FISHERY BULLETIN: VOL, 69. NO. 2 



to the nearest whole number, is entered as 

 "Equivalent 10-ft catch" in Table 3 and is di- 

 rectly comparable to the catch figures for the 

 6-ft IKMT in the row above it. Figures for the 

 overall fish concentrations, from data on total 

 catch and total volume filtered, are also given 

 in Table 3. 



When the catch data are compared on an equal 

 volume filtered basis, the superior sampling abil- 

 ity of the 10-ft IKMT is evident. With the ex- 

 ception of Porichthys in the lower stratum, the 

 10-ft IKMT caught more fish of each category 

 than did the 6-ft trawl and the resulting overall 

 concentrations estimated by the 10-ft IKMT are 

 likewise higher (Table 3). Pacific herring, a 

 major component of the mid-depth sonic scat- 

 tering layer in Port Orchard (Cooney, 1967; 

 Friedl, 1970), were not taken by the 6-ft IKMT 

 and apparently were capable of actively avoiding 

 the smaller trawl. The less active, Porichthys, 

 however, was sampled equally by the trawls in 

 the lower stratum. Though the catch and con- 

 centrations of Lepidogobius appear much lower 

 for the 6-ft IKMT in Table 3, the discrepancy 

 may reflect gear selection and loss through the 

 7.6-cm mesh of the 6-ft IKMT forward section 

 more than active avoidance of the trawl by the 

 fish. The Lepidogobius captured were small, 

 35 to 50 mm SL, and probably were filtered only 

 by the 1.3-cm mesh liner of the 6-ft IKMT inter- 

 mediate section. Assuming all gobies taken by 

 the 6-ft IKMT were filtered by the intermediate 

 section only, the concentrations above and be- 

 low 23 m would be 0.12 and 0.11 fish per 1000 m^ 

 respectively, and would more nearly appro.xi- 

 mate those of the larger trawl (Table 3). Thus, 

 as with the macroplankton, the finer mesh of 

 the forward section of the 10-ft IKMT enhanced 

 the ability of the trawl to sample small organ- 

 isms. 



The 10-ft IKMT caught more fish and sampled 

 active fish better than the 6-ft IKMT despite 

 the fact it was generally towed at lower speeds 

 (Table 4). The towing speed of the smaller 

 trawl, though only slightly greater than that 

 of the 10-ft IKMT, could have increased pres- 

 sure waves and vibrations associated with the 

 trawl and evoked greater avoidance responses 



Table 4.— Net speed data for 6- and 10-foot IKMT hauls 

 in January and February 1967. Speeds measured at the 

 surface while the trawls were at depth. Average speeds 

 are significantly different at the 99% level when com- 

 pared with Student's t distribution (Simpson, Roe, and 

 Lewontin, 1960). 



Hauls 

 (no.) 



Trawling speed (m/sec) 



Rcngg 



Average 



Confidence 

 interval (95%) 



10-fr 

 6-ft 



1.64 to 2.13 

 1.99 to 2.25 



1.64 

 2.14 



1.78 to 1.90 

 2.09 to 2.19 



in active fishes such as herring (Chapman, 1964; 

 Harrisson, 1967). At present, knowledge of 

 the pressure and vibration characteristics of 

 IKMT underway is lacking and further con- 

 clusions regarding the influence of such char- 

 acteristics on the sampling abilities of trawls 

 would be speculative and beyond the scope of 

 this paper. 



Comparison of the length frequencies of Por- 

 ichthys taken in deeper tows during the entire 

 winter period (November - March) indicates 

 a selection for larger fish by the larger trawl, 

 despite its slower towing speeds and finer mesh 

 (Figure 2). Nearly 70% of the Porichthys 

 taken between 20 and 35 m by the 6-ft IKMT 

 in the winter were less than 150 mm long (SL), 

 while half the Porichthys taken by the 10-ft 

 IKMT between 25 and 35 m in the same period 

 exceeded 180 mm SL (Figure 2). 



My results indicate the 10-ft IKMT catches 

 more fish, more active fish, and larger fish than 

 the 6-ft IKMT used in the study; similar con- 

 clusions, with respect to the ability of small 

 and large trawls to samjile mesopelagic fishes, 

 were reached by Harrisson (1967). Aron and 

 Collard (1969) studied the influence of net speed 

 on catch for a 6-ft IKMT fully lined with 1.2-cm 

 mesh netting and found that faster tows took 

 larger fish of certain types off the California 

 coast. My data, for inshore fishes, indicate 

 mouth size, towing speed, net mesh size, and, 

 perhaps, the dynamic characteristics of trawls 

 combined with the behavioral aspects of the or- 

 ganisms sami)led are all interrelated in a com- 

 plex way to ultimately determine the sampling 

 ability of a given trawl. 



Standardization of gear and techniques used 

 to sample midwater organisms would provide 



430 



