Dolfi Debrot

Summary

Habitat mapping is crucial for understanding habitat connectivity and for spatial planning, environmental management, conservation, and targeted research, including long-term change monitoring. However, such information has been lacking for many Dutch Caribbean islands, especially regarding marine habitats. This study used 2144 georeferenced images from different surveys to develop habitat models predicting the distribution of habitat types within the Saba Bank National Park. The habitat models link environmental factors to species or habitat occurrence, enabling predictions in unsurveyed areas with known covariates. Machine learning techniques (Random Forests, Gradient Boosting, and weighted K Nearest Neighbor) were applied to interpret and predict ten habitat types over the Bank. Three models were created for each technique: 1) utilizing only geographic coordinates; 2) incorporating covariables such as depth, distance to the edge of the Bank, Topographic Position Index (TPI), and Terrain Ruggedness index (TRI); 3) a combination of the previous two models. All models performed well, accurately predicting habitat types between 67 and 74% of the georeferenced images. However, the most natural representation occurred with models combining geographic and covariate variables. Predicted habitats include coral reef, patch reef, gorgonian reef, sargassum fields, cyanobacteria-dominated fields, Lobophora fields, Neogoniolithon- Lyngbya habitat, other macroalgae fields, sand with a mix of species, and bare sand. Habitat distribution appears to be related to the main currents in the area and depth, with coral reefs occurring mainly along the southern and eastern edge of the Bank, with gorgonians and other soft corals dominating there the shallow areas. Macroalgae, including fields of Sargassum, dominate the back-reef area. Extensive sand plains dominate the center of the Bank, and along the north-western and northern edge of the Bank, between 40 and 60m depth Lobophora fields can occur. In the south-eastern back reef area a number of mounds built up by the coralline alga Neogoniolithon occur. The Luymes Bank, the northeastern part of the Saba Bank, was the only area that was not correctly predicted, indicating that additional field-based observations are needed to refine results in this area.
 

Project description

Time: July 2023- July 2025

Project leaders: VHL (Alwin Hylkema), SCF (Kai Wulf), STENAPA (Erik Boman), WUR (Dolfi Debrot)

Project summary

Many Caribbean reefs have shifted from coral-dominated to algal-dominated ecosystems. The high algae cover reduces coral recruitment, making the reef unable to recover from other disturbances and resulting in flatter reefs with lower biodiversity. One of the reasons for the proliferation of algae is a mass die-off of the herbivorous sea urchin Diadema antillarum in the early 1980s. Natural recovery of Diadema populations is slow to non-existent, making active restoration of this important grazer a top priority in Caribbean coral reef management, especially since Diadema densities were reduced by another mass mortality event in 2022. The marine park organizations of Saba and St. Eustatius want to restore Diadema populations by restocking cultured individuals. However, important knowledge gaps need to be addressed before large numbers of Diadema can be restocked on the reef. Current culture methods can only produce a limited number of competent larvae. In addition, only 8% of the settlers survive and after restocking, survival on the reef is low as well. In the RAAK PRO Diadema II project, the bottlenecks in Diadema culture will be addressed by comparing larval survival across multiple culture methods and investigating the relation between larval size and post-settlement survival. Growing-out juveniles at sea is likely to help prepare them for life in the wild, while restocking at an optimal size might also increase survival. Finally, a thorough restocking site selection based on high shelter availability and settlement rates will increase the long-term Diadema densities. The acquired knowledge and developed practices will be verified in a larger scale restocking experiment involving at least 5000 Diadema urchins. By restoring Diadema populations through restocking, macroalgae will be more intensively removed and corals will have a chance to settle and to survive, increasing the ability of the reef to cope with other stressors.

The Saba black iguana, Iguana (guana/melanoderma) is a Lesser Antillean endemic variety of the green iguana that is (likely) only found on Saba and Montserrat. The species is the largest native land vertebrate of Saba. Hybridization with the introduced Green Iguana is now an imminent threat to this unique black island variety of iguana.

During fieldwork in 2021 non-native Green Iguanas were seen, captured, culled and genetically confirmed for Saba. If more animals remain on the island or have hybridized with the native Saban black iguana, there is an imminent threat that the latter will be displaced and disappear. A “Rapid Resonse Removal Campaign” is urgently needed to quickly and hopefully completely remove the invasive Green iguana. This Kennisdeskvraag therefore is aimed to swiftly design and rigorously execute such a campaign in close cooperation with local stakeholders. The project will be modelled after a similar successfully-executed campaign for Statia (Debrot et al. 2022).

As the number of Green iguanas is likely still very small, a Rapid Response Action is feasible and called for. The Ministry Agriculture Nature and Food Quality of the Netherlands carries ultimate responsibility for endangered species in the Caribbean Netherlands. In light of the local lack of capacity and expertise the ministry decided to commission the execution of a removal campaign by WMR. 

Together with local and Dutch expert iguana spotters two thorough surveys will be conducted this year in key risk areas to eliminate all iguanas that have Green Iguana or hybrid characteristics. The project will provide practical experience and lessons learnt that will be of great value to the further implementation of the Invasive Alien Species Joint Action Plan as previously also commissioned by the same ministry.

 The landing data for the lobster stock of the Saba Bank, collected annually since 2011, indicate that, despite reduced fishing pressure, there does not seem to be an increase in the stock from 2015 and that there seems to be structural overfishing and a decrease in the stock of egg-bearing female lobsters. Therefore, additional fishery-independent research into the lobster population on the Saba Bank is urgently needed. This research will use an underwater drone to map the occurrence and distribution of lobsters on the Saba Bank. This makes it possible to indicate any particularly important habitats and, based on a mapping of the bank's various habitats, to estimate the total stock of lobsters on the bank. Subsequently, it is possible to make a fishery-independent estimate of maximum lobster catches for the bank (MSY), which can serve as a better guideline for sustainable management than is currently possible on the basis of the catch data collected during regular port sampling.

This powerpoint walks through the recently published position paper entitled Nature based Solutions’: essential to safeguard the Dutch Caribbean against the consequences of climate change.

This report is an update of previously published reports presenting an overview of the trends in St.
Eustatius fisheries based on the fisheries data collected on the island from 2012 to 2020.
The fisheries of St. Eustatius remain mostly conducted by small open boats with outboard engines. The
number of fishing trips carried out by the fleet increased over 2014 peaking in 2015 with an average of
79 trips per months, and subsequently decreased in the following years to reach a minimum average of
25 trips per month in 2019. In 2020, the fishing effort per month increased to an average of 42 trips
per month.

The main activity is a Caribbean spiny lobster (Panuliris argus) fishery using traps, also catching a mix
of reef fish. This fishery is responsible for nearly 70% of the lobster landings on St. Eustatius. The trend
in the annual landings in this fishery broadly follows the trends in the fishing effort, with landings
reaching 30 tonnes in 2015 and since 2017 dropping to values comprising between 7 and 9 tonnes per
year. Landings of lobsters from the trap fishery show a strong seasonality with higher landings during
September-March, and low landings during June-July. The abundance index (derived by modelling the
landings per trip) indicates an overall increase in lobster abundance from 2012 to 2020, with temporary
declines in 2015 and 2018. The average carapace length (CL) shows interannual variations without any
specific trend, but is on average 94.5 mm for females and 102 mm for males. This means that an
average of 42% of the lobsters are landed below the legal size limit (95 mm). This problem is especially
acute for females of which 55% of the landings are of sublegal size. Estimations of F/FMSY proxies based
on the length distribution over time suggest an overexploitation of this stock with values of F/FMSY
between 1.25 and 1.375 for females and 1.125 and 1.25 for males.

The species composition of the bycatch of reef fish in the lobster traps is very diverse, and is dominated
by Acanthuridae (Blue Tang, Doctorfish, Surgeonfish), Ostraciidae (Honeycomb and Scrawled Cowfish),
Serranidae (Coney and Red Hind) and Holocentridae (Squirrelfish). The trends in the reef fish bycatch
in the lobster traps decreased from 2014 with values ranging from 9.9 tonnes in 2015 to 1.5 tonnes in
2019 and 1.6 tonnes in 2020. The biomass index calculated from the landings per trip suggests a
decrease in the combined reef fish stock size from 2014 to 2020. Length frequency data for the main
fish species caught in the lobster traps do not show any notable changes over the period studied. F/FMSY
proxies were estimated for the 7 most landed fish species.

The second most important fishing activities after trap fishing are scuba diving and free diving. Both
activities catch spiny lobster and fish though they both are largely dominated by lobster catches. Scuba
and free diving fleets composition and reporting varied considerably during the time period considered
making it impossible to extract a year effect from a GLM approach. Consequently, this approach was
not considered in this fishery. Scuba divers also conduct a fishery targeting Queen conch (Lobatus
gigas), representing roughly 34% of the trips. Estimates of the annual conch landings are highly
variable, and likely to be fairly uncertain due to the lack of information from logbooks in some years.
The mean length of the conch landed appears to be stable over time, at 24.5 cm and 23.7 cm for females
and males, respectively.

Next to the traps and diving fisheries, different line fisheries are conducted around St. Eustatius. A
handline fishery on reef fish produced landings of between 1.7 and 4.3 tonnes of fish per year in the
period 2014-2017, but with much lower estimates during 2018 and 2019, mainly due to a drop in effort.
In 2020, landings increased to 1.2 tonnes. Large pelagic fish are also caught by trolling, with landings
varying between 0.6 and 2.4 tonnes per year (2014 and 2016 respectively), 2020 landings were
estimated at 1.8 tonnes.

Our main recommendations in terms of both management and research and monitoring are as follows:
- Improve control of and compliance with lobster size-limit regulations.
- Develop a FAD (Fish Aggregation Device) fishery management plan as part of a St. Eustatius fisheries development plan.
- Improve port sampling monitoring and subsampling intensity to cover at least one third of the trips dedicated to each fishing métier.
- Keep collecting data on reef fish species to estimate their status. This can be done best by combining more intensive port sampling with fisheries-independent studies on the distribution
and abundance of these species around St. Eustatius.