Stanley et al.: Diel vertical migration by Sebastes flavtdus 



323 



The echo integrator was programmed to ana- 

 lyze the return echoes for a series of depth strata 

 (range slices) from the transducer to the bot- 

 tom (Kieser et al., 1987). Bottom tracking was 

 obtained with a 5-m bottom buffer An echo in- 

 tegration sequence was completed every 60 

 pings (1 minute), and the measured echo inten- 

 sities were stored on a personal computer. Echo 

 integrator and chart recorder thresholds were 

 set to 0.2 V; time-varied gain (TVG) was set to 

 20 log R. Thus all integrated echoes were dis- 

 played on the echogram. At this threshold level, 

 noise was negligible at all depths of interest. 

 Vessel speed was approximately nine knots. 



The integrator output was processed with cus- 

 tom software to exclude extraneous signals from 

 the ocean bottom, as well as noise and echoes from 

 other unwanted sources in the water column. 



A target strength (TS) of -32 dB/kg was used 

 to convert the measured backscatter cross sec- 

 tion to fish volume density (g/m'^) and fish sur- 

 face density tg/m'") estimates. This value was 

 obtained from a review of the literature (Foote, 

 1987; Kieser, 1992); no in situ rockfish TS were 

 available at that time. Only a relative TS was re- 

 quired because the study focused on the acoustic 

 availability of rockfish and relative rather than 

 absolute biomass and variance estimates. 



Biomass estimates were obtained by extrapo- 

 lating surface densities to cells that were 

 bounded by lines equidistant between adjacent 

 transects and by lines perpendicular to the 

 transect and passing midway between measure- 

 ments. An estimate of the total fish biomass for 

 the area was obtained by summing the prod- 

 ucts of surface densities and cell areas. 



B5-D 



1401 



B5-N 



2158 



B6-D 



1302 



B6-N 



2059 



B7-D 



1155 



B7-N 

 2002 



Figure 3 



Diurnal (D) and nocturnal (N) echograms from transects B.5, B6. 

 and B7 for November 17. "Sh" identifies the shallow end of both 

 long and short transects. "S" and "L" identify the deep ends of 

 the short and long sections of the transects. Time shown repre- 

 sents the approximate time the vessel passed over the cliffs. 



L S 



Fishing and sampling 



We conducted three bottom trawl tows (Table 1 ). Two 

 were conducted along the path of transect B5 and 

 the other along transect B6. All three targeted diur- 

 nal aggregations at the cliffs. The tows ran from shal- 

 low to deep and were terminated at the seaward side 

 of the cliffs, prior to encountering the aggregations 

 near the shelf break. Planned additional bottom and 

 midwater trawl fishing was curtailed owing to opera- 

 tional difficulties aboard the vessel. 



Analysis of variance 



Transect series 1, 2, 3, and 4 were conducted 17-18, 

 20-21, 22, and 24-25 November (Table 2). We ini- 

 ' tially tested for diel, transect, and series effects with 

 a three-factor ANOVA: 



where D 



,jk 

 I-' 



D,jf, =iJ + a,+lij+ 7^, + f , 



ijk' 



= surface density for transect /, day or 

 nighty, and observation (or series) k; 



= the overall mean; 



= the transect effect; 



= the diel effect. 



= the influence of the date (series); and 



/, = the error term, normally distributed 



with a mean of and variance of a'^. 



To test whether diel variances were heterogeneous, 

 we conducted a one-way randomization ANOVA lim- 

 iting the factors to the diurnal-nocturnal effect. We 

 conducted this test on a scaled version of the obser- 

 vations. Because it was obvious that the variance 



