Saba

Delft University of Technology (Master Thesis)

Abstract

Over the years, InSAR has become an indispensable tool in the study of ground deformation, including volcanic deformation, and this continues to be the case in times of improved technology. Since the volcanoes on the Caribbean islands of Saba and St. Eustatius are active, the implementation of an InSAR-based monitoring system is crucial to enhance the spatial resolution of volcano monitoring beyond the capabilities of the ground-based monitoring network, for instance in the case of localized deformations such as dike intrusions. However, technical challenges arise in these tropical settings, caused by dense rainforest, atmospheric artifacts and terrain variability, posing serious challenges to the use of InSAR. Time series InSAR, including SBAS and PSI, can be used to overcome these limitations. A previous study has explored the use of PSI for monitoring, using the already available DePSI software. In this research, an SBAS approach within the Delft InSAR software framework is developed using state-of-the-art Python packages, including (sar)xarray, dask and zarr, and is used to assess whether there is capability to develop SBAS into a volcanic monitoring tool for Saba and St. Eustatius. In addition, a preliminary comparison between the SBAS and PSI methodologies is performed based on a theoretical and (semi-)quantifiable approach. This study combines data from two satellites operating at different wavelengths: Sentinel-1 (C-band) and ALOS-2 (L-band).

Assuming no ongoing deformation, based on GNSS results, the variability of the results around zero can be used as an indicator of precision. The results obtained through SBAS are promising, in particular for L-band, on account of e.g., extensive spatial coverage, efficiency and relatively low variability even with the presence of atmospheric and DEM components. Overall, the results reveal mm order deviations. In the event of volcanic activity, the expected deformation signals are in the range of cm-dm's and can therefore be detected, i.e., with an estimated minimal detectable deformation of 1.5 cm/year in the worst-case scenario. The implementation of three different coherence-based masking approaches-water, single and individual-give an indication of the level of robustness and reliability of the results. Generally, a relatively high level of consistency can be observed among the different masking results of ALOS-2 for both islands, for St. Eustatius following the correction of the unwrapping errors using two testing approaches: an interferogram removal approach and an adaptive approach based on the DIA procedure. The latter procedure allows for retaining all observations and their residuals and is therefore preferred. In contrast, the Sentinel-1 results reveal a lower level of consistency. It is suspected that this inconsistency mainly arises on account of the numerous unwrapping errors within the single masking approach. The individual masking approach appears to be less susceptible to unwrapping errors, however is more prone to outliers than the single masking approach. Further research, following the correction of the atmospheric component and DEM errors, may offer insights into the preferred masking approach. Overall, the use of L-band imagery shows potential, offering spatial coverage where C-band does not, even with limited ALOS-2 data availability and large temporal baselines. The preliminary comparative analysis with the PSI approach, based on the respective strengths and limitations from literature, spatial coverage, processing steps, precision and computational requirements, suggests that a hybrid method could prove to be advantageous to minimize (potential) signal loss, e.g. either from limited spatial coverage or spatial resolution, and enhance volcanic risk assessment. SBAS excels in the extensive spatial coverage, especially using L-band, providing nearly homogeneous coverage of St. Eustatius, even on the flanks of the Quill, and on the outer flanks Mt. Scenery on Saba. However, regardless of the mask, wavelength or method, acquiring coverage around the summit of Mt. Scenery on Saba remains challenging.

The study contributes to advancing InSAR time series analysis for the volcanic monitoring on Saba and St. Eustatius through the successful implementation of an SBAS approach within the Delft InSAR software framework based on state-of-the-art packages, the implementation and evaluation of new approaches to enhance the method in terms of the efficiency and robustness and a comparison with existing software. In addition, the software can be applied in a generic sense for various applications and can be extended for further improvements.

The purpose of this report is to update recent catch trends in the Saba Bank fisheries with the data from 2016 and 2017.

For the lobster fishery (Panulirus argus), the number of fishing trips (and number of traps set) gradually grew from 2012 to 2015 but has since levelled off. The resulting landings of lobster have shown a similar pattern of increase up to 2015 but have now levelled off at around 70 tonnes annually. Increasing landings per unit effort indicate that the formerly reduced lobster abundance, which had been declining since 2000 and which had reached its lowest level in 2011, has subsequently increased relatively steadily all through 2017, and now has increased back to levels close to those of 2007. The average size of lobsters taken, continues to be large, which is favourable to gradual stock recovery.

Mixed landings of reef fish in the lobster fishery have fluctuated between 10 and 20 tonnes annually. The LPUE abundance index in bycatch species also shows a decrease of about 35% from high levels in 2000 and 2007 to lowest levels in 2011, followed by an increase up to 2013, after which landings have levelled off.

In the redfish trap fishery (Lutjanus spp.), the number of trips in the redfish trap fisheries had grown significantly during the period 2012 to 2015. In 2016 effort increased again to 625 trips. In 2017 the exerted fishing effort dropped down to about 350 trips, a level seen last in 2012. The landings of redfish have fluctuated over this period, with lower values in 2012 and 2017 and higher values in 2014 and 2016. The LPUE abundance index shows a decrease by 50% between 2007 and 2011, followed by an increase to slightly higher and constant values between 2012 and 2016, while in 2017 the LPUE abundance index continued increasing. The only snapper for which sufficient data was available, was the silk snapper, Lutjanus vivanus. Average size of the silk snappers landed continued its increasing trend, indicative of gradual stock recovery. The drop in snapper landings, despite the higher LPUE in 2017, can especially be ascribed to the 6-month closed season implemented that year (Graaf et al. 2017).

The overall conclusion is that for, both the lobster and redfish stocks, stock status, based on the LPUE index, and size-structure trends have continued in developing favourably. Bottom drop longline, pelagic and bycatch landings have remained much less important and have shown no significant new developments.

There are two matters of concern that require follow up. Firstly, the positive prognosis for the snapper “stock” status may partly be based on targeting geographically different stocks as well as different species. Hence the data as collected and analysed may actually be presenting a too optimistic assessment. To evaluate whether this is actually the case will require more detailed analysis based on individual species, as well as more accurate geographical recording of catches than as currently practiced. The second matter of concern regards the fate of the traps that were in the water on the Bank when the hurricanes Irma and Maria struck the area in September 2017. If trap loss was large, this may lead to higher detrimental impacts of ghost fishing in the coming years.

Effective management of the Saba Bank is of  utmost importance for the biological, ecological, and economic wellbeing of the area. Therefore, it is agreed that the management of the Saba Bank is being evaluated regularly. This report is the result of an in depth evaluation of the effectiveness of the current management. This document provides a holistic description of
the current situation and steadily builds towards concrete recommendations based on a comprehensive analysis of stakeholder perceptions, leading practices, and contextual facts.

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.

A B S T R A C T

Fish assemblages of different types of artificial reefs can differ greatly in abundance, biomass and composition, with some reef types harboring over five times more herbivores than others. It is assumed that higher herbivorous fish abundance results in a higher grazing intensity, affecting the benthic community by means of enhanced coral recruitment, survival and growth. Territorial fish species might affect this process by chasing away other fish, especially herbivores. In this study we compared the fish assemblage, territorial behavior and grazing intensity by fish on two artificial reef types: reef balls and layered cakes, differing greatly in their fish assemblage during early colonization. In addition, the effect of artificial reef type on benthic development and coral recruitment, survival and growth, was investigated. Although layered cakes initially harbored higher herbivorous fish biomass, this effect was lost during consecutive monitoring events. This seems to be the result of the higher territorial fish abundance around the layered cakes where almost four times more chasing behavior was recorded compared to the reef balls. This resulted in a more than five times lower fish grazing intensity compared to the reef-ball plots. Although macroalgae were effectively controlled at both reefs, the grazing intensity did not differ enough to cause large enough structural changes in benthic cover for higher coral recruitment, survival or growth. The high turf algae cover, combined with increasing crustose coralline algae and sponge cover likely explained reduced coral development. We recommend further research on how to achieve higher grazing rates for improved coral development on artificial reefs, for example by facilitating invertebrate herbivore. 

MSc intership report

Coral reef systems have been declining all over the world. The project “Restoration of resilience of nature and society in the Caribbean Netherlands” focuses on mitigating further decline by improving the resilience of the coral reef. This is done, among other things, through water quality monitoring. In this report, we focussed on the quality of the coastal waters of Saba.

The spatial and temporal variation of chlorophyll a, salinity and temperature was assessed and connected to possible land-based activities and anthropogenic stressors.

The local stressors were assessed through informal interviews. The water quality indicators were measured with sensitive sensor technology. This was done by boat, every two weeks, on 13 locations around Saba, at a depth of 1.5-10 meters.

Both a temporal and spatial variation in chlorophyll a, temperature and salinity have been found. The chlorophyll a values seem concerting when looking at the coral reef threshold. Temperature and salinity are not yet troubling within he period of this research. However, if their temporal trend persists, there would be cause for concern. A baseline for potential local stressors has been identified but more extensive research is needed. Prolonged monitoring of the water quality indicators and more research into local stressors and how these affect one another is needed to fully understand what is going on.
For full report or more information,  please contact erik.meesters@wur.nl or gulsah.dogruer@wur.nl

On January 17th, 2022, Mark Zagers, Managing Director of the Saba Electric Company (SEC), officially handed over the keys to Peter Johnson, President of the Saba Conservation Foundation (SCF), the new tenant of the old power plant at the Fort Bay harbor. Also present were commissioner, Bruce Zagers, other SCF board members, resident researchers, and staff. After the handover, the group toured and inspected the newly renovated facility.

Due to climate change, overfishing, coastal development and other stresses, coral reefs worldwide are drastically in decline and scientists are racing to save the “rainforest of the sea” from extinction. The facility will provide novel opportunities to conduct coral reef research at an ideal location, adjacent to the Saba Marine Park and near the Saba Bank. The project was initiated in collaboration and with support of the island government.

Kai Wulf, managing director of the SCF, explained: “Our plans are to enhance the building to provide a controlled environment, so called mesocosms, to grow and study reef organisms, to better understand and control factors that impact and promote their wellbeing, with the aim to develop practical solutions to restore marine ecosystems.”

Official key handover; from left to right: Ayumi Kuramae Izioka, Saba Bank Science Coordinator, Lynn Costenaro, Board Member, Saba Conservation Foundation Mark Zagers, Managing Director, Saba Electric Company Peter Johnson, President, Saba Conservation Foundation Alwin Hylkema, Resident Researcher, Van Hall Larenstein University, Bruce Zagers, Commissioner, Saba Island Government. Credit: Kai Wulf

Services provided by Saba’s marine and terrestrial ecosystems are vitally important to the island’s fisheries and tourism industries, with an estimated total economic value of US $29 million per year (TEEB Study, Wolf’s Company 2014).

For example, by cultivating and restocking Diadema sea urchins, important herbivores decimated in the Caribbean by a viral disease during the early 1980s, algae growth that is now smothering and choking corals, could be vastly reduced. Scientists may also be able to develop new techniques to propagate stony coral that is more resilient to rising ocean temperatures and C02 induced acidification.

Successful research and coral restoration practices could bring much publicity and elevate Saba’s visibility internationally. Additionally, the research facility will create a new form of sustainable tourism for Saba, attracting scientists, students and coral reef restoration practitioners to the island. These visitors usually stay for longer periods and often bring family and friends, contributing substantially to the economy to the island. Further benefits will not just include guided tours, but could also provide career options for local students interested in marine biology and engage interested residents in meaningful citizen science.

The finalization of the building modifications approved by SEC and full operational capacity depend on the ability of the SCF and its partners to raise the necessary funding.

Concept drawing of future research station. Credit: Kai Wulf

Published in BioNews 51

Abstract.– Intraspecific diversity is among the most important biological variables, although still poorly understood for most species. Iguana iguana is a Neotropical lizard known from Central and South America, including from numerous Caribbean islands. Despite the presence of native melanistic I. iguana populations in the Lesser Antilles, these have received surprisingly little research attention. Here we assessed population size, distribution, degree of melanism, and additional morphological and natural history characteristics for the melanistic iguanas of Saba, Caribbean Netherlands based on a one-month fieldwork visit. Using Distance sampling from a 38- transect dataset we estimate the population size at 8233 ±2205 iguanas. Iguanas mainly occurred on the southern and eastern sides of the island, between 180-390 m (max altitude 530 m), with highest densities both in residential and certain natural areas. Historically, iguanas were relatively more common at higher altitudes, probably due to more extensive forest clearing for agricultural reasons. No relationship was found between the degree of melanism and elevation, and few animals were completely melanistic. Furthermore, we found that body-ratio data collection through photographs is biased and requires physical measuring instead. Although the population size appears larger than previously surmised, the limited nesting sites and extremely low presence of juvenile and hatchling iguanas (2.4%), is similarly worrying as the situation for I. delicatissima on neighboring St. Eustatius. The island’s feral cat and large goat population are suspected to impact nest site quality, nest success, and hatchling survival. These aspects require urgent future research to guide necessary conservation management.

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.

Surveying subaqueous beach profiles and features beyond wading depths can be a costly process, requiring use of expensive equipment and boats. This paper describes the materials and methods of a simple, low-cost technique for underwater bathymetric surveying, herein named the diver depth-gauge profiling (DDGP) method. Although accuracy of depth data depends on the quality of the depth gauges used, it is commonly within 0.3 m. Data collection reliability was evaluated by repeated underwater beach profile surveys, and an example of its use in the Caribbean is provided.