Many fish species migrate to form fish spawning aggregations. The temporal and spatial predictability of these migrations and spawning aggregation locations makes species vulnerable to overfishing, as the majority of an adult population within a large region may be harvested quickly with minimal effort. Historically, the Nassau grouper, Epinephelus striatus (Bloch, 1792), was an important fishery species throughout its range, but due to spawning aggregation overfishing, it is now rare in many reef ecosystems. In The Bahamas, stocks continue to decline despite the implementation of spawning aggregation protections. While more Nassau grouper spawning aggregations have been reported in The Bahamas than any other country, very few have been validated, and the dynamics of spawning migrations to and from these sites is poorly understood. Here, we used acoustic telemetry to describe, for the first time, Nassau grouper migrations along Andros Island, The Bahamas, which is bordered by one of the longest barrier reefs in the world. We report the likely extirpation of a historically important spawning aggregation and suggest Nassau grouper are migrating to a previously undocumented spawning location. Fish migrated in groups during the January 2015 full moon along the barrier reef shelf edge traveling roundtrip distances of 71.5–260.3 km [x = 164.5 (SD 65.7) km, n = 9]. These results are critical to assess the efficacy of current management strategies in The Bahamas. Thus far, all known spaning aggregations have been reported to the scientific community by fishers. Data from our study, however, suggest the presence of a potential spawning aggregation informed by passive telemetry and warrants further investigation.

Nassau grouper (Epinephelus striatus, Bloch 1792) are globally critically endangered and an important fishery species in The Commonwealth of The Bahamas (hereafter The Bahamas) and parts of the Caribbean, with an urgent need for better management and conservation. Here, we adopted a combined approach, integrating restriction-site-associated DNA sequencing (RAD-seq) and acoustic telemetry to establish country-wide demographic structure, diversity and connectivity, and the origins of Nassau grouper using an active fish spawning aggregation (FSA) in the central Bahamas. RAD-seq analysis of 94 Nassau grouper sampled from nine locations in The Bahamas generated a working dataset of 13,241 single nucleotide polymorphisms (SNPs). Similar levels of genetic diversity were found among sampled locations. Evidence of population sub-structuring across The Bahamas was demonstrated and supported by discriminate analysis of principal components (DAPCs) along with analyses of molecular variance (AMOVAs). Associated acoustic telemetry data indicated Nassau grouper tagged at an active FSA in the central Bahamas during the 2016–2017 spawning season migrated to the Exumas at the conclusion of the spawning period. Telemetry data suggest the likely origins of five individuals, which traveled one-way distances of up to 176 km from the FSA in the central Bahamas to two sites within a no-take marine protected area (MPA). Analyses of high-resolution SNP markers (including candidate loci under selection) illustrated patterns of spatial structure and genetic connectivity not reflected by telemetry data alone. Nassau grouper from Exuma and Long Island appear to have genetic signatures that differ from other islands and from the Hail Mary FSA. Collectively, these findings provide novel information on the intraspecific population dynamics of Nassau grouper within The Bahamian archipelago and within an active FSA.

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 (N_e). 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 F_ST 0.00236, p = 0.0001). Temporal analyses of changes in N_e 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 N_e 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 N_e 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.

Because herbivory is a critical component of resilient reefs, there is a need to investigate the dynamics of herbivorous fish assemblages over various spatiotemporal scales. During the period of 2011–2019, 483 belt transect surveys were conducted across 26 sites around New Providence to assess the status of parrotfish populations across different reefs. Non-metric multidimensional scaling revealed two distinct parrotfish assemblages around New Providence temporally, differing between 2019 and earlier surveys, and spatially between fore and patch reef zones. Temporal analysis showed a significant decrease (59%) in parrotfish densities across reef sites and significant changes in mean density over time for three species Scarus iseri, Sparisoma aurofrenatum and Sp. atomarium. Changes in the size frequency distribution of parrotfish—particularly reductions in individuals ≥31 cm in size and the complete loss of fish >40 cm across all sites were found. Interactions between reef zone and size class were significant with the greater frequencies of larger individuals (≥21 cm) driving patterns (positive associations) on forereefs. These patterns also appear to be partly driven by variability in the abundance and size composition of Sparisoma viride, which is one of the species primarily targeted by Bahamian fishers, as well as one of the most common parrotfish across reef zones.