plankton during 1970. The objective of this cruise 

 was to identify those organisms associated with 

 sound scattering layers in the upper 150 m of 

 seawater and, if possible, to follow their day- 

 night migration patterns. Six successive inte- 

 grated tows of approximately 45 min each were 

 taken to a depth of 150 m (total time: 1852-2355). 

 A standard 6-foot (1.8-m) Isaacs-Kidd mid- water 

 trawl (IKMT) with a 2.9 m^ mouth opening [1%- 

 inch (3.8-cm) mesh with a V4-inch (0.6-cm) linear 

 nylon liner] was used for this series. The second 

 series of eight samples alternately sampled from 

 surface to 150 m and from 150 m to surface with 

 an eight-bar electronic multiple plankton sam- 

 pler (EMPS) attached to the IKMT (Pearcy and 

 Mesecar 1970) (total time: 0134-0514). The 

 cylinder-cone nets were approximately 2.9 m in 

 length with a mouth diameter of 0.4 m and made 

 of 0.57 1-mm nylon mesh. Another series also 

 used the EMPS to sample eight discrete layers 

 from the surface to 330 m depth covering three 

 bands of scatters (total time: 0640-1113). Scatter- 

 ing layers were located using 12 and 38.5 kHz 

 echo sounders. One automated STD cast was 

 made. 



Processing of Plankton Samples 



Samples from both mesh sizes of the 0.2-m 

 bongo nets were processed for the nearshore area, 

 stations NHOI-NHIO. Only one side of the 0.7-m 

 bongo net sampler was processed to examine the 

 offshore area, NH15-NH60. Generally, the entire 

 sample was sorted, however, many required sub- 

 sampling using an 8-cm diameter plankton split- 

 ter (Longhurst and Seiburt 1967). Approximately 

 22% of the 0.2-m bongo net samples and 39% of 

 the 0.7-m bongo net samples required subsam- 

 pling. Those samples which required splitting 

 were usually from stations NHOl and NH03. 



All crab larvae were removed from the samples 

 and positive identification of C. magister larvae 

 was made from the descriptions given by Poole 

 (1966) and from preserved specimens reared by 

 Thomas F. Gaumer, Fish Commission of Oregon, 

 Marine Laboratory, Newport, Oreg. Catches of 

 larvae were first converted by computer to 

 number per 1,000 m^ of seawater and ordered in a 

 format. Graphs of stage density against time 

 were plotted for the 0.2-m bongo net samples, 

 0.57 1-mm mesh, with the aid of a CalComp plot- 

 ter using the Oregon State University CDC3300 

 computer. 



FISHERY BULLETIN: VOL. 74, NO. 2 



SAMPLING VARIABILITY 



The detailed analyses of the various methods 

 by which sampling variability affected the esti- 

 mates of larval crab abundance are given by 

 Lough (1975b). Variability estimates and sam- 

 pler comparisons were made in this study on 

 other species of crab larvae than C. magister for 

 the most part, as limited ship time and weather 

 played an important role in determining the ob- 

 jectives and priorities of the sampling program. 

 Analysis of variance techniques were used to es- 

 timate the variance of a single observation in the 

 manner of Winsor and Clarke (1940). Confidence 

 limits for a single observation of either sampler 

 usually exceeded the 50-200% range reported by 

 Winsor and Clarke due to the relatively low den- 

 sities of crab larvae sampled during replicate 

 tows. A range of an order of magnitude was con- 

 sidered necessary to distinguish a real difference 

 between any two observations. There was no sig- 

 nificant difference between the total number and 

 kinds of crab larvae caught by the two sides of the 

 different sized samplers. The 0.7-m bongo net 

 sampler gave smaller confidence limits for larval 

 crab catches and was much superior in establish- 

 ing significant differences between stations than 

 the 0.2-m bongo net sampler. 



Most of the nearshore samples (NHOl-NHlO) 

 were taken during daylight hours; only 8.6% of 

 the 0.2-m bongo net samples were taken at night 

 between 1800 and 0600 h. More (26.7%) of the 

 0.7-m bongo net samples sorted beyond NHIO 

 were collected at night. Most larvae were caught 

 more abundantly in night tows than day tows for 

 both sized samplers. Day-night differences in lar- 

 val abundance were greater for the 0.2-m sampler 

 than the 0.7-m sampler. There was a nearly equal 

 distribution in the number of kinds of crab larvae 

 caught between day and night samples using the 

 0.7-m sampler; however, using the 0.2-m sampler, 

 significantly more kinds of larvae were caught at 

 night. 



The results of the Nekton Cruise showed that 

 the larvae of C. magister occur in relatively low 

 densities offshore as far as station NH45 during 

 early April 1970. They are most likely to occur in 

 the surface waters above 120 m, the depth of the 

 thermo- and halocline and are probably as- 

 sociated with the first sound scattering layer at 

 25 to 90 m depth. A Mann- Whitney two sample 

 rank test (Tate and Clelland 1957) retained the 

 null hypothesis that there was no significant dif- 



356 



