population

Abstract

Management of reef fish populations requires insight in their resilience to anthropogenic stressors. Studies on temporal variations in reef fish populations and their abundance responses to environmental changes are crucial to the development of ecosystem-based management.  Nineteen years of voluntary fish survey data of reef fish at the west coast of Bonaire Marine Park (Caribbean) were analysed to investigate the effect of environmental changes on local reef fish populations. Various anthropogenic stressors that influence the coral reefs of Bonaire were studied in recent years. This study studies to what extent these stressors and protective management measures can be related to the observed population trends. In addition, reef fish population responses to specific events were analysed. In general fluctuations in sighting frequencies were species-specific and difficult to interpret.  Although some noise in the data couldn’t be filtered out they show an overall negative trend in sighting frequencies in the 155 studied species.. A comparison of the average frequencies between the first five years of this study with the last five years resulted in an over 10% decrease in 64 of the species, while 30 species increased over 10%.  Three large parrotfish species decreased by approximately 50% over time and there were indications of a slight negative trend in mid-sized parrotfish as well. In fishery targets, the most intensive fished group, large to mid-sized groupers, decreased strongly. After the abundance of these groupers decreased to near zero, two species belonging to other fish families started to show signs of sequential overfishing.  It is unclear if larval import will compensate for the loss in reproductive capacity of the species in decline. Recent literature on recruitment distances suggest that the level of larval import is low on isolated islands like Bonaire, resulting in dependence of self recruitement.  Another event with strong effect on population dynamics are the mass mortality events in moray eels. Especially in two Enchelycore species these resulted in sigmoid like trends. The 2008 and 2022 mass mortality events were first noted at the onset of a period of sea water warming, paralleling various reef fish species in the Red Sea. This gives further support to the hypothesis that the current increase in warming events will lead to more frequent mass mortalities.  f.p.bennema@gmail.com 

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In their 2019 paper, Sea Turtle Conservation Bonaire (STCB) used sea turtle data collected between 2003 and 2018 to estimate abundance and predict future population trends for green and hawksbill turtles on Bonaire’s west coast. The non-profit organization has now expanded that research in their latest paper to determine the population trend between 2019 and 2022.

Hawksbill turtle. Photo credit: Brenda Kirkby

Sea turtles are an iconic species that face a wide variety of threats.  Since these species can travel vast distances, they are susceptible to a number of transboundary threats, including climate change and poaching as well as local concerns such as pollution and habitat loss.  Monitoring nesting and foraging grounds is a critical part in understanding long-term sea turtle trends throughout the Caribbean.

Unfortunately, sea turtles can be difficult to accurately monitor due to issues with detection and hence incomplete counts. To combat this, research statisticians have developed methods to estimate detection and population size, allowing for more realistic counts. STCB has been working many years to monitor local turtle populations, and in collaboration with Frank Rivera-Milan, to determine trends and test the accuracy of prediction models. For this particular research, the goal was to determine whether the population increased, decreased on remained the same between 2019 and 2022, compared to 2003-2018 and predictions for 2019-2030.

Results

Between 2019 and 2022, surveys were conducted along western Bonaire and Klein Bonaire. In total 703 green turtles and 56 hawksbill turtles were recorded.  Researchers also looked at how easy it was to spot the turtles, and they found that some factors, like the substrate (e.g., sand, rubble) or the level of disturbance (e.g., boats, divers), influenced the number of turtles that were detected.

Green turtle. Photo credit: Brenda Kirkby

When comparing historical data to the prediction data, this study yielded interesting results.  For green turtles, the number of turtles counted between 2019 and 2022 did not change much but was comparatively lower than data collected from 2003-2018 and lower than estimates predicted in the 2019-2030 model.  The number of hawksbill turtles, on the other hand, fluctuated between 2019 and 2022 but was similar to previous surveys (conducted between 2003-2018) and was closer to the predictive models.

Implications

The study’s density estimates aligned with previous research on sea turtles in other locations, and the researchers emphasized the importance of accounting for detection when estimating population numbers. This research concluded that this methodology provided reliable detection and population estimates for monitoring sea turtles within foraging grounds in the Caribbean, therefore this approach could be valuable for similar studies in coastal areas.

DCNA

The Dutch Caribbean Nature Alliance (DCNA) supports science communication and outreach in the Dutch Caribbean region by making nature related scientific information more widely available through amongst others the Dutch Caribbean Biodiversity Database, DCNA’s news platform BioNews and through the press. This article contains the results of one of those scientific studies, but this study is not a DCNA study. No rights can be derived from the content. DCNA is not liable for the content and the in(direct) impacts resulting from publishing this article.

In hun wetenschappelijk artikel uit 2019 gebruikte Sea Turtle Conservation Bonaire (STCB) gegevens over zeeschildpadden die tussen 2003 en 2018 waren verzameld om de populatie te schatten en toekomstige populatietrends voor groene en karetschildpadden aan de westkust van Bonaire te voorspellen. STCB heeft dat onderzoek nu uitgebreid in hun nieuwste wetenschappelijk artikel om de populatietrend tussen 2019 en 2022 te bepalen.

Karetschildpadden. Photo credit: Brenda Kirkby

Zeeschildpadden zijn een iconische soort die met een breed scala aan bedreigingen wordt geconfronteerd. Omdat deze soorten grote afstanden kunnen afleggen, zijn ze vatbaar voor een aantal grensoverschrijdende bedreigingen, waaronder klimaatverandering en stroperij, maar ook voor lokale problemen zoals vervuiling en verlies van leefgebied. Het monitoren van broed- en foerageergebieden is een cruciaal onderdeel om inzicht te krijgen in de langetermijntrends van zeeschildpadden in het Caribisch gebied.

Helaas kunnen zeeschildpadden moeilijk nauwkeurig te volgen zijn vanwege problemen met detectie en dus onvolledige tellingen. Om dit tegen te gaan, hebben onderzoeksstatistici methoden ontwikkeld om de detectie en de populatiegrootte te schatten, waardoor meer realistische tellingen mogelijk zijn. STCB werkt al vele jaren aan het monitoren van lokale schildpaddenpopulaties, en in samenwerking met Frank Rivera-Milan, aan het bepalen van trends en het testen van de nauwkeurigheid van voorspellingsmodellen. Voor dit specifieke onderzoek was het doel om te bepalen of de bevolking toenam, daalde of gelijk bleef tussen 2019 en 2022, vergeleken met 2003-2018 en voorspellingen voor 2019-2030.

Resultaten

Tussen 2019 en 2022 zijn tellingen uitgevoerd langs westelijk Bonaire en Klein Bonaire. In totaal werden 703 groene schildpadden en 56 karetschildpadden geregistreerd. Onderzoekers keken ook naar hoe gemakkelijk het was om de schildpadden te spotten, en ze ontdekten dat sommige factoren, zoals de ondergrond (bijvoorbeeld zand, puin) of de mate van verstoring (bijvoorbeeld boten, duikers), van invloed waren op het aantal schildpadden dat werd gedetecteerd.

Groene schildpad. Photo credit: Brenda Kirkby

Bij het vergelijken van historische gegevens met de voorspellingsgegevens leverde dit onderzoek interessante resultaten op. Voor groene schildpadden veranderde het aantal getelde schildpadden tussen 2019 en 2022 niet veel, maar was relatief lager dan de gegevens verzameld van 2003-2018 en lager dan de schattingen die in het 2019-2030-model waren voorspeld. Het aantal karetschildpadden fluctueerde daarentegen tussen 2019 en 2022, maar was vergelijkbaar met eerdere onderzoeken (uitgevoerd tussen 2003-2018) en lag dichter bij de voorspellende modellen.

Implicaties

De schattingen van de dichtheid tijdens de studie kwamen overeen met eerder onderzoek naar zeeschildpadden op andere locaties, en de onderzoekers benadrukten het belang van het verantwoorden van detectie bij het schatten van populatieaantallen. Dit onderzoek concludeerde dat deze methodologie betrouwbare detectie- en populatieschattingen opleverde voor het monitoren van zeeschildpadden in foerageergebieden in het Caribisch gebied, daarom zou deze aanpak waardevol kunnen zijn voor soortgelijke studies in kustgebieden.

DCNA

De Dutch Caribbean Nature Alliance (DCNA) ondersteunt wetenschapscommunicatie en outreach in de Nederlandse Caribische regio door natuurgerelateerde wetenschappelijke informatie breder beschikbaar te maken via onder meer de Dutch Caribbean Biodiversity Database, DCNA’s nieuwsplatform BioNews en via de pers. Dit artikel bevat de resultaten van een van die wetenschappelijke onderzoeken, maar dit onderzoek is geen DCNA-onderzoek. Aan de inhoud kunnen geen rechten worden ontleend. DCNA is niet aansprakelijk voor de inhoud en de indirecte gevolgen die voortvloeien uit het publiceren van dit artikel.

Published in BioNews 67

Dutch below

Queen conch are facing immense pressure leading to a dramatic decrease in their populations.  Luckily, a new project, started at the Curacao Sea Aquarium in 2020, will explore ways to reduce mortality rates and develop new methods for effective restocking of natural populations with hatchery-reared conch. 

Photo credit: Julia Ehrenheim

The large marine gastropod queen conch (Strombus gigas) is a charismatic and iconic symbol of nature and one of the most important demersal fishery resources in the Caribbean region. Trade of the species (largely for consumption) is regulated by the CITES treaty, to which the Kingdom of the Netherlands is party. 

The Problem

Today, these iconic marine snails are threatened by over-exploitation and loss of shallow water habitats, throughout the Caribbean as well as in the ABC islands.  Human activities have led to degradation of nearshore conch habitats causing breeding failure, decreased growth and survival of juvenile conch. Excessively low densities mean that animals of breeding age fail to find mates and therefore also fail to breed (the so-called Allee Effect). Queen conch are also particularly vulnerable to overfishing due to their unique life history (pelagic phase, density-dependent survival and reproduction). The exploitation of queen conch throughout large parts of its natural range has led to population collapses and decrease in population densities to levels where mating success is diminished.

Photo credit: Julia Ehrenheim

Hope on the Horizon

In 2020 the Queen Conch Hatchery Project was started at the Curacao Sea Aquarium with Julia Ehrenheim as the key scientist. The science of hatchery production has been well established for many years but rewilding to bolster natural populations has consistently failed. Therefore, the main goal of this project is to find ways to reduce and/or eliminate high mortality rates that occur in early life stages and develop methods for effective restocking of natural populations with hatchery-reared juvenile conch. In order to find answers to various elusive questions, Julia has started the research that she will use for her PhD degree at the University of Wageningen under Professors Tinka Murk and Dolfi Debrot.

The Experiments

Her first experiments aim to address the movement behavior of juvenile conch. For this she and her students tracked the activity of juvenile conch in a controlled hatchery environment. The results show that juvenile conch are more active at sunset than in the early morning or during midday. They also appear to preferentially move towards the current. Such experiments need to be done in the field but already suggest ways to facilitate the relocation of released hatchery conch and that juvenile conch will be most vulnerable to predators at sunset.

Photo credit: Julia Ehrenheim

A second experiment showed that juvenile conch often aggregate together in the wild. Therefore, she studied if there was a beneficial density for growth. The first finding showed that juvenile conch grew the most in shell length with a density of 200 conch per m2. Lower and higher population densities gave lower growth rates and lower total shell length, even though all were grown under the same conditions of food availability.

Correlations between weight and growth (in total length) indicate that there are distinct phases of growth. It seems that growth alternates between predominant growth in the thickness of the shell and in total shell length. Both do not occur simultaneously. The significance of this phenomenon to juvenile survival is not understood.

Implications

These findings for hatchery-reared conch will be further investigated and verified in wild grown juvenile conch. Taken together the results of this PhD project will help clarify why juvenile hatchery-released conch are so vulnerable and what needs to be done to enhance their survival so that they can be effectively used to restore natural populations.

Photo credit: Julia Ehrenheim

DCNA  

The Dutch Caribbean Nature Alliance (DCNA) supports science communication and outreach in the Dutch Caribbean region by making nature related scientific information more widely available through amongst others the Dutch Caribbean Biodiversity Database, DCNA’s news platform BioNews and the press. This article contains the results from several scientific studies but the studies themselves are not DCNA studies. No rights can be derived from the content. DCNA is not liable for the content and the in(direct) impacts resulting from publishing this article. 

Kroonslakken staan onder enorme druk, wat leidt tot een dramatische afname van hun populaties. Gelukkig is er in 2020 een nieuw project gestart in het Curaçao Sea Aquarium, waarin manieren worden onderzocht om de sterftecijfers te verlagen en nieuwe methoden te ontwikkelen voor het effectief uitzetten van natuurlijke populaties met in kwekerij gekweekte kroonslakken.

Foto: Julia Ehrenheim

De kroonslak (Strombus gigas) is een charismatisch en iconisch symbool van de natuur en een van de belangrijkste demersale visbestanden in het Caribisch gebied. De handel in de soort (grotendeels voor consumptie) wordt gereguleerd door het CITES-verdrag, waarbij het Koninkrijk der Nederlanden partij is.

Het probleem

Tegenwoordig worden deze iconische zeeslakken bedreigd door overexploitatie en verlies van ondiepe leefgebieden, zowel in het Caribisch gebied als op de ABC-eilanden. Menselijke activiteiten hebben geleid tot achteruitgang van leefgebieden van kroonslakken aan de kust, waardoor het voortplanten mislukt en de groei en overlevingskans van jonge kroonslakken afneemt. Te lage dichtheden zorgen ervoor dat dieren op geslachtsrijpe leeftijd geen partner vinden en dus ook niet voortplanten (het zogenaamde Allee-effect). Kroonslakken zijn ook bijzonder kwetsbaar voor overbevissing vanwege hun unieke levensloop (pelagische fase, dichtheidsafhankelijke overleving en voortplanting). De exploitatie van de kroonslak in grote delen van zijn natuurlijke verspreidingsgebied heeft geleid tot het instorten van de populatie en het afnemen van de populatiedichtheden tot niveaus waarop het paringssucces is verminderd.

Foto: Julia Ehrenheim

Hoop aan de horizon

In 2020 werd het Queen Conch Hatchery Project gestart in het Curaçao Sea Aquarium met Julia Ehrenheim als belangrijkste wetenschapper. De wetenschap van de productie van kweekerijen is al vele jaren goed ingeburgerd, maar het opnieuw verwilderen om natuurlijke populaties te versterken, is consequent mislukt. Daarom is het belangrijkste doel van dit project om manieren te vinden om de hoge sterftecijfers die zich in de vroege levensfasen voordoen te verminderen en/of uit te bannen, en om methoden te ontwikkelen voor het effectief uitzetten van natuurlijke populaties met in kwekerij gekweekte jonge kroonslakken. Om antwoorden te vinden op verschillende moeilijke vragen, is Julia het onderzoek gestart dat ze zal gebruiken voor haar promotieonderzoek aan de Universiteit van Wageningen onder professoren Tinka Murk en Dolfi Debrot.

De experimenten

Foto: Julia Ehrenheim

Haar eerste experimenten zijn gericht op het bewegingsgedrag van jonge kroonslakken. Hiervoor volgden zij en haar studenten de activiteit van jonge kroonslakken in een gecontroleerde kwekerij omgeving. De resultaten laten zien dat jonge kroonslakken actiever zijn bij zonsondergang dan in de vroege ochtend of middag. Ze lijken ook bij voorkeur naar de stroming toe te bewegen. Dergelijke experimenten moeten in de natuur worden gedaan, maar suggereren al manieren om de verplaatsing van uitgezette kroonslakken te vergemakkelijken en dat jonge kroonslak het meest kwetsbaar zijn voor roofdieren bij zonsondergang.

Een tweede experiment toonde aan dat jonge kroonslakken in het wild vaak bij elkaar komen. Daarom onderzocht ze of er een gunstige dichtheid voor groei was. De eerste bevinding toonde aan dat jonge kroonslakken het meest groeiden in schelplengte met een dichtheid van 200 kroonslakken per m2. Lagere en hogere dichtheden zorgden voor lagere groeisnelheden en een lagere totale schaallengte, ook al werden ze allemaal gekweekt onder dezelfde omstandigheden van voedselbeschikbaarheid.

Correlaties tussen gewicht en groei (in totale lengte) geven aan dat er verschillende groeifasen zijn. Het lijkt erop dat de groei afwisselt tussen overheersende groei in de dikte van de schaal en in de totale lengte van de schaal. Beide komen niet gelijktijdig voor. De betekenis van dit fenomeen voor de overleving van jonge kroonslakken wordt nog niet begrepen.

Implicaties

Deze bevindingen voor in kwekerij gekweekte kroonslakken zullen verder worden onderzocht en geverifieerd bij in het wild geboren jonge kroonslakken. De resultaten van dit doctoraatsproject zullen helpen verduidelijken waarom jonge kroonslakken die in kwekerijen zijn uitgezet zo kwetsbaar zijn en wat er moet worden gedaan om hun overlevingskans te vergroten, zodat ze effectief kunnen worden gebruikt om natuurlijke populaties te herstellen.

Foto: Julia Ehrenheim

DCNA

De Dutch Caribbean Nature Alliance (DCNA) ondersteunt wetenschapscommunicatie en outreach in de Nederlandse Caribische regio door natuurgerelateerde wetenschappelijke informatie breder beschikbaar te maken via onder meer de Dutch Caribbean Biodiversity Database, DCNA’s nieuwsplatform BioNews en de pers. Dit artikel bevat de resultaten van verschillende wetenschappelijke onderzoeken, maar de onderzoeken zelf zijn geen DCNA-onderzoeken. Aan de inhoud kunnen geen rechten worden ontleend. DCNA is niet aansprakelijk voor de inhoud en de indirecte gevolgen die voortvloeien uit het publiceren van dit artikel.

Published in BioNews 64

Bonaire is facing major challenges including (mass) tourism, population growth, urban expansion, climate change, biodiversity loss and the unilateral dependency on tourism. In thirty years, Bonaire will inevitably look different. Here, two different possible futures are presented, to form a basis for dialogue amongst stakeholders and to stimulate a positive change and sustainability on Bonaire. One of these scenarios follows current trends (business-as-usual), and the other bends those trends into a nature-inclusive future after a vision developed by a trans-disciplinary team of researchers, local experts and stakeholders. For both scenarios drivers and impacts are visualized and documented on climate, tourist numbers, population, infrastructure, resources, land use, erosion and nature. 
Visualizing scenarios is one important piece in creating awareness about the future as it allows to shed light of the difficult to grasp long-term effects, and explicitly showcases current trends. It gives opportunities to imagine a future that looks different from the prognosis, and to inspire to work towards a sustainable and desirable future.

DISCUSSION AND FUTURE PERSPECTIVES (excerpt from the report)

With this study, we shed light on measurable impacts to Bonaire if current trends continue as usual; and provided an alternative which can be visualized as a result of nature inclusive policies, actions and land use changes. However steering changes towards nature inclusivity is not only a change in land (and sea) use and indicator values, but to make it a reality it is a change in mindset of an entire community. This cannot happen without the awareness of the trade-offs that nature inclusive actions can bring to the many different stakeholders involved. In this discussion we make a case for the importance of nature inclusivity on Bonaire, and make a start towards a dialogue about the risks, trade-offs and opportunities that may lie ahead.
We have documented the current trends: Bonaire has experienced a quadrupling in population size over the past half century. Cruise tourism started growing exponentially in the mid-2000s, and stay-over tourism steadily increasing. All while access to freshwater with the climate change projections becomes more difficult and costly. While some Dutch Caribbean islands might have experienced a much more explosive increase in tourism and population than Bonaire (e.g. Aruba), other islands experience more stability (e.g. Saba). Anecdotal evidence from other islands (see Bonaire reporter, 2022), as well as the projections showcased in this report imply that if Bonaire wants to stay relevant as a tourist destination and support its growing population the island needs to focus on its long-term assets.
The scenarios in this report were described using indicators that progress along the trends and rates of the past several years. While these assumptions include some climate change parameters like gradual warming, and gradually reduced precipitation, they fail to consider implications of unforeseen natural disasters, or increasing severe weather conditions which will take a toll on the island. Neither do the scenario projections consider any changes in world trade processes for food or fuel. An honest look at the state of the island for the next 30 years under the Business as Usual projections indicates that sustaining such growth under the current (environmental or political) conditions of the planet are relatively short-lived, and are built on a set of fragile assumptions.
Naturally, trends described in the nature inclusive scenario imply (policy) choices with varying effects on each sector as shown using several indicators. In some cases, the rates compared to the BaU scenario will be slower (population growth due to immigrants, stay-over tourism, urbanisation), while with other indicators/sectors growth rates will increase (greenness, agricultural land, green and wind energy use and water collection). Specific implications of a scenario can be beneficial, while others can be unfavourable, depending on the agenda of each particular stakeholder. An example of such a trade-off is the extensive local food production under the nature inclusive scenario: in the foreseen closed agricultural system there is far less need for off-island nutrient imports. As fewer nutrients are brought onto the island, this reduces the harm from foreign particles to the environment and the reef. As such, the reef is more likely to stay healthy and can continue to be a major tourist attraction. Nevertheless, individuals currently working in the food import logistics sector may experience a decrease in business. This may be overcome by jobs created through the growing local food production industry, but awareness of this trade-off is important when instigating changes. Attractiveness of the nature inclusive scenario in the short-term depends on the stakeholder. While in the long-term, the implication of the nature-inclusive scenario is of an island prosperity that is inclusive for everyone and ensures sustainability. One which is much less dependent on the few precarious pillars on which it is currently built: food and fuel importation.
Another crucial trade-off of the nature inclusive scenario is the implication of water and waste collection. This requires significant infrastructural investments (sceptic tank collection, or rooftop collection installations, appropriate facilities and road ways to ensure this, and home fitting) which requires not only government support, but individual support and repeated actions. It means a change in routine, from linear to circular consumption. Routine is a difficult circuit to make or break. This will require a cultural understanding of the benefits and wholehearted will to change the norm. It may require a big investment in time and energy in the short-term to create a long term self-sustaining infrastructure

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.

Dit rapport is het resultaat van een in de maanden mei tot en met augustus 1994 gehouden onderzoek naar de verwilderde huisezel (Equus asinus) op Bonaire. 

In samenwerking met andere instanties wordt er op dit moment door STINAPA getracht om een natuurbeleidsplan voor het eiland te ontwikkelen, waarin het adequaat beheren van de natuurlijke waarden van het eiland centraal zullen staan. Om dit te kunnen bewerkstelligen is het van belang dat er accurate en recente gegevens beschikbaar zijn. Aangezien er over de verwilderde ezels nagenoeg niks bekend is is het noodzakelijk dat er onderzoek naar deze dieren wordt gedaan. Dit rapport is een start hiervoor en kan gaan dienen als een handreiking voor verder onderzoek.