Protecting threatened Over-seas biodiversity

Review of more than 10 years of study on the Atlantic Goliath Grouper in Guyana

After 4 months of internship at GEPOG association, Noémie SALIBA carried out an assessment of the studies on the Atlantic Goliath Grouper in Guyana, in order to put in place suitable management tools. So what can we conclude from these 10 years of study on the Atlantic Goliath Grouper ?

Atlantic Goliath Groupers © Geraldecarroll

A highly threatened population

The Atlantic Goliath Grouper has seen its world population drop drastically in recent decades, for example with more than 84% decline in numbers in Florida since 1950. In Brazil, the same observation is made, the species has been wiped out there. As a result, moratoriums banning fishing for Atlantic Goliath Grouper were put in place in the United States and Brazil, allowing the start of stock recovery, particularly in Florida. Thus, the species classified “critically endangered” according to the IUCN in the 1980s changed to “Vulnerable” status in 2018.

In Guyana, the species remains present, but is largely unknown due to the turbid waters and the size of the area which make monitoring complex to set up. The lack of knowledge about the species, its demography, the size of its population, its movements or the fact that no breeding site is known in Guyana and that no larval stage has been observed pushes researchers and associations to take a closer interest in this species.

The first initiatives appeared in Guyana in 2007, notably at the Ile du Grand-Connétable National Nature Reserve. The completion of a thesis between 2010 and 2014, the objective of which was to describe the biology and ecology of the species in Guyana, then made it possible to collect most of the data. The Capture-Marking-Recapture (CMR) technique has been widely implemented. The GEPOG association, manager of the reserve, then continued the marking work, thus making it possible to have data collected over a period of 11 years.

11 years of data review

Biometry and biopsy on a Atlantic Goliath Grouper during scientific studies © GEPOG

Since 2007, a great deal of data has been collected on three main sites in Guyana: the Grand-Connétable Nature Reserve, the Ilets de Rémire and the Battures du Connétable.
A total of 166 boat trips were carried out between 2007 and 2018 and 745 Atlantic Goliath Groupers were captured and tagged using an external tag positioned on the dorsal part of each animal. Of these 745 groupers, only 88 were recaptured. Rather, individuals recaptured over long periods of time are young groupers that appear to settle and grow in place. The Ilets de Rémire, closer to the coast, seem to be home to smaller groupers.

Population dynamics and survival

During the monitoring by CMR, little movement was detected between the different sampled sites. Groupers were generally recaptured at the site of first capture. This can be explained by a too low rate of recaptures not allowing to show this type of movements, but also by a migration of individuals over great distances, a high mortality rate or the non-transmission of tags by fishermen.
Individuals captured were of average size compared to individuals measured in Florida. However, this observation is surely linked to the fishing method used in Guyana which limited the capture of very large individuals (line fishing) unlike in Florida where observations are made by diving.

According to the data collected, the further individuals move away from the coast, the more their size increases. However, some small fish were also seen at rocky sites further offshore when juveniles were thought to be only present in mangroves. This is perhaps related to the characteristics of Guyana where the waters are turbid and shallow for miles.

Individuals were recaptured over rather long time steps (sometimes 4 years later) on the same site. This therefore demonstrates a territorial behavior of the species. However, apparent survival appears to be very low in adults and higher in small individuals.

Large-scale population movements and structuring

The genetic analyzes carried out on the captured individuals show that there is no genetic structuring on the scale of the South American continent and that the Guyanese sites are totally part of the South American population. This therefore suggests a very strong population dynamic leading to a high migration rate which was not however observed during these studies on the sites monitored in Guyana. It is possible that the species performs occasional large-scale movements followed by long periods of inactivity.

Estimated population size

Given the low number of recaptures, the data do not allow a population size to be calculated. The latter seems to be under the influence of migration processes, so it is difficult to make such an estimate via a CMR protocol on an unclosed population.

Answers leading to new questions

The lack of knowledge and the still fragile state of conservation on a global scale largely justify the implementation of management measures for the protection of the Atlantic Goliath Grouper in Guyana. Currently, no regulations exist in Guyana except for recreational sea fishing where a prefectural decree authorizes the capture of a grouper by boat and by trip.

Studies that raise new questions

  • What populations depend on larval production in Guyana?

Even if the hypothesis of larval stocks from Brazil would explain the absence of breeding sites in Guyana, this does not exclude other possible avenues such as the presence of these sites in brackish / fresh water or on deeper bottoms.

  • Where the other sites with a high presence of grouper in Guyana are located and where are the adults?

Guyanese waters present numerous wrecks and rocky sites further offshore that may harbor adult Atlantic Goliath Groupers, but no surveys have yet been carried out at such sites.

  • Where do the individuals who grew up in Guyana breed?

The few recaptures and the absence of a breeding season raise the hypothesis of a definitive dispersal of large adults towards Brazil or the northwest towards the Caribbean region or possibly Florida.

How to answer these new questions?

The CMR method does not seem suitable for answering all the questions asked. It would also require the involvement and commitment of all fishermen to succeed in obtaining large datasets. However, methods exist and can be implemented:

– Satellite beacons and acoustic telemetry

The equipment available on the market today, PSAT beacons and VEMCO transmitters, would allow small and large scale movement tracking. This would therefore provide a lot of data on the dynamics of populations and exchanges with other territories.

– Monitoring by photo-identification

Each grouper has a unique coloring and markings that allow them to be recognized. Thus, photo-identification could be implemented on a few very frequented sites (Battures du Constable for example). Since most of the fishermen are used to taking pictures of their catches, their transmission and processing would make it possible to monitor and estimate the number of people on a small site.

– The use of environmental DNA

This method involves using genetics to demonstrate the presence of species in different environments. This is done by collecting and filtering water samples and then carrying out laboratory analyzes that identify traces of DNA and identify species. Thus, several sites can be prospected without having to capture the animals.

Pending the establishment of new scientific monitoring, some of which should see the light of day in the next two years, GEPOG association coordinates locally the LIFE BIODIV’OM program which includes a consultation process with fisheries stakeholders, decision-makers and managers to sustainable management of the Atlantic Goliath Grouper in French Guiana.