Baseline assessment of the coral reef fish assemblages of St. Eustatius
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.