LEBER AND GREENING: COMMUNITY STUDIES IN SEAGRASS MEADOWS 



150 450 750 



Decapods 



10 



Molluscs Echinoderms 



NUMBER OF INDIVIDUALS 



Fishes 



Figure 2— Rarefaction curves for crab scrape (closed circles) and trawl data (open circles) from 1979 night samples. Expected numbers 

 of species ( ± 2 SD) are plotted against numbers of individuals. Length of curves indicates maximum number of individuals taken in any 

 single tow. 



Hence, with only one exception (July fish abundance), 

 the otter trawl never outperformed the scrape dur- 

 ing daylight collections. 



The trawl was more effective in sampling a tax- 

 onomic group other than fish in only one case Sig- 

 nificantly more decapod individuals were taken in 

 July trawl samples at night, reflecting high densities 

 of two caridean shrimps, Tozeuma carolinense and 

 Periclimenes longicaudatus, which appear to be 

 more susceptable to night trawl, rather than scrape, 

 sampling. However, decapod abundances were 

 notably higher in July daytime collections made with 

 the crab scrape (see Figure 1), thus the highly sig- 

 nificant interaction term for the July analysis 

 (decapod individuals, Table 2). 



Relative Abundance 



Numerical rankings of the most abundant or- 

 ganisms in each taxonomic group (combined over all 

 sample dates) taken in night scrape samples are com- 

 pared with those from night trawl samples in Table 



3. Clear discrepancies exist between scrape and trawl 

 estimates of species ranks and relative abundances. 

 Relative to scrape samples, trawl collections over- 

 estimated the degree of dominance (DI = combined 

 proportions of the two most abundant species, {n x 

 + n 2 )/N, McNaughton 1967) contributed by the 

 most abundant shrimp Tozeuma carolinense and 

 demersal fish Gobiosoma robustum, and under- 

 estimated dominance of the most important crab 

 Pagurus maclaughlinae and mollusc Argopectin ir- 

 radians in our samples (Table 3). Relative to trawl 

 collections, the scrape underestimated dominance 

 for the most abundant water-column fishes, Lagodon 

 rhomboides and Bairdiella chrysura. Species ranks 

 of subdominants in trawl samples also differed from 

 rankings based on data from scrape samples. 



DISCUSSION 



Scrape-trawl and day-night differences in sampling 

 effectiveness were conspicuous and generally con- 

 stant throughout the year. Although more (by a fac- 



447 



