Stoner and Waite Habitat associations of Strombus gigss within seagrass meadows 



575 



SO transect 1 (the southernmost transect) was moved 

 between transects 2 and 3 for the February samphng. 

 Measurements, including station depth, conch density 

 and shell lengths, macrophyte shoot density and 

 biomass, and amount of macroscopic detritus, were 

 taken at each station along the transects within a cir- 

 cle of 2.5 m radius around each stake. Sediment grain- 

 size and organic content were measured in July 1988. 



Depths were measured at low water using an elec- 

 tronic depth sounder on a 17-foot Boston Whaler, ad- 

 justing for the depth of the transducer below the sur- 

 face. Using scuba, conch were gathered within each 

 2.5-m circle at each station, counted, and measured for 

 shell length using large calipers. 



A separate collection of 80 conch ranging from 17 

 to 196 mm in siphonal length was made to generate 

 a length- weight curve. After freezing, the conch were 

 extracted from their shells, washed to remove feces, 

 blotted to remove excess water, and weighed. The 

 following equation describes the significant correlation 

 (r2 = 0.987, F = 5233.4, /XO.OOl) between shell length 

 and wet weight: 



logio (wet weight) = 3.403 x logi„ (length) - 5.569 



The equation was used to convert length-frequency 

 data to biomass values. 



Two replicate samples of macrophytes and macro- 

 scopic detritus (mostly senescent seagrass blades and 

 debris) were collected at each station by haphazardly 

 placing a quadrat with 25-cm sides within each circle. 

 The number of shoots within the quadrat were counted 

 and then collected along with the macrophyte detritus 

 into nylon bags (3.0-mm mesh). Thalassia blades (using 

 only aboveground parts), and macrodetritus were 

 separated in the laboratory and dried at 80°C for ap- 

 proximately 24 hours to constant weight. Dry-weight 

 biomass was determined for the individual components. 



A core of 40 mm diameter, with a penetration depth 

 of 5 cm, was taken for determination of sediment grain- 

 size distribution and organic content. A subsample of 

 approximately 100 g wet weight was dried at 80°C to 

 constant mass and incinerated at 550°C for 4 hours to 

 determine sediment organic content. Organic content 

 was quantified as the percent difference between dry 

 weight and ash-free dry weight. Another sediment sub- 

 sample of approximately 50 g was used to determine 

 sediment grain-size. After washing to remove salts and 

 to extract the silt-clay fraction, sand-sized particles 

 were analyzed using standard Ro-Tap procedures. Silt- 



Reference to trade names does not imjily endorsement by tlie 

 National Marine Fisheries Service, NOAA. 



clay fractions were analyzed using standard pipet pro- 

 cedures (Folk 1966). Product moment statistics were 

 generated for mean grain size and sortedness (McBride 

 1971). 



Habitat preference experiments 



Experimental manipulations of plots of seagrasses 

 were made at the Shark Rock site to test for habitat 

 preference in conch of various size classes. Cages 3.6 

 m diameter (10.2 m^) were constructed of black plastic 

 mesh with 20-mm openings. The cages were 25 cm high 

 and supported by 1-m long pieces of reinforcement bar 

 driven into the sediment. The bottom edge of the 

 screen was pushed into the sediment approximately 3 

 cm to prevent escape of conch. No losses occurred dur- 

 ing the experiments. 



Four cages were built in locations similar to station 

 5 (see above), characterized by Thalassia shoot den- 

 sities of 700 shoots/m- (Moderate Density). Two cages 

 were built near stations 3 with approximately 300 

 shoots/m- (Low Density), and two cages were built in 

 high biomass habitats similar to station 7 with approx- 

 imately 750 shoots/m- plus a heavy layer of detritus 

 (High Density). Macrophytes in two of the moderate- 

 density cages were manipulated so that one-half of each 

 was reduced to 300 shoots/m^ (Low Density) (Table 2). 

 This was accomplished by placing a rope across the 

 cage through the center and parallel to the direction 

 of flood tide current. (Manipulations were oriented with 

 the current because juvenile conch at the study site are 

 known to move either up or down current during 

 migration.) Then shoots were systematically pulled 

 from one-half of the cage to achieve the desired shoot 

 density. The two cages for this Moderate/Low treat- 

 ment were manipulated to provide mirror images of 

 one another. The other two moderate-density plots 

 were manipulated in a similar fashion to provide 

 Moderate/Sand treatments, where all seagrass shoots 

 were removed from one side of each cage. Replicate 

 Low/Sand treatments were constructed by manipu- 

 lating the two low-density plots. High/Moderate treat- 

 ments were set up by manipulating the high-density 

 plots. About 80% of the loose detritus (comprised most- 

 ly of senescent seagrass blades) was removed from half 

 of the high-density plots. Final shoot densities in the 

 high- and moderate-density areas were similar (656 and 

 672 shoots/m^, respectively), and removal of detritus 

 yielded a macrophyte environment similar to the 

 moderate-density areas (Table 2). The manipulations 

 required maintenance such as removing new growth 

 and accumulated detritus. Except for the removal of 

 detritus which accumulated along the edges of the 

 cages, maintenance of the plots was performed be- 

 tween the short habitat preference runs. 



