Diversity, abundance, distribution and habitat use reef-associated sharks in the Dutch Caribbean

Shark are in serious decline in the Caribbean due to fishing pressure and slow reproductive life-history. The Dutch Caribbean Nature Policy Plan in 2013-2017 was implemented to gain more knowledge on sharks, ensure sustainable fisheries, built an adequate management framework and invest in communication, education and outreach. Distribution and abundance of sharks in the Dutch Caribbean is poorly known. Several studies have been recently carried out on the occurrence, diversity and relative abundance of shark and ray species in the Dutch Caribbean e.g. baited remote underwater video (BRUV) studies on Saba, Saba Bank and St Eustatius and a pilot acoustic telemetry study on reef associated sharks around Saba. 

In this study, three BRUV studies were conducted in the shallow coastal waters of Bonaire, Curaçao and St Maarten during 2015-2017 and the acoustic telemetry study on Saba was extended to St Maarten, St Eustatius and the Saba Bank. This study is part of the Save Our Shark (SOS) project carried out by the Dutch Caribbean Nature Alliance and financed by the Dutch Postcode Lottery (Postcodeloterij). 

The aim of the project is to conduct a base-line survey to describe diversity, abundance, distribution and habitat use of reef-associated sharks in the Dutch Caribbean. The BRUV surveys can serve as a reference point to evaluate management measures and marine parks. The acoustic telemetry study is to assess individual movement patterns of sharks that use coral reefs during different life stages. 

The BRUV study on St Maarten was carried out in March to May 2015 (133 BRUV deployments), on Bonaire in September to December 2016 (110 deployments) and Curaçao in September 2016 to January 2017 (164 deployments). For the acoustic telemetry study in addition to the existing array of 8 receivers around Saba, 8 receivers were placed on the Saba Bank, 8 on the Dutch side of St Maarten and 8 around St Eustatius. Thus the entire network comprised 32 detection stations. In addition to 12 sharks that were equipped with acoustic transmitters lasting 4.5 years in 2014 on Saba (8 Caribbean reef shark and 4 nurse sharks), 11 sharks were equipped with transmitters on Saba Bank, 4 on St Eustatius and 1 on St Maarten in October 2015 to January 2016, totalling 28 sharks (21 Caribbean reef sharks and 7 nurse sharks). Telemetry data retrieved until March 2018 is presented in this report. 

At the SOS BRUVs at St Maarten, a maximum number of sharks per frame per deployment (MaxN) of 37 sharks were observed, 21 were Caribbean reef shark, 15 nurse shark and 1 tiger shark; at Bonaire, 12 MaxN sharks, 11 Caribbean reef shark and 1 Great Hammerhead; at Curaçao, 9 MaxN sharks, 5 Caribbean reef shark, 3 blacktip shark and 1 great hammerhead and in addition a MaxN of 7 Cuban dogfish were observed with the submarine 300m deep BRUV pilot. More sharks were observed in marine parks and conservation zones than outside these areas, especially in Curaçao and St Maarten. 

When comparing these SOS BRUV surveys to earlier BRUV surveys at Saba, St Eustatius and Saba Bank and a BRUV survey at Aruba in 2017, shark species richness in these BRUV surveys was highest at Aruba with 8 species and lowest at Bonaire with 2 species. On Saba 5 shark species were observed, Saba Bank 4 shark species, Curaçao 3 shark species in the regular BRUV survey and 1 more species in the BRUV submarine pilot in 300m deep water, and St Eustatius and St Maarten all 3 shark species. At least 10 shark species were observed within all BRUV studies in the Dutch Caribbean combined. 

Acoustic telemetry revealed that both Caribbean reef sharks and nurse sharks showed strong residency to relatively small home ranges (order of magnitude of a few km). This was observed on all four reef systems studied (Saba, Saba Bank, St Maarten, St Eustatius), although numbers on some sites and species were low on St Maarten and St Eustatius, where they stayed within the borders of the marine parks for long periods. Larger movements were more scarce; two adult Caribbean reef sharks residing for years around Saba made short back trip excursions to the Saba Bank, one nurse shark tagged on Saba showed up more than two years later on the Saba Bank before moving back to Saba. The detection network is still in place and given the battery life of 4.5 years the tagged sharks will yield more data after March 2018. Tagged sharks from other studies were also observed within the network set-up: one nurse shark tagged in a study around the U.S. Virgin Islands moved ca. 160 km to the Saba Bank in 2017, and one juvenile tiger shark from another SOS project moved from St Maarten to the Northwest side of Saba. 

The SOS BRUV and acoustic telemetry showed higher presence of reef associated sharks within the marine parks combined with strong residence for longer periods within the relatively small home ranges. These results suggest that protecting smaller areas of the size of the current marine parks will help in the conservation of at least part of local populations of sharks. Also larger scale movements and connections between adjacent coral reefs over deeper waters (>500m) were found. For this, larger scale reserves, such as Yarari protecting a network of important habitats and safeguarding pathways between them might be necessary to protect entire populations of reef associated sharks.

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