FISHERY BULLETIN: VOL. 69. NO. 2 



Table 1. — Dimensions and material specifications of the 

 Isaacs-Kidd midwater trawls compared in this study. 



dimensional data are presented in Table 1. Dur- 

 ing each haul, speeds were measured at the 

 surface with a Tsurami-Seiki-Koshakusho Co. 

 flowmeter' while trawls were at depth. The 

 same engine speed was used for all hauls. Gen- 

 erally, trawls were at depth 10 min and in the 

 water less than 15 min total. In January, sev- 

 eral 10-ft IKMT hauls were at depth 15 min. 

 Net depth was monitored on deck from signals 

 transmitted through the towing cable by a pres- 

 sure-activated sensing unit (designed and built 

 by the Department of Oceanography, University 

 of Washington) mounted above the trawl. A 

 Marine Advisers bathykymograph attached to 

 the trawl bridle was read after each haul to 

 check sampling depth. 



The sampling distance was calculated from 

 the speed and duration of each haul. This 

 distance was multiplied by the appropriate trawl 

 mouth area (Table 1) to determine the maxi- 

 mum volume of water filtered during each haul; 

 the volumes so determined were used to calcu- 

 late the monthly fish concentrations. For deter- 

 minations of zooplankton concentrations, how- 

 ever, use of filtered volumes based on trawl 

 mouth areas would result in concentrations in- 

 ordinately low (Banse and Semon, 1963). In- 

 stead, volumes filtered should be based on the 

 trawl cross-sectional area eff'ective in sampling 

 zooplankton of the size considered. The effective 

 cross-sectional area, as used here, may be de- 

 fined as that area which yields the correct zoo- 

 plankton concentration, as measured indepen- 



' The use of trade names is merely to facilitate de- 

 scriptions; no endorsement is implied. 



dently, when divided into the zooplankton catch 

 per unit distance of tow. Thus, if the length 

 of tow is known, the number of animals caught 

 per unit distance towed can be converted to 

 concentration if the efl'ective cross-sectional 

 area is known. Banse and Semon (1963) com- 

 pared euphausiid catches from a quantitative 

 high speed catcher with those of the 6-ft IKMT 

 and determined the effective cross-sectional area 

 of the trawl to be not significantly diflS'erent 

 from the area of the opening of the middle 

 (1.3-cm mesh liner) section of the trawl, namely 

 1.26 m- (Table 1). This efi"ective area, multi- 

 plied by the sampling distance for 6-ft IKMT 

 hauls, produced the effective volume of water 

 filtered by the smaller trawl. Macroplankton 

 concentrations were calculated for January and 

 February from total 6-ft IKMT catch and total 

 effective volumes filtered each month. Total 10- 

 ft IKMT macroplankton catch was divided by 

 the total sampling distance to determine the 

 monthly macroplankton catch per kilometer by 

 the larger trawl. 



RESULTS AND DISCUSSION 



Macroplankton samples from each trawl were 

 comjiared to determine the effective cross-sec- 

 tional area of the 10-ft IKMT. The mysid, 

 Neomysis kadiakensis, represented 80% of the 

 total catch; the mysid (AcantJiomysis macrop- 

 sis), the euphausiids {Enphausia pacifica and 

 Thysanoessa raschii) , and decapods of the genus 

 Crago made up most of the rest. Most of the 

 individuals were between 2 and 2.5 cm long. 

 Plankton concentrations were considerably re- 

 duced in February (Table 2) and sejiarate esti- 

 mates of the 10-ft IKMT effective cross-sec- 

 tional area were made for each month. Using 

 the method of Banse and Semon (1963), divi- 

 sion of the 10-ft IKMT catch per kilometer by 

 the catch per 1000 m-' filtered by the effective 

 cross-sectional area of the smaller trawl yielded 

 an estimate of the 10-ft IKMT effective cross- 

 sectional area for each month (Table 2). The 

 mean effective area, weighted according to the 

 number of 10-ft IKMT hauls made each month, 

 is 1.75 m- (Table 2) . This is con.siderably larger 

 than the area of the intermediate section open- 



428 



