Lobatus gigas

As invasive seagrass continues to expand and replace native species, populations such as the queen conch are seeing significant changes to their habitat and subsequent negative impact in food source availability. With potential consequence for the resilience of such species in a changing world. A recently published study from St. Barthelemy, St. Eustatius and St. Maarten, worked to understand how native and invasive seagrasses influence juvenile queen conch’s development by studying both dietary composition and growth rate.

This article was published in BioNews29

More information: Boman, E.M., Bervoets, T., de Graaf, M., Dewenter, J., Maitz, A., Meijer Zu Schlochtern, M.P., Stapel, J., Smaal, A.C., Nagelkerke, L.A.J., 2019. Diet and growth of juvenile queen conch Lobatus gigas (Gastropoda: Strombidae) in native, mixed and invasive seagrass habitats. MARINE ECOLOGY PROGRESS SERIES. Vol. 621: 143–154, 2019 https://doi.org/10.3354/meps12990

Abstract:

The queen conch (Lobatus gigas) is under pressure and has decreased in distribution and abundance as a result of fisheries in the Caribbean region. To ensure sustainable fishery, knowledge on the population dynamics of this species is required. Therefore, this research focused on the status and the reproductive behaviour of the conch population around St Eustatius and provides information about the possibilities for a sustainable fishery. Conch distribution and abundance, population structure, reproductive activity and fishing pressure were determined in the coastal waters around the island of St Eustatius by dive surveys, towed video surveys, reproduction surveys and fishery catch surveys. The study covered the entire Statia National Marine Park waters and covered different habitats and depths up to 40m.

The study shows that the queen conch is abundant around St Eustatius, with mean densities of 57 (dive surveys) and 115 (video surveys) adults per ha. The total adult queen conch stock was estimated to be 184,100 (95% C.I.: 77,586-390,000) in 2,700 ha Marine Park. Further, a higher conch abundance was found on rubble habitats and at greater depths (17-31 m). Mainly mature conchs were detected, indicating a shortage of recruitment of young conch, or a difficulty in observing young conch. Further, reproductive activity started in March and declined after October, with peak reproductive activity during June and July. Minimum lip thickness reported for reproductive behaviour was 9 mm. However, this study also indicated that more research is necessary to qualify the exact spawning season and size at maturity of conch around St Eustatius.

In conclusion, the status of the queen conch could be qualified as good and does not seems to appear under direct pressure in the coastal waters of St Eustatius. With this population status, small- scale fishery may be possible without negative consequences for the queen conch population. However, to ensure long-term sustainable of a conch fishery, harvest restrictions as advised by the queen conch working group are recommended to be used; like a minimum of 15 mm lip thickness, closed areas and an annual harvest of maximal 6200 adult conch. Thus, non-detrimental fishing is possible for queen conch on St Eustatius, under the conditions that, proper management and harvest restriction regulations need to be developed, implemented and enforced in close co-operation with stakeholders. 

Abstract:

In 1999 a survey was carried out (Lott, 2000) to estimate the conch population and determine the status of the seagrass beds in Lac Bay, Bonaire. Since 1999 the recreational use of Lac has increased, new structures have been built around Lac and illegal conch fishing still goes on (conch fishing is forbidden by Marine Ordinance of 1991). In order to assess the effects of the moratorium and the impact of increased activity in and around Lac the 1999 survey has been repeated, applying the same methods. Fieldwork was carried out from the end of May 2007 through the beginning of October 2007.

Findings:

The main conclusion from this study is that the moratorium on conch fishing seems to be effective as Lac Bay’s conch population has increased; however, the cover of turtlegrass has diminished, which is most likely linked to an increase in anthropogenic disturbance of Lac.

• The conch population has increased: in the 51,000 m2 survey area, 223 live conch were found, against 111 in 1999.
• Adult conch were found (4% of the population) while in 1999 no adult conch were recorded.
• The cover of turtlegrass has diminished in most of the 17 grids (2,142 quadrats of 1 m2 ) studied; these grids were spread over Lac Bay. Grid Cai showed no reduction in cover.
• Trampling from windsurfers, snorkelers and other users of Lac Bay plays a major role in the reduction of turtlegrass.
• Cover by macro algae has changed but not necessarily diminished; in some grids the cover increased, in others it decreased.
• The number of most invertebrates has decreased.

Management Recommendations:

• Further study on Lac Bay’s species composition.
• Further study on Lac Bay’s water quality.

Abstract:

Queen Conch, Lobatus gigas, is a large, herbivorous, marine gastropod found primarily in the Western Atlantic Caribbean region. Adult conch range in length anywhere from 143-264 mm and are typically found in depths up to 25 meters, but are more often found between 10 – 18 meters. Due to its commercial importance and high market value, queen conch populations have dwindled to extremely low levels within its range. St. Eustatius Marine Park was established in 1996 and became actively managed in 1998 to conserve and protect the marine environment surrounding St. Eustatius up to and including the 30-meter (100 feet) depth contour and regulates the conch fishery on the island. Within the park, two no-take reserves are established on both the northern and southern ends of the island. The study revealed numerous sites that are ideal conch habitat from depths of 60-110 feet. Average density of conch was 0.043 conch/m2. A total of 86 individuals were included in the survey. The average length of conch was 22.1 cm with average lip thickness was 1.0cm. Individuals were on average 170.4 cm apart and were uniformly distributed. Density of Lobatus gigas in St. Eustatius is three times greater than densities reported for Turks and Caicos but drastically lower than densities reported for beds at similar depths in the Bahamas and Dominican Republic. The results of this survey suggest that Statia’s conch population may be very unique because the shallowest bed is located at 60 feet and conchs have been observed in depths up to 110 feet. Conch may be aggregating in certain areas for nutritional, reproductive or protective benefits. Typical locomotory and reproductive behaviour were observed during the study. Most conch surveyed met regulation size, which is encouraging because it means that juveniles are being left and allowed to reach sexual maturity thereby replenishing the population. Because catches are not reported it is not known if most conchs being collected are greater than 19cm (siphonal length) and have a fully-grown lip, which may mean immature individuals may be being removed prior to reproducing. There are numerous opportunities for further study on the L. gigas population in the St. Eustatius Marine Park including larval, behavioural and migration studies as well as collection of basic oceanographic data, which would have widespread applicability.