Life History of California Sheephead: Historical Comparisons and Fishing Effects
Marine Science Institute
University of California
Marine Science Institute
University of California
Dept. of Biological Sciences
CSU Long Beach
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May 7, 2012
May 3, 2012
Contact: Christina S. Johnson, firstname.lastname@example.org, 858-822-5334
R/OPCFISH–05 Feb. 2007–Feb. 2010
The California sheephead (Semicossyphus pulcher) is a sequentially hermaphroditic species. All fish are born female. Social cues trigger the largest females to turn into males. The largest “alpha” males, in turn, defend territories and harems. Fishermen preferentially target both females and males: Commercial fishermen seek out the small platter-sized females (which can be sold at a premium live to Asian markets) while “trophy” sport fishermen tend to selectively remove the biggest males. One of the main findings of this project is that recreational fishing is hammering populations of large male sheephead. Off Catalina Island, the most visited of the Channel Islands, there is a virtual absence of alpha males. Notably, the current legal size limit would allow all males to be fished out legally at this site.
For the project, researchers combined historical and diver survey data from nine study sites in Southern California to document fish abundances, sizes and other life history characteristics. Predation studies were also conducted, as were gut-content analyses. Where possible, fish statistics were compared to historical records of fish size and age of sexual maturity. Scientists examined both effects of fishing pressure on sheephead life-history characteristics – size, growth, survivorship and age of sex change – and how they varied, irrespective of fishing activity, with geographic locale. In the project's final stages, a computer model was developed, based on field data from the nine study sites, to explore management options for improving fishery yield and sustainability. With the model, scientists were able to identify some management options for protecting the species and improving fishery production.
Need for Research
The average California sheephead is smaller than it once was; females are reaching sexual maturity earlier, and they are transforming into males younger, and at odd times in the year. Researchers were funded to investigate why fish size and reproductive structure has changed. The species warrants such focused study, as it is both recreationally and commercially fished and is a key member of the giant kelp forest ecosystem. Additionally, large sheephead (but not small ones) are, in the absence of sea otters, a main natural control against overgrazing of kelp by sea urchins. Sheephead populations are thus critical to maintaining the predator-prey relationships that sustain the kelp forests.
This study shows that repeatedly removing large dominant males triggers early sex change in females, resulting in both smaller males and smaller females. Because larger females produce more eggs, continuous fishing pressure on males actually (and somewhat paradoxically) reduces total egg output. The gender-bending life history characteristics of sheephead make them especially vulnerable to overfishing. The observed declines in fish sizes, at sites like Catalina, are likely due to heavy fishing pressure (as opposed to water quality issues, which was once theorized).
The California Department of Fish and Game is considering re-doing its 2004 sheephead stock assessment in light of what has been learned during this project and others. There have, however, been no changes in the fisheries’ management or catch limits based on recent research. The department's “Update on Nearshore Fishery Management Plan Implementation” states that changes "have not been implemented pending further research."
The results of this project fulfill many of the department's goals for improving Essential Fishery Information and could also be considered in the pre-assessment of the fishery, as part of the California Sustainable Seafood Initiative initiated by OPC and its partners.
Results also underscore why reevaluating the management plan should be a priority: The current minimum-size limit (30 centimeters for both recreational and sport fishers) is ineffective at protecting the full-life history of a fish with both male and female life stages. A larger size limit or slot limits (establishing upper and lower bounds on legal-sized fish) are options for ensuring that both sexes are present to reproduce.
New studies also suggest that only sheephead greater than the 30-centimeter size limit are capable of preying on urchins. Smaller sheephead feed on softer, smaller benthic invertebrates. This means that current regulations could encourage, or allow, the creation of urchin barrens in some places.
Output from the computer model mentioned above suggests that highly localized management approaches, albeit impractical, might increase fishery yields by about 25 percent without depleting spawning biomass. More significantly, a similar increase in yield might be achieved by subdividing the fishery into two zones, each with different minimum size limits. In the northern portion of the Southern California Bight and northward up the coast (i.e., the northern Channel Islands, San Nicolas, and Santa Barbara Islands, and along the northern mainland coast of Santa Barbara and Ventura counties), the minimum size limit would need to be increased 90 millimeters. The 30-centimeter size limit is sufficient in the southern portion of the bight (i.e., south of Palos Verdes and the southern Channel Islands, including San Clemente and Catalina). Alternately, a uniform 50-millimeter size-limit increase could boost yields by about 15 percent.
Results highlight the need for a mechanism to enable CDFG staff to incorporate data sets and innovative approaches to fisheries management from outside entities. This challenge was identified in the “Lessons Learned from California’s Marine Life Management Act” report and is referenced in other literature (See Phipps, Fujita and Barnes, 2010).
Scientists have held two sheephead workshops, at which academics shared research findings with CDFG staff currently managing the fishery. The daylong meetings were held in April 2009 and January 2011, with funding for the latter provided by California Sea Grant. Managers have said that the new research will be included in the next stock assessment, when funds for that effort become available.
A graduate student on the project, Kerri Loke, was recently hired by CDFG and is currently preparing a manuscript on the differential fecundity of sheehead at the project’s nine study sites. (Reference: Loke KA, Floyd, AJ, Lowe CG, Hamilton SL, Caselle JE, Young KA. In review. Fecundity of California Sheephead, Semicossyphus pulcher, revisited. Marine and Coastal Fisheries.)
California Sea Grant is currently supporting a follow-up study on sheephead. This project is being led by a former graduate student on the project, Scott Hamilton, now a professor at Moss Landing Marine Laboratories. He and colleagues are examining the effects of fishing on sheephead behavior and reproduction by comparing key attributes of sheephead populations inside and outside marine reserves. Of interest is whether fishing disrupts the ability of males to establish territories and court females. In the first year of that project, researchers assessed the effects of the Catalina Island marine reserve on fish density, biomass, size structure, sex ratios and habitat preferences. The Sea Grant trainee is leading manipulated predation experiments inside and outside the marine reserve to document sheephead prey preferences and predation rates on sea urchins as a function of sheephead size.
Following are peer-reviewed journal articles published since 2010. A list of earlier publications is available in the 2007–2010 Ocean Protection Council/California Sea Grant Research Report.
Hamilton SL, Wilson J, Ben-Horin T, Caselle JE. Utilizing spatial demographic and life history variation to optimize sustainable yield of a temperate sex-changing fish. PLoS ONE 6(9): e24580. doi:10.1371/journal.pone.0024580
Hamilton SL, Caselle JE, Lantz CA, Egglof TL, Kondo E, Newsome SD, Loke-Smith KA, Pondella DP, Young K, Lowe C. 2011. Extensive geographic and ontogenetic variation characterizes the trophic ecology of a temperate reef fish on southern California rocky reefs. Marine Ecology Progress Series 429: 227-244
Caselle JE, Hamilton SL, Schroeder DM, Love MS, Standish JD, Rosales-Casian JA, Sosa-Nishizaki O. 2011. Geographic variation in density, demography, and life history traits of a harvested temperate sex-changing reef fish. Canadian Journal of Fisheries and Aquatic Science 68: 288-303
This project was funded by the California Ocean Protection Council and administered by California Sea Grant.