facebook or Twitter.


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