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Award Detail

Doing Business As Name:San Jose State University Foundation
  • Scott Hamilton
  • (408) 924-1434
Award Date:08/11/2014
Estimated Total Award Amount: $ 207,252
Funds Obligated to Date: $ 207,252
  • FY 2014=$207,252
Start Date:03/01/2015
End Date:02/28/2019
Transaction Type:Grant
Awarding Agency Code:4900
Funding Agency Code:4900
CFDA Number:47.050
Primary Program Source:040100 NSF RESEARCH & RELATED ACTIVIT
Award Title or Description:Collaborative Research: RUI: Impacts of size-selective mortality on sex-changing fishes
Federal Award ID Number:1436545
DUNS ID:056820715
Parent DUNS ID:056820715
Program Officer:
  • Daniel J. Thornhill
  • (703) 292-8143

Awardee Location

Street:210 North Fourth Street
City:San Jose
County:San Jose
Awardee Cong. District:19

Primary Place of Performance

Organization Name:Moss Landing Marine Laboratories
Street:8272 Moss Landing Road
City:Moss Landing
County:Moss Landing
Cong. District:20

Abstract at Time of Award

Many marine fish species change sex during their lifetimes, and many of them are targets of commercial and recreational fishing. The timing of sex change in these animals is often related to body size, so populations typically consist of many small fish of the initial sex (usually female) and few large fish of the other sex (usually male). In nature, smaller fish are at a greater risk of mortality due to predation, but fishermen tend to seek larger fish. Thus fishing that targets larger individuals may skew sex ratios, removing enough of the larger sex to hinder reproduction. However, the extent to which size-selective mortality affects sex-changing fishes is poorly understood. This research will explore the effects of size-selective mortality on the population dynamics of sex-changing species using an integrated set of field experiments and mathematical models. It will provide the first experimental exploration of the sensitivity of different sex-change patterns and reproductive strategies to selective mortality. The results will advance our knowledge of the susceptibility and resilience of sex-changing organisms to different types of size-selective mortality and will reveal how sex-changing species can recover after size-selection ceases, as in populations within marine reserves where fishing is suddenly prohibited. The findings will inform fisheries management policies, which do not currently consider the ability of a species to change sex in setting fisheries regulations. The project will provide professional training and research support for three graduate students and multiple undergraduates from California State University Northridge, Moss Landing Marine Laboratories, and University of North Carolina Wilmington. The research findings will be communicated with fisheries managers, via national and international scientific meetings, posted on institutional websites and social media, and through publicly available modeling exercises. This project will consist of a three-year study of the effects of size-specific mortality on sex-changing fishes. Field experiments will use three closely related rocky-reef fishes that differ in sex-change pattern and are amenable to field manipulation and direct measurement of reproductive output. The species include a protogynous hermaphrodite (a female-to-male sex-change pattern common among harvested species) and two simultaneous hermaphrodites that differ in their ability to switch between male and female. Two types of experiments will be conducted on populations established on replicate patch reefs at Santa Catalina Island, California: (1) sex ratios will be manipulated to determine when the scarcity of males limits population-level reproductive output; and (2) experiments cross-factoring the intensity of mortality with the form of size-selection (i.e., higher mortality of large or small individuals) will test the demographic consequences of size-selective mortality. In concert with the field experiments, size- and sex-structured population models (integral projection models) will be developed for use in three ways: (1) to evaluate how different types of selective mortality should affect population dynamics; (2) to predict outcomes of the field experiments, testing/validating the model and allowing direct prediction of the ecological significance of short-term selection; and (3) to fit to existing survey data for a fourth species, a widely fished, sex-changing fish, inside and outside of marine reserves. Part (3) will evaluate whether and how quickly the mating system and reproductive output of that species (not directly measurable in the field) is recovering inside reserves. This integrated set of field experiments and models will yield novel insight into the effects of size-selective mortality on the population dynamics of sex-changing marine species.

Project Outcomes Report


This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.

The goals of the project were to investigate the effects of size-selective harvesting practices on the demography of sex-changing fishes, through a combination of: (1) large-scale field experiments manipulating the densities, size structure, and sex ratios of fishes to simulate the effects of different types (i.e., removal of large or small body sizes) and intensities of fishing mortality and (2) ecological modeling studies designed to compare and contrast the effects of fishing on single sex (i.e., gonochoristic) and sex-changing (i.e., protogyny or female to male sexual transition) reproductive strategies and the transient population dynamics that occur following the adoption of management strategies, like marine reserves, which prohibit fishing. 

Over three years, we conducted field experiments at Santa Catalina Island, CA, using two species of gobies (blackeye gobies, Rhinogobiops nicholsii, and bluebanded gobies, Lythrypnus dalli) as model organisms. We constructed an array of 69 patch reefs (2 x 2 m in size) out of natural rocks and cinderblocks in a large sandy bay, with reefs separated by 10 m, and covered each reef with predator exclusion cages. We then stocked the reefs with fish (tagged to indicate their size and sex) in a series of experiments separately testing how the type and instensity of size-selective harvesting or skewed sex ratios (holding density constant) will impact reproduction, growth, and sex change. 

Experiments testing the effects of size-selective harvesting on population demography found that size-selective removal of large individuals, as is common in many fisheries, caused a much larger reduction in reproductive output for our two sex-changing fish than different intensities of removal (i.e., reductions in population abundance). However, populations were able to compensate somewhat for the removal of large individuals as the remaining fishes grew significantly faster and changed sex from female to male at high rates when subjected to selective removal of large bodied individuals. These results will have important implications for fisheries management of sex-changing fishes. Surprisingly, sex change is a common reproductive strategy in many commercially important species (e.g., groupers, snappers, parrotfish, wrasses, etc.), but most models and stock assessment do not take it into account.

In the experiments testing the effects of skewed sex ratios, we found that for both species total egg production, female per capita production, and the number of nests per reef were not affected by sex ratio. By contrast, male per capita production and the percentage of males guarding nets significantly increased as sex ratios became more female-biased. For blackeye gobies, growth rates were highest for individuals that completed sex change during the experimental period and the frequency of sex change was highest on reefs that were strongly female-biased. Both species defend demersal nests and are highly resilient to sex ratio skew, potentially because intrasexual competition between males (i.e., territory and mate monopolization) or females (i.e. competition for nest space) limit reproductive potential. 

To set expectations for the adaptive management of Marine Protected Areas (MPAs) we need models that predict shorter-term, transient population dynamics. We designed the first population dynamic models to address this question for a protogynous (female-first) sex-changing fish. In these populations, fishing prior to MPA implementation both reduces abundance and skews sex ratios, because fishing removes more males than females. The post-MPA trajectory then depends on how much fishing there had been, and how quickly the sex ratio returns to its unfished state. We used a two-sex age-structured model parameterized for a generic protogynous species to simulate a range of possible scenarios. We found that in general, protogynous species should recover faster than non-sex-changing (gonochore) species. However, late-maturing protogynous species that required higher male:female sex ratios for fertilization exhibited longer, more oscillatory recovery trajectories. Importantly, we found that changes in sex ratio inside MPAs were not indicative of population recovery.

The project trained 9 graduate students from MLML who participated in the field experiments. Stephen Pang was the lead graduate student on the project and completed his Master's degree from MLML in May 2019, studying the effects of skewed sex ratios on reproduction in sex-changing fish. He also gave poster and oral presentations at the Western Society of Naturalists meeting in 2016 and 2017. 6 undergraduate students from CSU Monterey Bay served as research assistants for 3 months during each of the 3 summer field seasons. 5 of these students were part of the Undergraduate Research Opportunity Center (UROC) program and each student developed and conducted an independent research project, related to the broad goals of the experiments. Those projects resulted in 2 poster presentations at the Western Soecity of Naturalists meeting in 2016 and 2017. In addition, we had 3 NSF REU students participate through the University of Southern California REU program. Three of the participating undergradautes were subsequently accepted to Master's programs following their involvement in this project. We held outreach events for the public at Catalina Island.

Last Modified: 05/31/2019
Modified by: Scott Hamilton

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