Kimani Kitson-Walters

Abstract

The seagrass Halophila stipulacea is native to the Red Sea. It invaded the Mediterranean over the past century and most of the Caribbean over the last two decades. Understanding the main drivers behind the successful invasiveness of H. stipulacea has become crucial. We performed a comprehensive study including field measurements, a mesocosm experiment, and a literature review to identify ‘superior growth traits’ that can potentially explain the success story of H. stipulacea. We assessed meadow characteristics and plant traits of three invasive H. stipulacea populations growing off the Island of Sint Eustatius (eastern Caribbean). We compared similar parameters between native (Eilat, northern Red Sea) and invasive (Caribbean) H. stipulacea plants in a common-garden mesocosm. Lastly, we compared our field measurements with published data. The newly arrived H. stipulacea plants from St. Eustatius were characterized by higher percent cover, higher below- and above-ground biomasses, more apical shoots, and faster leaf turnover rates than those measured in both native and older invaded habitats. These results were further confirmed by the mesocosm experiment where the invasive H. stipulacea plants grew faster and developed more apical shoots than the native plants. Results suggest that increased growth vigour is one of the main invasive traits that characterize successful invasive H. stipulacea populations in the Caribbean and potentially in other invaded areas. We encourage long-term monitoring of H. stipulacea in both native and invaded habitats to better understand the future spread of this species and its impacts on communities and their ecosystem functions and services.

The 1983-1984 die-off of the long-spined sea urchin Diadema antillarum stands out as a catastrophic marine event because of its detrimental effectson Caribbean coral reefs. Without the grazing activities of this key herbivore, turf and macroalgae became the dominant benthic group, inhibiting coral recruitment and compromising coral reef recovery from other disturbances. In the decades that followed, recovery of D. antillarum populations was slow to non-existent. In late January 2022, a new mass mortality of D. antillarum was first observed in the U.S. Virgin Islands. We documented the spread and extent of this new die-off using an online survey. Infected individuals were closely monitored in the lab to record signs of illness, while a large population on Saba, Dutch Caribbean, was surveyed weekly before and during mortality to determine the lethality of this event. Within four months the die-off was distributed over 1,300 km from north to south and 2,500 km east to west. Whereas the 1983-1984 die-off advanced mostly with the currents, the 2022 event has appeared far more quickly in geographically distant areas. First die-off observations in each jurisdiction were often close to harbor areas, which, together with their rapid appearance, suggests that anthropogenic factors may have contributed to the spread of the causative agent. The signs of illness in sick D. antillarum were very similar to those recorded during the 1983-1984 die-off: lack of tube feet control, slow spine reaction followed by their loss, and necrosis of the epidermis were observed in both lab and wild urchins. Affected populations succumbed fast; within a month of the first signs of illness, a closely monitored population at Saba, Dutch Caribbean, had decreased from 4.05 individuals per m2 to 0.05 individuals per m2. Lethality can therefore be as high as 99%. The full extent of the 2022 D. antillarum die-off event is not currently known. The slower spread in the summer of 2022 might indicate that the die-off is coming to a (temporary) standstill. If this is the case, some populations will remain unaffected and potentially supply larvae to downstream areas and augment natural recovery processes. In addition, several D. antillarum rehabilitation approaches have been developed in the past decade and some are ready for large scale implementation. However, active conservation and restoration should not distract from the primary goal of identifying a cause and, if possible, implementing actions to decrease the likelihood of future D. antillarum die-off events.

Summary

Over the last 10 years, the Caribbean Netherlands fisheries on Saba and St. Eustatius have been monitored and multiple assessment reports have been made by Wageningen Marine Research (WMR) in collaboration with local Data Monitoring Officers (DMOs). However, due to challenges in collecting the necessary data, there are gaps in the data which can lead to large uncertainties in the current stock assessments and make it difficult to deliver a more detailed assessment of the fisheries and the state of the stocks.

The specific objectives of this report were to present the data challenges and provide recommendations to address the shortcomings in the current data collection. By addressing these and providing solutions, improvements of the Caribbean Netherlands fisheries monitoring program can be made.

The main gaps identified in the data are:

- Limited coverage by the logbook data, especially the case in St. Eustatius. This implies that large raising factors are applied when estimating total effort and landing estimates, which leads to more uncertain estimates.

- Landings not reported by species (at least for the main species) and port sampling for species composition not frequent enough to be able to produce landing estimates and abundance indices at the species level (instead of species groups). For instance in Saba, the number of trips sampled to estimate the length-composition of the landings was on average 60 per year (excluding 2011), with mainly lobster and redfish trips being sampled. On average, around 40 trips per year were sampled for species composition, taken representatively from the different fishing methods. This is less than one catch sampled per week. This is too low and needs to be intensified if data availability and quality are to improve.

- While some species are over-sampled for length-composition, others are not sampled enough to be able to compute reliable length-based indicators.

Our key recommendations are:

o Port sampling and biological data collection-frequency must be stepped up to meet minimum targets.

o Going along with fishers on the vessels, in order to measure catches on location. (Then fishermen won’t have to wait at the harbor for the DMOs work to be done.)

o Facilitate working in morning/midday/evening shifts. This enables data collection after regular working hours, e.g. when fishers come home late in the day (5-6pm).

o Set quantitative targets for data collection. We suggest targeting for a minimum of 70% logbook declarations, activity surveys, catch species composition and weight data (tonnes), while doubling the effort on selected species of importance

o Data collection will now need to include exact biometric data to establish length-weight and fecundity curves, sex ratios and reproductive seasons for individual species, as well as the collection of otoliths from a range of sizes for each species as a basis for age and growth studies by the WMR otolith lab.

o Have DMOs sit in a workspace with a clear view of the harbor where fishers arrive with their catches, so they can immediately act when boats arrive with their catches. This is mainly an issue for the St. Eustatius DMO.

o For bycatch measurements photographing the fish on a cm grid surface can save measuring time in port or on vessels. o Increase willingness of fishers to participate in data collection. o Incentivize fishers to participate by organizing regular (bimonthly or quarterly) gettogethers where the DMOs update fishers on some monitoring results, providing snacks and drinks.

o Provide dedicated freezer storage space for fishers at the harbor, enabling DMOs more time for the port sampling. Fishers willingness to wait for port sampling is understandably limited. By providing dedicated freezer storage facility, the DMOs can take extra time needed for sufficient biological sampling (i.e. species composition, length, sex) while the catch of the fishers stays fresh. The same can be done for lobster catches if a port-based holding area is provided. 

o Provide modern technologies to the fishers and/or DMOs, e.g. Electronic Reporting Systems (ERS) such as electronic logbooks, and GPS systems such as the Vessel Monitoring System (VMS).

o Arrange for closer involvement of WMR in work planning for the island DMO’s

The massive die-off of the herbivorous sea urchin Diadema antillarum in 1983 and 1984 resulted in phase shifts on Caribbean coral reefs, where macroalgae replaced coral as the most dominant benthic group. Since then, D. antillarum recovery has been slow to non-existent on most reefs. Studying settlement rates can provide insight into the mechanisms constraining the recovery of D. antillarum, while efficient settlement collectors can be used to identify locations with high settlement rates and to collect settlers for restoration practices. The aim of this study was to compare pre and post die-off settlement rates and to determine possible settlement peaks in the Eastern Caribbean island of St. Eustatius. Additionally, we aimed to determine the effectiveness and reproducibility of five different settlement collectors for D. antillarum. D. antillarum settlement around St. Eustatius was highest in May, June and August and low during the rest of the study. Before the die-off, settlement recorded for Curaçao was high throughout the year and was characterized by multiple settlement peaks. Even though peak settlement rates in this study were in the same order of magnitude as in Curaçao before the die-off, overall yearly settlement rates around St. Eustatius were still lower. As no juvenile or adult D. antillarum were observed on the reefs around the settlement collectors, it is likely that other factors are hindering the recovery of the island's D. antillarum populations. Of all five materials tested, bio ball collectors were the most effective and reproducible method to monitor D. antillarum settlement. Panels yielded the least numbers of settlers, which can partly be explained by their position close to the seabed. Settler collection was higher in mid-water layers compared to close to the bottom and maximized when strings of bio balls were used instead of clumps. We recommend research into the feasibility of aiding D. antillarum recovery by providing suitable settlement substrate during the peak of the settlement season and adequate shelter to increase post-settlement survival of settlers. The bio ball collectors could serve as a suitable settlement substrate for this new approach of assisted natural recovery.

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.