556 
Fishery Bulletin 95(3), 1 997 
and with the interaction of these terms with the con- 
trol net number, with tow as the experimental unit. 
Length-frequency distributions of abundant organ- 
isms collected by the control nets in each area were 
visually examined. 
IBRD's versus control nets Catches of shrimp, fish, 
and the nine most abundant species were analyzed. 
An AN OVA model was used to compare the differ- 
ence between control and BRD net catches between 
areas and seasons. ANOVA was also used to detect 
differences caused by towing a BRD on the port or 
starboard side of the twin trawl. 
The number of individuals and biomass of each 
species (or group) caught in a BRD net was compared 
with the number caught by the control net by using 
a univariate paired t-test. The univariate procedure 
is appropriate if one can assume that the probability 
of one species being retained within the net is inde- 
pendent of another species being retained. The short 
tow duration contributes to the chance that this as- 
sumption is valid. Paired Ctests were conducted on 
untransformed and log-transformed differences be- 
tween BRD- and control-net catches. Percent catch 
differences of untransformed data were calculated 
to compare device nets: 
Percentage catch Device net - Control net 
= x 100. 
difference Control net 
Percent catch difference values could range from 
-100 to infinity. 
Differences between areas and seasons 
A univariate paired t-test was also used to compare 
differences between a BRD net and the control net 
within each area. This test had less power because 
the sample size was reduced by two-thirds and be- 
cause the test was not able to detect as small a dif- 
ference as the test with the areas combined. A simi- 
lar analysis was conducted to examine device per- 
formance in each season. 
For all analyses, differences between means with an 
alpha of 0.05 or less were considered significant. How- 
ever, the exact probabilities are presented in the tables. 
Results 
Control nets 
The control nets collected 88 species of fishes and 
invertebrates; fewer species were collected in the 
spring than in the fall in all areas (Table 1). More 
than 64% of the 84,919 organisms collected in the 
control nets were caught during the spring. Nine 
species represented nearly 89% of the total control- 
net catch. Bay anchovy, white shrimp, and hardhead 
catfish catches were higher in the fall, but the other 
Table 1 
Numbers of most abundant organisms collected in the control nets in inshore waters of Louisiana during the spring and fall of 
1992. 
Spring 
Fall 
Area Area 
Combined 
Species 
Borgne 
Bar re 
Calcasieu 
Total 
Borgne 
Barre 
Calcasieu 
Total 
total 
Brown shrimp 
Penaeus aztecus 
2,842 
5,375 
10,593 
18,810 
400 
481 
245 
1,126 
19,936 
Atlantic croaker 
Micropogonias undulatus 
597 
8,346 
5,162 
14,105 
240 
207 
2,015 
2,462 
16,567 
Bay anchovy 
Anchoa mitchilli 
712 
976 
1,106 
2,794 
1,011 
2,387 
2,166 
5,564 
8,358 
White shrimp 
Penaeus setiferus 
47 
51 
732 
830 
1,954 
3,425 
1,752 
7,131 
7,961 
Hardhead catfish 
Arius felis 
423 
363 
2,638 
3,424 
1,087 
1,432 
1,864 
4,383 
7,807 
Spot 
Leiostomus xanthurus 
846 
2,059 
1,603 
4,508 
180 
161 
339 
680 
5,188 
Sand seatrout 
Cynoscion arenarius 
353 
196 
2,097 
2,646 
153 
689 
408 
1,250 
3,896 
Blue crab 
Callinectes sapidus 
149 
1,610 
154 
1,913 
111 
907 
13 
1,031 
2,944 
Gulf menhaden 
Brevoortia patronus 
86 
278 
1,533 
1,897 
54 
68 
623 
745 
2,642 
Other species 
273 
2,108 
1,152 
3,533 
1,263 
2,273 
2,551 
6,087 
9,620 
Total 
6,328 
21,362 
26,770 
54,460 
6,453 
12,030 
11,976 
30,459 
84,919 
Number of species 
38 
51 
53 
66 
47 
65 
57 
82 
88 
