Queen conch (Lobatus gigas) populations living deeper than 20 m are rarely studied, because of the limitations of conventional survey methods using divers [i.e., belt transect (BT), towed-diver]. A crucial management goal for conch populations is to maintain adult densities at adequate levels to ensure reproduction, which is highly density dependent. Therefore, accurate estimates of adult conch densities, both in shallow and deep areas, are essential. The rapid technical progress of video systems has made it possible to develop new cost-effective ecological sampling tools, which can be used to survey areas previously hardly accessible. A lightweight towed video array was used, which was able to survey adult conch throughout the species entire depth range (ca. 0–60 m depth), in a safe and efficient manner. The towed video method (TVM) was compared with a conventional BT method using scuba divers, in its ability to identify adult live and dead conch. A series of intercalibration transects was conducted in a high-complexity (HC) and in a low-complexity (LC) habitat by having the towed video followed by a diver conducting a concurrent standard BT, covering the exact same surface area as the towed video. In both the HC and LC habitat, adult live queen conch had similar counts with both methods. Adult dead conch were not mistaken for live conch but were significantly underestimated with the towed video compared with the BT. The results validate the use of TVM as a reliable sampling tool to estimate densities of live adult conch in both HC and LC habitats throughout the species depth range.
Rijn, J. van
The large marine gastropod, Queen conch (Strombus gigas) was studied over a period of 3 months on the island of St. Eustatius. Strombus gigas are widely distributed throughout the Caribbean and are important to local communities due to its nutritional and traditional value. Because of an increase in conch landings since the seventies Strombus gigas populations have dramatically declined causing it to be included in the Appendix II of CITES. Fishermen on St. Eustatius have seen Strombus gigas populations move to deeper waters. Strombus gigas have been actively protected since 1996 by the St. Eustatius Marine Park in all surrounding water up to a 30-meter depth contour; no fishing is allowed in the two marine reserves and regulations state that Strombus gigas may only be caught free diving to a max of 20 conch per person per year and with a minimum shell length of 19 cm. This study focuses on estimating a trend in Strombus gigas populations and testing a towed video method for benthic (queen conch) surveys. Dive surveys covered a transect of 50 meters long and 10 meter wide, noting species, shell length and lip thickness. The video array was constructed of light weight materials with a live feed to the towing vessel to check alignment. Towed video transects covered a length of 500 meters with a width of 1 meter only noting species and numbers. To calibrate this towed video method, a diver followed the video array on 5 occasions to establish a correction factor. Fisheries samples were taken whenever the fishermen would land his catches and notify us. Population trend surveys showed no clear trend and were unusable due to the limited sample size. For the dive surveys a total of 25 sites were sampled of which 11 sites met the minimum density for Strombus gigas to reproduce, of these 11 sites at least 5 are within the marine reserve and thus unfished. Towed video calibration proved promising, in the future better observer training and greater sample size is necessary. Of the 11 towed video sites, Strombus gigas clearly prefers the rubble/algae sites over bare sandy sites. Of the 11, 7 sites met the minimum required density for reproduction. But the towed video calibration made clear that the Strombus gigas numbers noted during the towed video analysis are likely to be an over estimate as adult Strombus gigas numbers seem to include dead Strombus gigas and Strombus costatus who are misinterpreted. In future studies better observer training is necessary, a larger sample size for trend indications and the towed video calibration is needed as well. Strombus gigas reproduction and fishing pressure also need to be studied.
Data on beach debris and tar contamination is provided for 21 natural beach sites in Bonaire, Southeastern Caribbean. Transects amounting to a combined length of 991 m were sampled March–May 2011 and a total of 8960 debris items were collected. Highest debris and tar contamination were found on the beaches of the windward east-coast of the island where geometric mean debris concentrations (± approx. 70% confidence limits) were 115 ± 58 items m-1 and 3408 ± 1704 g m-1 of beach front. These levels are high compared to data collected almost 20 years earlier on the nearby island of Curaçao. Tar contamination levels averaged 223 g m-1 on windward beaches. Contamination levels for leeward west-coast beaches were generally two orders of magnitude less than windward beaches.