FRIEDL: PERFORMANCE OF ISAACS-KIDD TRAWLS 



10-FT IKMT 



350 



250 



350 



LENGTH,MM-5MM GROUPS 



Figure 2. — Porichthys notatus length frequencies (SL) 

 from hauls made on monthly cruises from November 

 1966 through March 1967. Ten-ft IKMT catch from 

 five hauls between 25 and 35 m; volume filtered 57,400 

 m3. Six-ft IKMT catch from 12 hauls between 20 and 

 35 m; volume filtered 58,000 m^. Total catch (N) and 

 mean length (X) given on graphs for each trawl. 



data). Thus, discussion of the biolog-ical "uni- 

 verse" defined by samples from a given trawl 

 must acknowledge the limitations of the gear 

 employed and avoid conclusions beyond the 

 scope of the data available from the sampler. 

 In general, my results indicate the 10-ft IKMT 

 to be preferable to the 6-ft IKMT for biological 

 surveys emphasizing fishes in inshore waters, 

 provided the vessel employed is capable of han- 

 dling the large trawl on a regular basis. For sur- 

 veys of macroi)lankton, however, the 6-ft IKMT 

 is adequate and generally easier to deploy. The 

 larger mouth opening and overall finer mesh of 

 the forward section apparently enable the 

 10-ft IKMT to sample more fish, larger fish, 

 and more active fish better than the 6-ft trawl. 

 Fully lining the trawls with fine-mesh netting 

 (UNESCO, 1968) would help simplify analysis 

 of results by eliminating the need for estimating 

 effective sampling cross-sectional areas when 

 calculating concentrations of small fishes and 

 macroplankton. Such lining would probably 

 also increase the trawls' ability to sample smaller 

 fishes (Backus et al., 1970), but the overall ca- 

 pabilities of the trawls to sample large or active 

 forms would likely change little and utilization 

 of the 10-ft IKMT would still be recommended. 



a more valid basis for comparison of difl'erent 

 samples, but limitations of resources and equip- 

 ment often determine the manner and means 

 by which samples are taken. In this study, 

 for instance, the strain of towing the 10-ft IKMT 

 severely taxed the running rigging of the re- 

 search vessel and prevented the use of the larger 

 trawl on some cruises. At best, results of stud- 

 ies, such as this and that of Aron and Collard 

 (1969), illuminate the limitations and capabil- 

 ities of sampling gear, characteristics which 

 must be recognized even with widely employed 

 equipment such as the 6-ft IKMT. That trawls 

 may sample only a limited portion of the fauna 

 present is obvious and must be recognized. For 

 instance, recent work in Puget Sound with large 

 trawls indicates concentrations of herring, de- 

 termined by IKMT hauls in winter sonic scat- 

 tering layers, may be at least two orders of 

 magnitude low (T. S. English, unpublished 



ACKNOWLEDGMENTS 



This project was conducted in partial fulfill- 

 ment of the requirements for a degree of Master 

 of Science in the Department of Oceanography, 

 University of Washington, Seattle, Wash., with 

 the guidance of Dr. T. Saunders English. The 

 efforts of the crew of R/V Hoh are gratefully 

 acknowledged as is the critical review of the 

 manuscript by Dr. Eric G. Barham, Naval 

 Undersea Research and Development Center, 

 San Diego, Calif. Final preparation supported 

 by the U.S. Naval Ship Systems Command under 

 SR 104 03 01 Task No. 0588. 



LITERATURE CITED 



Aron, W. 



1959. Midwater trawling studies in the North Pa- 

 cific. Limnol. Oceanogr. 4: 409-418. 



431 



