Abstract
Severe declines of endangered Nassau grouper (Epinephelus striatus) across The Bahamas and Caribbean have spurred efforts to improve their fisheries management and population conservation. The Bahamas is reported to hold the majority of fish spawning aggregations for Nassau grouper, however, the status and genetic population structure of fish within the country is largely unknown, presenting a major knowledge gap for their sustainable management. Between August 2014–February 2017, 464 individual Nassau grouper sampled from The Bahamas were genotyped using 15 polymorphic microsatellite loci to establish measures of population structure, genetic diversity and effective population size (Ne). Nassau grouper were characterized by mostly high levels of genetic diversity, but we found no evidence for geographic population structure. Microsatellite analyses revealed weak, but significant genetic differentiation of Nassau grouper throughout the Bahamian archipelago (Global FST 0.00236, p = 0.0001). Temporal analyses of changes in Ne over the last 1,000 generations provide evidence in support of a pronounced historic decline in Bahamian Nassau grouper that appears to pre-date anthropogenic fishing activities. M-ratio results corroborate significant reductions in Ne throughout The Bahamas, with evidence for population bottlenecks in three islands and an active fish spawning aggregation along with apparent signs of inbreeding at two islands. Current estimates of Ne for Nassau grouper are considerably lower compared with historic levels. These findings represent important new contributions to our understanding of the evolutionary history, demographics and genetic connectivity of this endangered species, which are of critical importance for advancing their sustainable management.
Highlights
Anthropogenic activities, most notably the over-exploitation of species and habitat degradation has led to significant global declines in biodiversity and genetic diversity (Worm et al, 2006; Allendorf et al, 2008; Paddack et al, 2009; Pinsky and Palumbi, 2014)
Patterns of weak or no genetic subdivision at varying spatial and temporal scales using microsatellites and other molecular markers have been reported for a number of Epinephelids, e.g., Camouflage grouper, E. polyphekadion (Rhodes et al, 2003); Hawaiian grouper, E. quernus (Rivera et al, 2004); Red grouper, E. morio (Zatcoff et al, 2004); Dusky grouper, E. marginatus (Maggio et al, 2006; Schunter et al, 2011; Buchholz-Sørensen and Vella, 2016) and Goliath grouper, E. itajara (Silva-Oliveira et al, 2008)
Nassau grouper sampled throughout the archipelago were overlapping in discriminate analysis of principal components (DAPC) analysis and no geographical clustering was apparent in either PCoA or Bayesian-based Structure analyses
Summary
Anthropogenic activities, most notably the over-exploitation of species and habitat degradation has led to significant global declines in biodiversity and genetic diversity (Worm et al, 2006; Allendorf et al, 2008; Paddack et al, 2009; Pinsky and Palumbi, 2014). Nassau grouper (Epinephelus striatus, Bloch, 1792) are among the top predatory reef fish species, naturally distributed throughout the Tropical Western Atlantic including Bermuda, Florida, The Bahamas and Yucatan Peninsula, the Caribbean Sea and parts of the Gulf of Mexico (Heemstra and Randall, 1993; Albins et al, 2009; Froese and Pauly, 2014) They have experienced severe population declines and even local extirpations throughout their native range over recent decades (Olsen and LaPlace, 1979; Sadovy and Eklund, 1999; Sadovy De Mitcheson et al, 2008). Stock assessment data are deficient and marked declines in commercial landings, FSA abundance and densities on reef habitats have been documented in The Bahamas despite regulatory policies (Cheung et al, 2013; Dahlgren et al, 2016b; Sherman et al, 2016; Stump et al, 2017)
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