Queen conch (Lobatus gigas) populations living deeper than 20 m are rarely studied, because of the limitations of conventional survey methods using divers [i.e., belt transect (BT), towed-diver]. A crucial management goal for conch populations is to maintain adult densities at adequate levels to ensure reproduction, which is highly density dependent. Therefore, accurate estimates of adult conch densities, both in shallow and deep areas, are essential. The rapid technical progress of video systems has made it possible to develop new cost-effective ecological sampling tools, which can be used to survey areas previously hardly accessible. A lightweight towed video array was used, which was able to survey adult conch throughout the species entire depth range (ca. 0–60 m depth), in a safe and efficient manner. The towed video method (TVM) was compared with a conventional BT method using scuba divers, in its ability to identify adult live and dead conch. A series of intercalibration transects was conducted in a high-complexity (HC) and in a low-complexity (LC) habitat by having the towed video followed by a diver conducting a concurrent standard BT, covering the exact same surface area as the towed video. In both the HC and LC habitat, adult live queen conch had similar counts with both methods. Adult dead conch were not mistaken for live conch but were significantly underestimated with the towed video compared with the BT. The results validate the use of TVM as a reliable sampling tool to estimate densities of live adult conch in both HC and LC habitats throughout the species depth range.
The shallow marine habitats surrounding St. Eustatius fulfil critical ecosystem services in terms of fishery production, recreation, dive tourism and coastal protection. In this a key role is played by the fish communities. In this report we document the relative finfish community composition, density and distribution in the shallow coastal waters of the St. Eustatius Marine Park based on 104 baited video stations distributed among two management sectors and five habitat classes ranging in depths from 8-30 m. In doing so we introduced to the Dutch Caribbean the use of a technology-based method that provides more precise length data than common visual transects, is applicable to wide range of depths and sampling conditions, and is more sensitive for detecting and monitoring apex (top) predatory fish species such as shark.
Compared to earlier survey results our findings highlight the virtual disappearance of large grouper species from the reefs of St. Eustatius. The natural absence of mangrove nursery habitat is one key driver of fish community structure in St. Eustatius and the loss of former seagrass beds is a second key factor probably accounting for the lack of typical mangrove and seagrass-associated scarids (e.g. Scarus coeruleus and guacamaia), snappers (e.g. Lutjanus apodus and griseus) and grunts (e.g. Haemulon sciurus).
The most important local determinant of fish community structure was found to be habitat three- dimensional structure while the measured effect of designated fishing reserve zones was much less pronounced. Nevertheless, mean overall fish size was slightly higher in the fishing reserves. However, our community sampling was insufficient to meaningfully compare densities and size structure of main commercial target species. Community abundance of planktivores and herbivores were notably inversely related, with low-structured sandy habitat being dominated by planktivores and higher-structured hard- bottoms being dominated by herbivores. Low-structured habitat which offered little critical shelter to small fish had the highest mean fish size of all five habitats.
Our results indicate a relatively low quantifiable effect of the present fishing reserves. This may have to do with either or a combination of a) low finfish fishing effort and/or low fishing selectivity, b) problems in the enforcement of the fishing reserves or, c) geographic scale issues due to the movement of fish between defined zones which act to blur potential effects of stated management regimes, and finally our sampling design as a fish community baseline lacking focus on targeted commercial species. Further directed research is needed to properly evaluate and enhance the functioning of the marine park reserves which are accorded an important role in the future socio-economic development of St. Eustatius.
The reefs of St. Eustatius are characterised by very low levels of three-dimensional structure (Risk’s Index: 1-1.3, see Debrot et al. 2014), which was found to be the most important local determinant of fish abundance and distribution. The potential for habitat enhancement to jointly help achieve fishery and conservation goals seems evident. We recommend that measures to enhance such three- dimensional structure may be useful to help increase fish abundance to the benefit of both fishing and biodiversity stakeholders such as the conservation, dive and tourism sectors.
The relatively high presence of sharks (Caribbean reef sharks and nurse sharks) around St. Eustatius is encouraging in the context of conservation, valuable for dive tourism, and interesting for research. As top predators, these sharks play an important ecological role in healthy reefs and their higher abundance around St Eustatius compared to most other areas of the Caribbean may contribute to and be a useful indicator of overall coastal ecosystem health. Further studies of these important species are called for.
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Our assessment of the use of the BRUV method for fish community assessment showed that, due to the deployment strategy used, our test power to detect changes in both numerical and species changes in the communities studies was relatively low. A combination of extensive BRUV surveys (once every 3 years) in combination with yearly fish surveys (i.e. protocol Global Coral Reef Monitoring Network) at a small number of permanent sites with high structural complexity is recommended to ensure the timely detection of trends in reef fish populations.
Knowledge on the density, distribution and occurrence of whales and dolphins in the Caribbean Netherlands is sparse. This knowledge is needed as basic input for conservation and management of cetaceans in the area. Especially in the long term, dedicated data is needed to provide a base-line for monitoring the effect of policy decisions, such as a proposed implementation of a whale sanctuary.
Until recently, knowledge was mainly based on strandings and opportunistic sightings of whales and dolphins. Dedicated data collection, such as obtained from designed aerial or shipboard surveys, will provide reliable and unbiased estimates of abundances and describe distribution patterns and habitat use. However, these types of studies are costly. To explore options on how to bridge the gap between costs (high-low) and data quality (high-low), we investigated a method that could potentially provide long term and cost effective, albeit low quality (in certain aspects), data. In this report we present the results obtained using a port sampling programme used to monitor the fisheries of Saba, St. Eustatius and Bonaire.
During port sampling fishermen were interviewed after returning to the harbour from a fishing trip. The fishermen described their fishing activities and in addition they reported any sightings of whales or dolphins. The study on Saba has included the recording of cetacean sightings since July 2012, the same method has been applied in St. Eustatius since November 2012 and on Bonaire since January 2014. In total 59 different fishing vessels participated in the study (9 Saba, 15 St. Eustatius and 35 Bonaire). The waters around each island were divided into sub-areas to provide data on where the fishing effort took place and where sightings were made. Effort was described as "fishing trips" per sub-area, per month and per island. A total of 1428 days at sea were monitored, with 1020 from Saba, 292 from St. Eustatius and 116 from Bonaire.
During the study a total of 42 whale sightings of 71 individuals was made, of these 36 (62 animals) were recorded in Saba, 2 (2 animals) in St. Eustatius and 4 (4 animals) in Bonaire. There were 93 dolphin sightings consisting of 1362 individual animals. Of these, from Saba there were 71 sightings (877 animals), from St Eustatius 3 sightings (144 animals) and from Bonaire 19 sightings (341 animals).
The relative density (sightings per "fishing trip") showed a pronounced difference in occurrence of cetaceans between islands. The highest relative density of dolphins was found in Bonaire with 0.16 dolphin sightings/fishing trip. The highest relative density of whales was found in Saba with 0.04 whales/fishing trip. Occurrence of whales and dolphins indicated seasonal patterns, in particular for Saba waters where the monitoring ran for several years and most whale and dolphin sightings were in March. The spatial distribution in the Saba study area indicated that dolphins occur regularly on the Saba Bank. In Bonaire the data indicated that an area on the west side of the island and close to shore (<1 km) with high fishing effort also had a high occurrence of cetacean sightings.
An evaluation of the method used indicated that the sampling methodology could be adapted to improve data quality. Most important hereby is a standardization of data collection and data storage between the islands. It also showed that the information provided by the fishermen is very useful in identifying areas of research needed to further investigate cetacean distributional patterns and habitat use around these three islands.
This research was performed within EZ-program Beleidsondersteunend Onderzoek (BO). BO-11-011.05- 034, BO-11-011-05-008.
Quantitative habitat mapping and description form the basis for understanding the provisioning of ecosystem services and habitat connectivity, and hence provide an essential underpinning for marine spatial planning, management and conservation. Based on 869 video stations in a 150 x 200 m grid, we mapped 25.3 km2 of the near-shore island shelf of St. Eustatius at depths ranging 5-30 m. This yielded a coarse-grained map of the principal habitat classes of St. Eustatius’ seascapes. A total of nine principal seafloor habitats were distinguished. Gorgonian reefs amounted to 22% of the Statia Marine Park habitats sampled and were concentrated in the shallow wave-exposed eastern parts of the island (7.7 m average depths). The densest coral “scapes” and seagrass beds of St. Eustatius were concentrated at depths of about 24 m and only amounted to 4 and 5 percent resp. of the island shelf habitats studied. Whereas coral areas were essentially limited to the southern and south-western island shelf areas, seagrass beds were confined to the northern island shelf area. Including patch reef habitats, total hard coral-scape habitat for the St. Eustatius Marine Park amounted to about 19% of the area surveyed and about 475 ha of habitat. Sargassum reef habitat typically occurred at the seaward edge of communities dominated by hard coral growth.
To assess the status and current population densities of the endangered Lesser Antillean Iguana (Iguana delicatissima) on the island of St. Eustatius, we spent more than 80 h searching for iguanas and covered more than 63 km of trails and tracks — but found only 22 iguanas, for an overall average of 3.7 h per iguana. Overall population density was 0.35 iguanas per hectare, which represents 0.5–1.0% of densities documented elsewhere in healthy populations. Population densities have declined across all habitats since the last assessment in 2004. The lack of nesting sites and high iguana mortalities due to anthropogenic causes were the two core factors limiting recovery of iguanas on St. Eustatius.
Our principal recommendations are to:
- (a) Train and equip border officials to prevent potential entry of mongooses and Green Iguanas from neighboring islands;
- (b) implement enforcement and upgrade protective legislation;
- (c) develop and maintain new additional nesting habitat, a measure that is both easy and inexpensive; and
- (d) establish a program to promoto "iguana-friendly" gardens as the main means of reducing cumulative mortality.
Finally, we propose the development of an in situ husbandry and breeding program to help bolster the overall recovery program, a move that would also benefit islanders by offering a relaxed setting in which they could better learn to appreciate this emblematic island species.
The endangered Lesser Antillean Iguana, Iguana delicatissima, is an emblematic species for the island of St. Eustatius and in Caribbean Netherlands it is only found on St. Eustatius. In this study we conducted an extensive population survey for the iguana and compared densities in different areas to densities documented most recently in 2004. We conducted 39 field surveys and spent a total of 80 hours and 21 minutes searching for iguanas. We covered 63,672 m of trails and tracks and found only 22 iguanas. An overall average of 3.70 hours were searched for each iguana found. Due to the low encounter rates, detailed estimation and comparison of population densities remain problematic. Overall population density was 0.35 iguanas per hectare which represents 0.5-1% of densities documented elsewhere in healthy populations. Current population densities have declined across all habitats since the 2004 survey. Iguana encounter rates and densities in natural habitat were highest for the region where the northern hills abut onto the central plain. Island-wide, those areas provide the best combination of sun, shelter, food and potential for nesting sites. The population of the Lower Town sector, indicated in 2004 as the most dense and promising subpopulation, has all but disappeared. Island-wide, the residential estate subdivisions remains the second-most important area for the iguana.
We conclude that even though several valuable conservation measures are in place (e.g. establishment of legally protected parks, designation as a legally protected species and a successfully-run awareness campaign), the status of the iguana has not improved significantly in the last 8 years. Our results show that compared to 2004 when the population was estimated to number 425 (275-650) animals, current population size certainly lies on the low side of this range. This is far below the required minimum viable population size of 5000 animals and means that the iguana is critically endangered on St. Eustatius. It is readily vulnerable to extirpation on the island. Human hunting is likely a minor problem, shelter and food availability on the island are abundant, and invasive predator densities in the wild are relatively low. Of the 28 documented instances of death or endangerment of iguanas during the study period, most were attributable to anthropogenic causes. Suitable nesting sites for the iguana appear very limited, especially due to a combination of geology and vegetation. Therefore, lack of nesting sites and high iguana mortalities due to anthropogenic causes are suggested as the two core factors limiting recovery of the iguana on St. Eustatius .
The following management measures are proposed:
1. Protect current populations by:
- Prevention of introduction of invasive species
(Train and equip border officials to prevent potential entry of the mongoose and the Green Iguana from neighbouring islands),
- Enforcement and upgrading of legal protection
(Implement enforcement and upgrade protective legislation),
- Development and protection of additional nesting sites
(Develop and maintain new additional nesting habitat, a measure that is both easy and inexpensive),
- Establishment of an “iguana-friendly yard” programme
(Establish a programme to promote “iguana-friendly” gardens, as the main means of reducing cumulative mortality).
2. Increase the biological knowledge about the iguana by conducting studies for a better knowledge of the critical biological parameters,
3. Create public awareness for the plight of the species,
4. Establish a small, local husbandry project.
(Development of an in situ husbandry and breeding project could serve a pivotal role in bolstering the other core program themes and especially offers a relaxed setting in which islanders can experience the iguana as the gentle and beautiful animal that it is).
This report is part of the Wageningen University BO research program (BO-11-011.05-004) and was financed by the Ministry of Economic Affairs, Agriculture and Innovation (EL&I) under project number 4308701004.