THE RELATIVE SAMPLING PERFORMANCE OF 6- AND 

 10-FOOT ISAACS-KIDD MIDWATER TRAWLS 



William A. Friedl' 



ABSTRACT 



The relative abilities of 6- and 10-ft Isaacs-Kidd midwater trawls (IKMT) to sample macroplankton and 

 fishes were assessed from comparable hauls taken with graded mesh nets during January and February 

 1967, in central Puget Sound. The plankton catch, mostly individuals 2 to 2.5 cm long, was dominated 

 by the mysid, Neomysis kadiakensis. To quantify zooplankton data from the larger trawl, its cross-sec- 

 tional area effective in filtering macroplankton was estimated for each month. The mean eflfective cross- 

 sectional area of the 10-ft IKMT is 1.75 m^. This implies a significant funneling of macroplankton by 

 the forward section of the trawl. 



The fishes taken were dominated numerically by Pacific herring, Clupea harengus pallasi; bay gobies, 

 Lepidogobius lepidus; and plainfin midshipmen, Porichthya notatus. Herring was not taken by the 6-ft 

 trawl; there was little apparent difference in the ability of the two trawls to capture midshipmen 

 and gobies. Overall, the 10-ft IKMT caught more fish, more active fish, and larger fish than the 6-ft 

 trawl. Though the 6-ft IKMT is probably adequate for studies with an emphasis on macroplankton, use 

 of the 10-ft IKMT to sample fishes in inshore waters is preferable. 



Interpretation of net haul data depends upon 

 the capabihties and limitations of the sampling 

 gear employed. With the plethora of equipment 

 presently available for sampling the larger 

 plankton and smaller nekton, comparative infor- 

 mation on the relative sampling abilities of dif- 

 ferent gear is needed to equate results obtained 

 with different nets. 



This report deals with the relative sampling 

 abilities of two sizes of Isaacs-Kidd midwater 

 trawl (IKMT), a type of net widely employed 

 in marine and freshwater investigations. The 

 results apply to IKMT in general ; the assess- 

 ment elucidates the degree to which data ob- 

 tained with different ti'awls are comparable. 



METHODS AND MATERIALS 



Samples were taken from the University of 

 Washington 65-ft research vessel Hoh at night 

 along a N-S track in Port Orchard, a narrow 

 channel west of Bainbridge Island in central 

 Puget Sound with a maximum depth of slightly 

 over 40 m. Comparable IKMT hauls were made 



' Marine Biology Branch Code 5042, Naval Undersea 

 Research and Development Center, San Diego, Calif. 

 92132. 



on two cruises in January and February 1967. 

 Length frequency data are from five cruises 

 made each month from November 1966 through 

 March 1967. 



Two sizes of IKMT with graded mesh nets 

 were compared. The mouth area of the 10-ft 

 IKMT (Figure 1) is 7.68 m^ and that of the 6-ft 

 IKMT (Aron, 1959) is 2.94 m^. Mesh sizes 

 for the various sections of the trawls and other 



COLLECTINS BUCKET. 10cm DIAMETER. 33 mm SCREEN 



COD END- l/B in. (0 32cml MESH KNOTLESS NYLON 



mJERMEOiATE SECTION: 



I Ve in. (2 9 cm ) MESH NYlON PANELS 



1/2 in (13 cm) MESH KNOTLESS NYLON LINER 



FORHaRD SECTION: 

 1 1/2 in 13.8 cm 1 MESH 

 NYLON PANELS 



Manuscript received January 1971. 



FISHERY BULLETIN: VOL. 69, NO. 2, 1971. 



Figure 1. — Dimensions and construction details of the 

 10-ft Isaacs-Kidd midwater trawl used in this study 

 (after Cooney, 1967). 



427 



