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Marine Depth Shallow

This Strombus gigas study was conducted in the Cayos Cochinos Marine Reserve (Fig 3.1) centred on the Honduran island Cayos Menor, (GPS co-ordinates 15^o57'07.38”N, 86^o30'02.83”), in the marine laboratory of the Honduran Coral Reef Foundation, for 30 days, from July 18^th to August 20^th 2007.

Fig. 3.1 Map of Honduras, dark circle showing position of Cayos Cochinos Marine Reserve.

3.1 Distribution and population characteristics of Strombus gigas

Due to limitations enforced by safety regulations of the NGO Operation Wallacea (see appendix 1) dives were restricted to a maximum depth of 18m, for 50-minutes duration and to dive sites with a fixed buoy system already in place. This meant the method plan was changed from the intended true random system (see appendix 2) as it would have produced a majority of sites in excess of the maximum 18m depth. A system based on the shallow water areas of the marine park, on sites predetermined to comply with Operation Wallacea regulations, that expand on sites that previous conch specimens were found by Tewfik and Guzman (1997) and sites suggested by the local fishermen. 17 sites were chosen, ranging across the marine park area that was accessible by boat, all less than the 18 meter maximum depth, concentrated across the shallow water of the 13 coral cays. This provided a sufficient cross section for shallow water distribution and population characteristics, as the majority of areas outside the cays are in excess of 30m in depth and below the stated range of S. gigas.

Fig 3.2. Map of Cayos Cochinos Bay Islands showing predetermined dive sites A-Q, plotted on Google Earth 2008 application.

At each dive site A-Q plotted in Fig 3.2 above, the start point was recorded by taking a surface GPS co-ordinate (accurate to 30m) when moored to the buoy, shown visually in Fig 3.3.

Table 3.1. List of dive sites and corresponding GPS co-ordinates

Fig. 3.3. Map of GPS co-ordinate points taken, plotted in ArcGIS v9.1

Visual assessment of Strombus gigas was then conducted by 2 scuba divers, using 50m belt transects. Upon descending to the bottom surface, the transect was started at the mooring line, following a direction dictated and maintained by following the ocean floor/reef contour line to maintain concurrent depth. During each transect the 2 divers surveyed 3 meters each side of transect, logging initial and final depth and recording a dive profile every 60 seconds using a Sunnto Vyper dive computer. This belt method was adapted from methods appropriate in clear waters with irregular bottom types suggested by the 2 guides; The Mesoamerican Barrier Reef Systems Project Manual of Method for Monitoring (Almada-Villela et. al2003) and as outlined in the Queen Conch Stock Assessment and Management Workshop (Belize 2007), and based on methods used by similar studies (Tewfick & Guzman 1997, Berg).

Habitat description every 10 meters, was noted from the following 4 categories (taken from Tewfik and Guzman 1997) and the dominant type recorded for each transect: AP = ALGAL PLAIN substrate of various texture from fine mud to coarse sand dominated by green algae (Halimeda, Penicillus, Caulerpa, Udotea, Rhipocehalus), RF = REEF - variety of morphologies (heads, patch, fringing) and are dominated by large living colonies of stony corals of the head forming and branching types (Acropora, Montastraea, Diploria), Rk = ROCK - low relief, hard bottom and dead, eroded reefs dominated by soft corals, small solitary corals and sponges with small areas occasionally covered by a thin later of sediment where sea greases and green algae may form patches, SG = SEAGRASS MEADOWS - Turtle and Manatee grasses dominated on sediments of varying texture and depth.

During each transect, Strombus gigas specimens encountered were classified into maturity and age categories and observations on copulation and spawning, solitary egg masses were also noted.

Fig 3.4 Age categorisation (adapted from Tewfick and Guzman 1997).

The use of these categories allowed an estimation of age within the population structure, and allowed for statistical comparison of a definitive point to distinguish juveniles from adults.

Morphometric data of each of the S. gigas specimens' shells was taken by measuring the following dimensions using a measuring tape:

Shell length (SL) - tip of spire to siphonal canal, shell width (SW) - anterior base of spine extreme left to same extreme right & lip thickness - mid-lateral region on the lip side of shell - see Fig 3.4.

Fig. 3.4 Cross-section of the shell of adult Strombus gigas showing growth in shell and lip thickness. (A) Recently matured adult, lip thickness = 5 mm; (B) old adult, lip thickness = 27 mm. Arrows represent position where lip thickness is measured. (Appeldoorn 1988a)

As conch grow in length & width within the first 3 years (Berg 1976) and then continue into maturity only by thickening of lip, the lip measurement was the most important in dictating the adult categories. All S. gigas specimens encountered were photographed (top, inside exposed and lip thickness) to compile a photo library of individual conch specimens to eliminate repeat measures and facilitate opportunities for further studies of changes in shell, shape, & epibiotic fouling on tagged specimens. Other species of conch encountered on transect swims were also measured and recorded for species comparison.

3.2 Quantifying Daily Movement of Strombus gigas

No previous method or research has suggested a method for quantification of S. gigas movement, so the following exploratory method was developed:

Repeated for 5, 24 hour intervals, at sites A, E and K, the position of numerically tagged S. gigas specimens were marked by placing plastic tubing in sand adjacent to conch equal to visual centre of length. After 24 hours, accurate to within 15 minutes, distance travelled in a straight line^* was measured using a tape measure to the same central position on the conch, and, along with direction travelled taken by a compass, was recorded for each S. gigas specimen. The recorded distance between the two points was averaged over 5 days to produce an estimated quantification of S. gigas daily movement. At site A, which was a sand bowl, measurements were taken each day by sweeping the transect in a circle, using the mooring line as a fixed centre point, recording the distance and direction from the mooring line. Using a circular sweep enabled an accurate map of conch movement to be drawn using ArcGIS version 9.0.

Fig. 3.2 - Distance (A-B) and direction (C) from mooring line (centre) were recorded as position of S.gigas specimen.

3.3 Statistical Analysis

Multiple regression analysis using GenStat version 10.9 was used to detect significant differences in siphonal length, width, lip thickness, and estimated area of Strombus gigas, as well as for significant differences in morphometric measurements for habitat grouping and measured depths. Using binomial linear regression, exploring an estimated area required for development of lip, to continue investigating a reliable method of aging Strombus gigas specimens. Movement results for sites E & K was averaged from the 5 distances calculated. For site A, S. gigas position each day was plotted using ArcGIS distance-direction tool, then distance between the two plotted day points was measured using the measure tool and similarly averaged. Numbers of individuals recorded at each site and number of 300m² transects were used to estimate density distribution numbers per hectare at each site, then One-way ANOVA was used to test for significant differences between site transect means. Multiple regression techniques using GenStat version 10.9, tested for differences in abundances between species and depth/habitat strata. The Index of Dispersion (Krebs 1999) was used to determine the distributional pattern of Strombus gigas at each site to determine if conch were distributed randomly, uniformly, or aggregated in clumps over the ocean floor. The same analysis was used to analyse significant differences in habitat and depth selection between the other 4 species found.

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