FRIEDL: PERFORMANCE OF ISAACS-KIDD TRAWLS 



Table 2. — Summary of macroplankton catch data used to compute the 10-ft IKMT cross-sectional area effective 

 in sampling macroplankton. Separate estimates were made for each month. The mean 10-ft IKMT effective area 

 is computed from the monthly estimates and is weighed according to the number of hauls made with the larger 

 trawl each month (see text). 



ing (0.32 m=; Table 1) and indicative of a sig- 

 nificant funneling by the forward section of the 

 trawl. 



The effective macroplankton sampling area of 

 the 6-ft IKMT corresponds to the area of the 

 opening of the intermediate section (1.3-cm 

 mesh liner) ; effects of funneling by the for- 

 ward section (7.6-cm mesh) of the trawl are 

 not obvious (Banse and Semon, 1963). My re- 

 sults indicate that the forward section of the 

 10-ft IKMT, with 3.8-cm mesh, is relatively more 

 important a factor in the ability of the net to 

 sample macroplankton than the corresponding 

 section of the smaller trawl. Given forward 

 sections of the same mesh for both trawls, how- 

 ever, the effective macroplankton sampling area 

 of the 6-ft IKMT would probably equal or ex- 

 ceed that of the 10-ft IKMT, for organisms of 

 the size considered here. The ratio of the ef- 

 fective cross-sectional area to total trawl mouth 

 area is 0.43 for the 6-ft IKMT and 0.23 for the 

 10-ft trawl. Thus, a larger percentage of the 

 water entering the mouth of the smaller trawl 

 is filtered for macroplankton. For this reason, 

 and because the 6-ft IKMT is generally easier 

 to handle and deploy, the smaller trawl would 



be preferred for studies with primary emphasis 

 on macroplankton or small fishes. 



The relative ability to each trawl to sample 

 fishes was also assessed. Total catch figures 

 are presented in Table 3. Numerically, Pacific 

 herring, Chipea harengus pallasi; bay gobies, 

 Lepidogobius lepidus; and plainfin midshipmen, 

 Porichthys notatus, dominated the overall catch. 

 Herring and gobies were common in hauls above 

 23 m while midshipmen were most abundant in 

 deeper tows. A few shiner perch, Cymatogaster 

 aggregata. Pacific cod, Gadus macrocephalus, 

 spiny dogfish, Sqjialus acanthias, and miscella- 

 neous flatfishes were also taken and are included 

 in the category "Others" in Table 3. Though 

 the total catch of the 10-ft IKMT exceeds that 

 of the 6-ft IKMT for each category of fishes, 

 the data are not directly comparable because 

 the larger trawl filtered more water in each 

 stratum. To equate catch data between trawls 

 on the basis of equal volumes filtered, the 10-ft 

 IKMT catch for each category of fish was 

 multiplied by the ratio of 6-ft to 10-ft IKMT 

 volumes filtered for each stratum (0.31 for hauls 

 above 23 m; 0.47 for deeper hauls. Exact vol- 

 umes given in Table 3) . The product, expressed 



Table 3. — Summary of fish catch data, by trawls, from hauls made in January and February 1967. Total catch fig- 

 ures are in whole numbers and concentrations of fishes (Individuals/1000 m^ of water filtered) are in parentheses. 

 Estimated 10-ft IKMT catch for volumes filtered equal to those of the 6-ft IKMT within each stratum is entered 

 as "Equivalent 10-ft catch" and is directly comparable to the 6-ft IKMT catch data (see text). 



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