isms or P. setiferus. Significantly different residual 

 variances were found for S. empusa and Trachy- 

 penaeus sp. (Table 4). 



The 12.2 m trawl caught more individuals and 

 more species than the 4.3 m trawl (Table 5). The 

 large trawl caught 30,000 organisms during the day 

 and 46,000 during the night. The small trawl caught 

 3,000 during the day and 3,800 during the night. The 

 large trawl caught 99 species during the day and 

 107 during the night, while the small trawl caught 

 63 species during the day and 82 during the night. 

 The trend of more species caught in the large trawl 

 was apparent for vertebrates both day and night and 

 invertebrates during the day. The same number of 

 invertebrate species were caught at night in both 

 trawls. Species caught exclusively in one trawl were 

 usually represented by fewer than 30 individuals 

 during the entire study. 



Only 26 of 125 species were represented by a 

 mean catch ^5/tow in either trawl (Table 6). These 

 26 species comprised 95% of the total catch. 



small trawl it was 0.03. The fishery manager must 

 decide if an increase in species diversity helps 

 manage a particular fishery and ultimately whether 

 it is cost effective to go after these "rare" in- 

 dividuals. 



Catch in the large trawl may be higher than in the 

 small trawl because of higher efficiency. Kjelson and 

 Johnson (1978) reported higher catch efficiencies for 

 a 6.1 m trawl than for a 3.0 m or 4.6 m trawl. Loesch 

 et al. (1976) reported 5% efficiency for Leiostomus 

 xanthurus in a 4.0 m trawl while Kjelson and John- 

 son (1978) reported 32% for the same species in a 

 6.1 m trawl. 



The relationship between trawl width and catch 

 may be asymptotic. This study showed the 12.2 m 

 trawl caught more organisms than the 4.3 m trawl. 

 Cody and Fuls (1985) found the same trend but 

 reported that the catch in the 12.2 m trawl was not 

 significantly less than the catch in the 13.7 m trawl. 

 Matthews (1982) found no difference in mean total 

 weight caught in 12.2 m and 13.7 m trawls. He did 



Table 4.— Summary of ANCOVA for catch per tow of total organisms and selected species. 



"P< 0.01. 



Discussion 



Catches in the large trawl were consistently 

 higher than in the small trawl. Chittenden and Van 

 Engel (1972) stated there must be some relationship 

 between catch and tow duration because of the 

 amount of bottom sampled, but they found that in- 

 creased tow duration (which increases area covered) 

 did not significantly increase the catch of blue crabs 

 in a 9.1 m trawl. However, they tested only a small 

 range of tow durations (5-15 min) and concluded that 

 variation in the trawl catches was a significant fac- 

 tor. Tow duration in this study was constant, so 

 higher catches were most likely a result of more area 

 being sampled by the larger net. 



It also seems reasonable that a large trawl would 

 have a greater chance of encountering organisms 

 especially if they have patchy distributions such 

 as found with shrimp (Matthews 1982). The large 

 net caught more species than the small net in this 

 study. The highest mean catch per tow was 0.37 for 

 species found exclusively in the large trawl; for the 



not, however, compare the total number or size of 

 organisms. Because of the inherent variation found 

 in sampling with trawls, the inability to detect differ- 

 ences in the 12.2 m and 13.7 m trawls would be 

 expected. 



Implications of this may apply to the commercial 

 trawl fishery. Through the years shrimp fishermen 

 have been reducing the size of trawls and increas- 

 ing the number of trawls used in order to increase 

 catch efficiency (Christmas and Etzold 1977). These 

 changes may reflect the asymptotic relationship of 

 trawl width and at the same time help reduce un- 

 wanted by catch. 



Cody and Fuls (1985) reported a regression coef- 

 ficient of 2.52 for the catch vs. catch relationship 

 for P. setiferus in daytime samples in contrast to 

 5.37 for this study. Only 13 data points over a much 

 wider range of X i (0-136/tow vs. 0-55/tow) were 

 used by Cody and Fuls. When the ranges of X { 

 were made comparable the slopes of the two regres- 

 sions were not significantly different. 



The use of small trawls and determination of rela- 



988 



