de Graaf, M.

Understanding how reef-associated sharks utilise coastal reef habitats throughout their ontogeny is essential for their effective management. In this study, we assessed the distribution of sharks in the northern Caribbean Netherlands (Saba, Saba Bank and St Eustatius) and ontogenetic shifts in habitat and depth use of the 2 most observed species: Caribbean reef shark Carcharhinus perezi and nurse shark Ginglymostoma cirratum. We used stereo baited remote underwater video (stereo-BRUV) surveys at 376 sites. A total of 126 shark sightings (6 species) were recorded, with C. perezi (n = 72) and G. cirratum (n = 42) most frequently observed. The probability of recording at least 1 shark per deployment ranged from 0.19 to 0.37 and is comparable with shark occurrences in the small number of other stereo-BRUV studies in the Wider Caribbean Region. Habitat type was the most important factor driving reef-associated shark occurrences, with the highest probability of observing C. perezi and G. cirratum in soft-coral habitat. Additionally, occurrences of C. perezi were significantly influenced by the management zone, with highest probabilities of occurrence in no-fishing zones. Almost all observed reef-associated sharks (95.6%) were juveniles, indicating that the study area may be a nursery area. Overall, both species were observed in deeper waters with increasing size, indicating ontogenetic shifts in depth use. Our findings imply that protected areas should not be limited to a single habitat or depth, but focus on protecting a large area with the range of habitats and depths necessary for reef-associated sharks to complete their life cycle.

KEY WORDS: Elasmobranchs · Conservation · Carcharhinus perezi · Ginglymostoma cirratum · Marine reserve · Baseline study · Caribbean Netherlands · BRUV · Baited remote underwater video

Student Report

Species composition, distribution, and relative abundance 

Elasmobranchs are suffering from habitat loss and are declining at a rapid pace. They are listed as “Data Deficient” on the IUCN Red List of Threatened Species, as there has not been enough research conducted on elasmobranchs. Contemporary and historical data are very limited. The worldwide decline of elasmobranchs is largely due to the Asian shark fin trade.

However, in the Dutch Caribbean, sharks do not get targeted for artisanal fisheries, but are mostly killed as bycatch. To conserve elasmobranch species, it is necessary to collect information related to their diversity, distribution, and abundance. Baited Remote Underwater Video (BRUV) is used to study marine environments and their inhabitants; it has become the standard approach to learn about elasmobranchs in their representative habitats. BRUV deployments were done along the East coast and a part of the West coast of Bonaire.

From the videos, the maximum numbers of individuals (MaxN) were counted. Also, a citizen science project was undertaken and the presence or absence of elasmobranch species was noted. Data from the video footage and the citizen science project were collected and used to compare Bonaire’s shark and ray species compositions, distributions and abundances of the East coast with the West coast.

Juvenile queen conch are primarily associated with native seagrass such as Thalassia testudinum in large parts of their range in the Caribbean and the southern Gulf of Mexico. Here, a number of non-native seagrass species have been introduced including Halophila stipulacea, which is natural to the Red Sea and the Indo-Pacific. In the Caribbean, H. stipulacea often creates dense continuous mats with little or no sediment exposed, compared to native seagrass, which grows much less dense. We examined the diet and growth of juvenile conch in both native, mixed, and invasive seagrass beds using stable isotope analysis and an in situ growth enclosure experiment. Organic material in the sediment (i.e. benthic diatoms and particulate organic matter) was found to be the most important source of carbon and nitrogen for juvenile queen conch in all 3 habitats investigated, and there was a significantly higher probability of positive growth in the native seagrass compared to the invasive seagrass. Due to the importance of the organic material in the sediment as a source of nutrition for juvenile conch, limited access to the sediment in the invasive seagrass can potentially cause inadequate nutritional conditions to sustain high growth rates. Thus, it is likely that there is a negative effect on juvenile queen conch growth currently inhabiting invasive seagrass beds, compared to native seagrass beds, when other potential sources of nutrition are not available.

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.

The Saba Bank supports a vibrant local fishing industry. Fishing effort focuses mainly on a trap fishery, targeting lobster and deep water red fish (snapper) (Dilrosun 2000, Toller 2008, van Gerwen 2013, Boonstra 2014), as well as bottom drop lining for red fish (snapper). Other common fishing activities include hand lining and trap fishing for red hind (grouper) as well as trolling for pelagic species.

The download contains:

• presentation given by Martin de Graaf at the Saba Bank Symposium in December 2016.
• bioNews article based on the presentation 

BioNews is produced by the Dutch Caribbean Nature Alliance and funded by the Ministry of Economic Affairs.