Influence of protogynous sex change on recovery of fish populations within marine protected areas

Demographic Consequences of Small-Scale Fisheries for Two Sex-Changing Groupers of the Tropical Eastern Pacific

The effects of fishing on the demography and population ecology of sex-changing fishes are largely unknown, despite the fact that their fisheries provide important economic and nutritional resources in coastal communities throughout the tropics, especially in Latin America. Species with female-first sex change often have naturally skewed sex ratios in the adult population, and fishing pressure can alter this natural bias, limiting egg production and fertilization success. How fishing alters demography and population vital rates depends on which sizes and sexes are selected. We consider two types of fishery selectivity “asymptotic (selecting the largest fish) and plate-sized (selecting fish between in a narrow, but small, size range)” that to represent fisheries for two important fish species of the Tropical Eastern Pacific, the Pacific goliath grouper (Epinephelus quinquefasciatus) and the endemic sailfin grouper (Mycteroperca olfax) of the Galapagos Islands, known locally as bacalao. Each of these large, long-lived species support small scale fisheries of significant value, but there is limited information on fishing effort, selectivity, or population trends. Using a population model, we estimate how the biology of these species contributes to their risk of overexploitation under different possible scenarios of fishing and reproductive biology. Specifically, we consider how variation in growth rates and fertilization rates interact with selectivity to affect age structure and sex ratios. We compare two metrics of population status: the spawning potential ratio (SPR), and the relative standing biomass after fishing (BF/B0). In our modeled populations, when fertilization rates were reduced, fishing rapidly decreased the spawning potential of both species, but did not affect biomass at moderate levels of fishing mortality. However, we predict low fertilization success, fast somatic growth, and asymptotic selectivity of fisheries for sex-changing species decreases both spawning potential and biomass, even at low levels of exploitation, suggesting these factors can cause rapid depletion of sex-changing species. Our findings highlight key gaps in our knowledge of spawning behavior and fertilization success of sex-changing fishes that must be filled if we are to sustainably manage these culturally and economically significant fisheries.

Recruitment variability and sampling design interact to influence the detectability of protected area effects

Correctly identifying the effects of a human impact on a system is a persistent challenge in ecology, driven partly by the variable nature of natural systems. This is particularly true in many marine fishery species, which frequently experience large temporal fluctuations in recruitment that produce interannual variations in populations. This variability complicates efforts to maintain stocks at management targets or detect the effects of rebuilding efforts. We address this challenge in the context of no-take marine reserves by exploring how variable larval recruitment could interact with the timing of reserve establishment and choice of sampling design to affect population dynamics and the detectability of reserve effects. To predict population changes in the years following a no-take reserve implementation, we first tested for periodicity in larval recruitment in an important U.S. Pacific coast recreational fishery species (kelp bass, Paralabrax clathratus) and then included that pattern in a population model. We also used this model to determine the detectability of population increases under alternative sampling approaches and minimum age sampled. Kelp bass larval recruitment in the Channel Islands, California, peaked every about six (major) and about two (minor) years. Our model showed that establishing a reserve during a peak or trough enhanced or delayed, respectively, the post-reserve population increases. However, establishing a reserve during a recruitment peak could obscure a failing reserve, that is, a reserve that is unable to secure longer-term metapopulation persistence. Recruitment peaks and troughs also interacted with sampling design to affect the detectability of reserve effects. Designs that compared inside-outside were the most robust to variable recruitment, but failed to capture whether the reserve has improved metapopulation growth. Designs that included a time element (e.g., before-after) are more suited to assessing reserve effectiveness, but were sensitive to recruitment variation and detectability can change year-to-year. Notably, detectability did not always increase monotonically with reserve age; the optimal time for detectability depended on the minimum age of organisms sampled and was greatest when the cohort of a major recruitment peak first appeared in the sampling. We encourage managers to account for variable recruitment when planning monitoring and assessment programs.

Development of reproductive potential in protogynous coral reef fishes within Philippine no-take marine reserves

Empirical evidence for increases in the reproductive potential (egg output per unit area) of coral reef fish in no-take marine reserves (NTMRs) is sparse. Here, we inferred the development of reproductive potential in two species of protogynous reef fishes, Chlorurus bleekeri (Labridae: Scarinae) and Cephalopholis argus (Epinephelidae), inside and outside of Philippine NTMRs. We estimated key reproductive parameters and applied these to species-specific density and length data from 17 NTMRs (durations of protection 0-11 years) and paired fished sites (controls) in a space-for-time substitution approach. For C. argus, we also used density and length data collected almost annually over 29 years from a NTMR and an adjacent control at Apo Island. The results suggest that C. bleekeri can develop 6.0 times greater reproductive potential in NTMRs than controls after 11 years of protection, equivalent to approximately 582,000 more eggs produced 500 m^-2 inside NTMRs. Enhancement of reproductive potential in C. argus was not evident after 11 years in the space-for-time substitution. At Apo Island NTMR, reproductive potential of C. argus increased approximately 6-fold over 29 years but NTMR/control ratios in reproductive potential decreased through time (from 3.2 to 2.4), probably due to spillover of C. argus from the NTMR to the control. C. argus was estimated to produce approximately 113,000 more eggs 500 m^-2 inside Apo Island NTMR at the 29th year of protection. Ratios of reproductive potential between NTMR and controls in C. bleekeri and C. argus were often greater than corresponding ratios in density or biomass. The study underscores the importance of species-specific reproductive life history traits that drive variation in the development of larval fish subsidies that originate from NTMRs.

Minimum size limits and the reproductive value of numerous, young, mature female fish

Fisheries management relies on various catch and effort controls to preserve spawning stock biomass and maximize sustainable yields while limiting fishery impacts on marine ecosystems. These include species-specific minimum or maximum size limits to protect either small non-reproductive subadults, a portion of reproductively mature adults, or large highly fecund individuals. Protecting size classes of mature fish is expected to yield a viable source of larvae for replenishing populations and reduce the risk of recruitment overfishing, yet size-specific recruitment contributions have rarely been assessed empirically. Here, we apply genetic parentage analysis to measure the reproductive success of a size-structured population of a commercially important species of coral grouper (Plectropomus maculatus-Serranidae) in no-take marine reserves (NTMRs) in the Great Barrier Reef Marine Park, Australia. Although the per capita reproductive success of individual fish increases rapidly with body length, the numerous young mature female fish, below the minimum size limit (MSL) (38 cm total length), were responsible for generating disproportionately large contributions (36%) towards larval replenishment of both fished and reserve reefs. Our findings indicate that MSLs are an effective harvest control measure to safeguard a portion of the spawning stock biomass for coral grouper and supplement recruitment subsidies assured from NTMRs.