Stoner and Sandt: Habitat quality for Strombus gigas in seagrass meadows 



695 



Table 1 



Habitat characteristics of eight sites where juvenile queen conch were transplanted. Depth is the mean at low water. Values for sediments 

 are mean ± standard deviation (n 2). Asterisks and plus signs indicate mean values which are not different statistically (ANOVA and 

 Neuman-Keuls test; P<0.05). 



Site 



C3 



Nl 

 L3 

 Wl 



CI 

 C2 

 LI 



L2 



Resident 

 conch 



No 



Yes 



No 



No 



Yes 



No 

 No 



No 



Seagrass 

 biomass 



Zero 



Low 

 Low 

 Low 



Moderate 

 Moderate 

 Moderate 



High 



Depth Sediment grain-size 



(m) (+) 



1.3 



2.3 

 2.2 

 2.0 



3.6 

 3.7 

 3.4 



3.7 



1.50 + 0.02* 



1.52±0.10* 

 2.02±0.30* 

 1.82 ±0.20* 



2.56±0.29* 



1.15 + 0.11* 

 2.26 + 0.23* 



2.40 + 0.19* 



Sediment sorting 



(40 



0.84±0.10* 



1.14±0.06* 

 1.08 ±0.20* 

 1.47±0.28* 



1.39 ±0.22* 

 1.62±0.19* 

 1.40±0.23* 



1.04±0.07* 



Sediment organics 

 (%) 



2.38±0.33* 



2.68±0.14* 

 3.66±0.33* 

 3.83±0.47* 



3.76 ±0.33* 

 2.80 ±0.33* 

 2.92 ±0.32* 



5.36±0.48 



cages were 30cm high, 5m in diameter, and pushed into 

 the sediment approximately 2.0cm to prevent loss of 

 animals. Exact positions of the cages were chosen to 

 provide uniformity in macrophyte cover at each site, 

 and to insure that sites selected for similar character- 

 istics (CI, C2, LI and Nl, L3, Wl) had equivalent 

 seagrass and detrital cover as well as sediment qual- 

 ity (Table 1, Fig. 2). 



Previous experiments with the same cage design at 

 Children's Bay Cay site 1 showed that the cages did 

 not effect sediment grain-size, sediment chlorophyll, 

 or accumulation of detritus. Enclosed animals had 

 growth rates equivalent to individuals tagged and 

 released in the field surrounding the enclosures (Stoner 

 1989a). 



Animals used in this experiment were 1 -year-old 

 Strombus gigas collected from the sand bank near 

 Children's Bay Cay. At the beginning of the experi- 

 ment, all of the conch were between 82 and 105 mm 

 total shell length. Mean lengths in individual treat- 

 ments were not different at the beginning of the ex- 

 periment (ANOVA, F 1.90, P>0.05), ranging from 92.0 

 to 94.3mm. After measuring habitat characteristics 

 (see below) and clearing all noticeable macroinverte- 

 brates from the 16 enclosures, 24 individually tagged 

 and measured conch were placed in each cage (1.2 

 conch/m 2 ), yielding a density equivalent to mean sum- 

 mer population density at CI and Nl. Dead or lost 

 conch were replaced to maintain the population den- 

 sity within cages (Table 2). Replacements were of a size 

 similar to the mean conch size in a particular treatment 

 at the replacement time. Conch were marked with vinyl 

 spaghetti tags (Floy Tag & Mfg., Inc.) tied around the 

 shell spire. 



All transplants were made by 26 April 1988, and 

 measurements of total shell length (spire to siphonal 



canal) were taken with calipers at 35, 75, and 120 days. 

 Growth rate was determined on the basis of mm shell 

 growth per day. Each enclosure was examined at ap- 

 proximately 2-week intervals to determine mortality 

 rates over time, to replace dead conch, and to remove 

 invading invertebrates. One of the cages at C3 was 

 destroyed by wave action in June 1988, and all of the 

 enclosed animals were lost. This cage was rebuilt and 

 new animals were introduced on 23 June. 



At the end of the experiment, soft tissue weight of 

 individual conch was determined by drawing the animal 

 from its shell after freezing and subsequent thawing. 

 Wet weight was measured after washing away feces 

 and light blotting of the tissues. Body condition was 

 determined by the ratio of wet weight:shell length. 



Living macrophytes and macroscopic detritus were 

 collected from each enclosure at the beginning and end 

 of the experiment. Four replicates were taken from 

 25 x 25cm quadrats into nylon bags (3.0mm mesh) for 

 determination of aboveground biomass. Individual 

 samples were divided into green Thalassia testudinum 

 blades and detritus (senescent blades and blade 

 fragments). The only other macrophytes collected were 

 the seagrasses Syringodium filiforme and Halodule 

 wrightii found in very small amounts, and an occasional 

 calcareous green alga Rhipocephalus phoenix. The 

 aboveground fractions were dried at 80 °C to constant 

 weight. Rhipocephalus phoenix was not included in 

 analysis of macrophytes because of the large bias 

 created in dry weight and because the alga is not con- 

 sumed by juvenile conch. 



Sediments to 5cm depth were sampled with a 3.5cm 

 diameter core tube, one sample per enclosure. These 

 samples were frozen until laboratory analyses were 

 performed. Sediment organic content was determined 

 by drying a subsample of approximately 50 g wet 



