First indication of the abundance and distribution of protected seagrass associated species in the introduced Halophila stipulacea meadows on Sint Eustatius

Student Report

The present study investigated the abundance and distribution of protected seagrass associated species within the introduced Halophila stipulacea meadows on St. Eustatius. These species are the queen conch, Caribbean spiny lobster, slender seahorse, green turtle and several species of grouper (Epinephelus striatus, Mycteroperca interstitialis, Mycteroperca venenosa), snapper (Lutjanus cyanopterus, Lutjanus analis) and the rainbow parrotfish. These species are commercially and ecologically valuable through fisheries, tourism and their contribution to biodiversity which maintains important ecosystem services. The introduction of H. stipulacea altered the seagrass meadows as it has significantly shorter shoots compared to native seagrasses, potentially affecting these protected seagrass associated species as they benefit from high complexity e.g. shelter. Current nature management however is based on outdated research which describes the situation prior to the introduction of H. stipulacea and no monitoring efforts are being made to update this. Furthermore, no baseline knowledge regarding the effect of H. stipulacea on the protected seagrass associated species or even the location of meadows was known prior to this study, resulting in a knowledge gap. Effective nature management is hindered by this knowledge gap and the aim of this research is to take the first step in providing an indication of the abundance and distribution of protected seagrass associated species to help close it.

Data was collected from May 5th to July 6th,2020, in Oranje Bay, St. Eustatius Marine Park. Visual surveys along belt transects using SCUBA were conducted at 31 sampling sites after verification of the meadow’s location by manta tow. Sampling sites were selected using random stratified sampling. Most of the research species were absent in the meadow. Only the queen conch (n = 10) and the slender seahorse (n = 2) were observed. Most of the queen conch were found in the southern half of the meadow, whereas the slender seahorses were found more to the north. Both species were most abundant in homogeneous characterised habitat. The absence of the other protected seagrass associated species was unexpected. This may be the result of overfishing and habitat degradation as population numbers have been declining for years prior to the introduction of H. stipulacea and are currently quite low. Associated grouper and snapper species with similar habitat requirements as the protected seagrass associated grouper and snapper species were found in the meadow, indicating that it might be suitable (nursery) habitat. The green turtle is known to interact with H. stipulacea, and their absence from this study is likely coincidental. It is recommended that the seagrass associated species distribution and preferred habitats will be identified on St. Eustatius to create baseline knowledge regarding the species and its threats. Executing this additional research will enable responsible authorities to focus the island specific management plans on conservation and recovery of these protected seagrass associated species to ensure effective nature management.