Research and Monitoring Results for the Size Class Distribution and Abundance of the Queen Conch, Strombus gigas, and Seagrass Characterization in Lac Bay, Bonaire
This report provides a comprehensive coverage of accomplishments completed to date by Environics for the Lac Bay queen conch (Stombus gigas, known locally as “karko”) monitoring project, which was completed over the period June - December 1999. The consultant activities were proposed to the Bonaire Marine Park and VOMIL in the Terms of Reference (May 1999, Appendix 1) to respond to the management needs listed in the Bonaire Marine Park Terms of Reference LAC001/98, by investigating the following research questions:
• What is the current health and status of the bay area and surrounding environs, and how has it changed from the past to present, as determined through the interpretation of aerial photographs, scientific research and local historical knowledge?
• What is the status of the globally endangered species, the queen conch (Strombus gigas), which inhabit Lac Bay?
• How is the bay currently being used by marine life, wildlife, and by humans?
• Are there indications of threats on the bay’s natural carrying capacity?
For this report, the following activities were conducted to provide baseline information about the status of the conch population and also to describe the current health and status of the greater bay environs, through the process of the following:
• assessing change from the past to present with historical interpretation through prior literature search;
• conducting rapid ecological assessments of the bay benthos, including queen conch population status, seagrass abundance and epiphyte density, and
• detecting potential threats of the conch in the Lac Bay environs.
Field monitoring to determine the conch population size class distribution, in addition to characterization of the seagrass beds, were completed in seventeen 200 x 200 meter grid sites in the bay. Each grid contained three 5-m belt transects, to assess a total of 3,000-m2 area per grid. The completed ecological assessment would result over coverage of 51,000-m2 area. The sites were non-randomly selected for ground truthing extensive coverage from aerial photographs. Each transect start point was placed with compass rose headings, followed by assigning latitude/longitude coordinates on a line map of Lac Bay (grid map - Map 2). In the field, the grids and transect lines were located using a Garmin GPS unit to conduct the monitoring activities with trained volunteers. Rapid Ecologicial Assessment protocols were adapted from The Nature Conservancy Caribbean Program (1996), and the Southwest Florida Water Management District (1998) seagrass monitoring methods (adapted in the BMP volunteer monitoring draft manual in Westmacott, 1999). This project was completed through the scientific support of trained volunteers from the Bonaire Underwater Research Assistants (BONURA) group. Following mastery training skills, the resultant protocol and species identification training were field tested in Lac with the volunteers during June 1999; routine surveys were underway in July 1999. Monitoring activities were completed at the end of September 1999. GIS map products of the survey results are currently being completed at Duke University, through the co-operation of the Dept. of the Environment, with graduate student Phyllis Dermer, under the supervision of Dr. Patrick Halpin. Draft maps are contained in this report.
The final map products will include a GIS zonation map, with management recommendations for the greater bay area. In brief, the statistical results indicate that for both conch and invertebrate species in general, the spatial distribution patterns are clumped or uniform, implying potential constraints on the system. For conch, fishing pressure may be likely, as the map results show the remnant conch population is located in the deeper bay channel or boat routes, that have least potential for anthropogenic accessibility. The conch mean size class is approximately 17.8-cm, or approximately 2.5 years; no adults were found during this assessment period. In the 51,000-m2 bay area surveyed, a total of 111 conch were found, averaging only 7.4 conch per 3,000-m2 grid area. Surveys were conducted during the normal breeding period when adult conch usually migrate into shallow sandy areas to mate (personal observation, 1987, 1991; Stoner and Waite, 1990). Anecdotal reports in Bonaire indicate that adult conch remain on the deep reef sandy terraces to mate, at approximately 50-70-m depths, likey due to Bonaire’s limited shallow water bank systems available for breeding purposes (personal communication, Newton, 1999).
A main resource for conch larval recruitment into Lac Bay may generate from upstream sea current sources that are located in the neighboring Aves Island and Los Roques archipelago region. During October 9–13, 1999, Environics visited Los Roques to observe the area’s conch fishery management program, and its implications on conch fisheries for Bonaire. Approximately 6 years ago, the Los Roques archipelago also realized a near complete collapse of conch. They immediately initiated a “no-take” moratorium to manage the remaining stock, with strict consequences to poachers. Environics observed an impressive recovery, with many adult size conch concentrated in the shallow (<3-m) seagrass banks of Los Roques. In the Los Roques conch sanctuary areas, there were reported an estimated 0.42 individuals per m2 in unfished areas (Weil and Laughlin, 1984). (Compare the Los Roques estimation to the Lac Bay estimation of 0.0021 individuals per m2). Los Roques Inparques marine management program is anxious to network with Bonaire, to share its program as a “peer demonstration model” with Bonaire the benefit from the Los Roques experience, to save the Lac Bay conch population. Historical research conducted by Hummelinck and Roos (1969) described the discarded conch piles at Cai as changed very little since Hummelinck’s prior study period that was conducted during the early 1930’s. The reference inferred that the conch fishing pressures had been impacted approximately since this time, and the result of the conch distribution patterns are similar to this present day GIS map depiction for conch distribution - that remaining conch are found only in areas of least access by fishers.
The results of this study, and the historical research by Hummelinck and Roos (1969), lead to the conclusion that conch populations once were, and can be, viable in Lac Bay. In addition, according to conch size class and habitat viabilitiy studies (Stoner and Waite, 1990, 1991; Weil and Laughlin, 1984), Lac bay’s abiotic conditions (bottom depths, temperatures), as well as its biotic features (seagrass habitat abundance, epiphyte density, short shoot abundance), are suitable to sustain conch stock. However, the results from this survey activity indicates that few, or no conch were found in the greater seagrass areas, but, as found
in 1969, stock are still distributed in areas that may be least accessible to fishing pressure – in the deeper boat channels, or in shallower, high energy wave zones. However, moderate numbers of young conch (less than the mean 2.5 year size class found in this study) do persist in some areas of Lac, but are quickly taken out. Evidence shows the removal of young recruits due to fishing pressure, by the discarded piles of recent takes of brightly pink small (<17.5-cm) cracked conch shells found in the bushes near the shores of Sorobon.
Adult conch reach reproductive size during the 3 to 5 year size class (Berg, 1975). Therefore, a 6 year moratorium is recommended to support the Lac Bay conch population recovery, modeled after the Los Roques conch recovery program. Public education and outreach programs, supplemented with peer visits from the Los Roques park management team to describe their conch recovery program, is recommended, that would drive the local Bonairean community into supporting the Lac management program. After analyzing the GIS mapping results for seagrass habitat abundance, etc., a zonation plan of the bay is recommended to demarcate a sanctuary preservation area for viable conch and other wildlife habitats, as well as to designate mulit-purpose recreational use areas. Thereafter, a fisheries management program could be implemented, enforced through issuance of permits and size class catch limits is recommended, that would sustain conch population and commercial fisheries, as well as for local family consumption.