No abstract

Shark populations have declined across the Caribbean region, with negative associations between shark abundance and human population density, open access to fishing, and proximity to large markets (‘market gravity’). This decline is frequently attributed to fishing mortality, which increases in areas closer to humans and outside marine reserves. Although it is difficult to disentangle the effects of fishing mortality from other anthropogenic pressures on sharks, comparing shark abundance and diversity in jurisdictions with near zero fishing mortality versus prevalent shark fishing can demonstrate the role of overfishing. We used baited remote underwater video systems to compare shark abundance and diversity on coral reefs in 2 Caribbean nations with contrasting levels of shark exploitation: Belize (shark fishing) and The Bahamas (shark sanctuary). The abundance of targeted shark species and diversity were significantly higher in The Bahamas than in Belize. Caribbean reef and nurse shark abundance in Belize were best predicted by fishing-related factors (marine reserves, market gravity, their interaction). In The Bahamas, abiotic factors (depth, sea surface temperature) best predicted nurse shark abundance, while depth, market gravity, and its interaction with marine reserves predicted Caribbean reef shark abundance. These results indicate that fishing mortality reduces shark abundance and diversity in Belize, while lower fishing mortality in The Bahamas has greatly reduced but not eliminated human impacts on sharks. Future work should elucidate the indirect effects of humans to develop holistic shark conservation plans. We suggest minimizing shark fishing through multinational management plans to improve shark abundance and diversity, especially on reefs near densely populated areas.

Spatial comparisons of reef shark abundance inside and outside marine protected areas (MPAs) are common and generally report positive MPA effects, yet few studies have tracked abundance trends over long time periods. This is problematic because inside:outside comparisons at a single point in time cannot evaluate whether populations are declining. In Belize, the Caribbean reef shark Carcharhinus perezi is one of the most fished shark species and is more abundant inside MPAs. Although the relative abundance of C. perezi was stable inside Glover’s Reef Marine Reserve (GRMR) from 2001 to 2013, using standard baited remote underwater video station surveys, we document a decline in relative abundance inside the no-take marine reserve from 2009 to 2019. We used a negative binomial generalized linear model and model averaging to test the effect of year, depth, and water temperature on C. perezi and nurse shark Ginglymostoma cirratum relative abundance. While model-averaged results indicated a C. perezi decline, G. cirratum remained stable from 2009 to 2019. We hypothesize that the C. perezi decline is a result of fishing along the edge of GRMR, while G. cirratum stability is related to their behavior and nationwide protection. Given the dynamic nature of fisheries regulations, economic pressures, and site-specific environmental conditions, our results emphasize the need for standardized long-term monitoring of reef sharks inside and around MPAs globally.

The great hammerhead (Sphyrna mokarran) is a highly mobile, large‐bodied shark primarily found in coastal‐pelagic and semi‐oceanic waters across a circumtropical range. It is a target or by‐catch species in multiple fisheries, and as a result, rapid population declines have occurred in many regions. These declines have contributed to the species being assessed as globally critically endangered on the IUCN Red List. Although conservation and management measures have yielded promising results in some regions, such as the United States, high levels of at‐vessel and post‐release mortality remain a major concern to the species population recovery. This examined the vertical space use and thermal range of pop‐off archival satellite–tagged S. mokarran in the western North Atlantic Ocean, expanding the understanding of the ecological niche of this species and providing insight into by‐catch mitigation strategies for fisheries managers. The results showed that S. mokarran predominantly used shallow depths (75% of records <30 m) and had a narrow temperature range (89% of records between 23 and 28°C). Individual differences in depth use were apparent, and a strong diel cycle was observed, with sharks occupying significantly deeper depths during the daytime. Furthermore, two individuals were confirmed pregnant with one migrating from the Bahamas to South Carolina, U.S.A., providing further evidence of regional connectivity and parturition off the U.S. East Coast. The findings suggest that S. mokarran may be vulnerable to incidental capture in the western North Atlantic commercial longline fisheries due to substantial vertical overlap between the species and the gear. The results can be incorporated into conservation and management efforts to develop and/or refine mitigation measures focused on reducing the by‐catch and associated mortality of this species, which can ultimately aide S. mokarran population recovery in areas with poor conservation status.

Oceanic whitetip sharks Carcharhinus longimanus aggregate at Columbus Point, Cat Island, The Bahamas, in at least April and May. We show that signs of putative mating activity (i.e. bite wounds) on mature females, which dominate the aggregation, are exceedingly rare at that time, but may be more common in July. Male plasma testosterone concentrations also suggest that spermatogenesis occurs in April and May and copulation is underway or has just ended by July. We support these preliminary data with direct observations of putative precopulatory behavior made at Columbus Point in July 2019. Using 25 min of video footage from the event, we identify common shark mating behaviors such as echelon or parallel swimming, following, shielding, nosing, and biting. We also describe the ‘circle-back’, a newly observed behavior where the male orients to the trail of the female, possibly to investigate olfactory cues. Based on these 3 parallel lines of evidence, we hypothesize that mating could occur in the eastern Bahamas as early as May, but is probably concentrated in midsummer (i.e. July), and that Columbus Point, Cat Island, may be a mating habitat for the oceanic whitetip shark.