population structure

The genus Alsophis used to be very abundant in the Lesser Antilles. Numbers have declined ascribed to anthropogenic causes and the introduction of invasive species to their habitat. Most Alsophis species are classified as endangered or critically endangered by the IUCN. Alsophis rufiventris, a species native to the islands Saba and St. Eustatius, is facing a constant threat of the Javan mongoose being introduced in its habitat. Also dogs, cats, rats and humans actively influence the abundance of A. rufiventris. Though all these threats, the population of A. rufiventris seemed stable and was classified as vulnerable by the IUCN in 2016. In 2017 hurricane Irma and Maria impacted on Saba and St. Eustatius. It was expected that the A. rufiventris population was altered. Therefore a new population assessment was done in the Quill and Boven National Park on St. Eustatius in 2018 and was repeated in this study. In 2018 the population size in the study area was 165.

Distance sampling with line transects was used to estimate the population size of A. rufiventris. In this study 1068 line transect surveys resulted in 60 snakes recordings. As the snake count was sparse, N-mixture models were used as well.

Distance sampling resulted in a population size of 464 and the N-mixture models resulted in a population size of 178. These results are preliminary, as covariates had no influence on the model in the program DISTANCE. While, according to earlier research by Savit et al. in 2015 the covariates should influence the results. Multipliers have to be added in the program DISTANCE to get more accurate results. From this study the habitat preference of A. rufiventris cannot be described, as the findings on habitat categories were not significantly different from each other.

A. rufiventris is becoming more rare in its native habitat and that is a cause for concern. As many threats to the species are present, but also actively influence their abundance, the species might become, or effectively already can be categorized as (critically) endangered. It is advised to continue monitoring of this species not only on St. Eustatius but on Saba as well. Also conservation actions like establishing biosecurity and conducting rodent control might be beneficial. As these conservation actions might not happen in the near future, a breeding program in a zoo might be beneficial to preserve the genetic variance in the population.

ABSTRACT: Effectiveness of a marine protected area (MPA) in supporting fisheries productivity depends upon replenishment patterns, both in supplying recruits to surrounding fished areas and having a sustainable spawning stock in the MPA. Surveys for queen conch Strombus gigas were made in 2011 at 2 locations in the Exuma Cays, The Bahamas, for direct comparison with surveys conducted during the early 1990s at Warderick Wells (WW) near the center of the Exuma Cays Land and Sea Park (ECLSP) and at a fished site near Lee Stocking Island (LSI). There was no change in adult conch density and abundance in the shallow bank environment at LSI where numbers were already low in 1991, but numbers declined 91% in the deeper shelf waters. At WW, the adult population declined 69% on the bank and 6% on the island shelf. Unlike observations made in the 1990s, queen conch reproductive behavior near LSI is now rare. Average age of adult conch (indicated by shell thickness) at LSI decreased significantly during the 20 yr period between surveys, while average age increased at WW and juvenile abundance decreased. These results show that the LSI population is being overfished and the WW population is senescing because of low recruitment. In 2011, the ECLSP continued to be an important source of larvae for down- stream populations because of abundant spawners in the shelf environment. However, it is clear that the reserve is not self-sustaining for queen conch, and sustainable fishing in the Exuma Cays will depend upon a network of MPAs along with other management measures to reduce fishing mortality.