FISHERY BULLETIN: VOL. 84, NO. 2 



Table 8.— Summary of estimates of school abundance (D), mean school biomass (MSB), and biomass (8) of widow 

 rockfish on the Halibut Hill ground during replicates on 26-27 March, 30 March, and 31 March-1 April 1982. 



Variations in the pattern of school abundance over 

 the course of a night were common. Echograms 

 recorded during the seven replicates of one transect 

 on the Halibut Hill ground (Fig. 11) illustrate one 

 case when abundance was high early in the night 

 and decreased toward dawn (26-27 March). The op- 

 posite trend of low abundance increasing toward 

 dawn is illustrated (31 March-1 April) in the same 

 figure. 



DISCUSSION 



The objectives of this 3-yr project were to study 

 the schooling behavior of widow rockfish to provide 

 the background needed to design effective abun- 

 dance estimating surveys; then to develop an appro- 

 priate survey methodology for the species; and, 

 finally, to test the feasibility of implementing such 

 a survey. Substantial progress was made toward 

 satisfying these objectives. The studies of widow 

 rockfish habits and distribution have provided a base 

 for designing surveys which cover its range and pro- 

 duce the best likelihood of encountering the ex- 

 ploitable population at a time when it will be most 

 available 



Understanding the schooling and dispersal be- 

 havior of widow rockfish was important to develop 

 an appropriate survey approach for estimating abun- 

 dance The nighttime aggregations which are the 

 targets of the commercial fishery tend to disperse 

 about daybreak, perhaps scattering throughout the 

 water column or seeking shelter near the bottom. 

 If the latter had been the case, more conventional 

 survey methods (i.e, bottom trawl or conventional 

 echo integration surveys) might have been more 

 appropriate 



Although daytime concentrations of widow rock- 

 fish were observed, bottom trawl catches during the 

 1980 and 1981 surveys showed that this species is 

 relatively unavailable to bottom trawls in an area 



where widow rockfish are known to aggregate at 

 night. 10 This is substantiated by low incidences of 

 widow rockfish in catches of other bottom trawl 

 surveys during periods when midwater trawlers were 

 making large landings. Consequently, when mid- 

 water schools disappear during the day, it is unlike- 

 ly that they disperse along the bottom. In recent 

 years, skippers of midwater trawlers have com- 

 mented that widow rockfish are becoming more 

 evasive and dive below their nets to avoid capture 

 Some skippers have taken advantage of this behavior 

 by purposely driving the schools toward bottom with 

 engine noise where they capture them with bottom 

 trawls equipped with roller gear. Although these are 

 classified as bottom trawl landings, the fishermen 

 are, in a sense, capturing midwater schools. Fisher- 

 men have also reported encountering daytime aggre- 

 gations of this species over the continental slope in 

 waters deeper than they are usually found at night 

 (>500 m) and some have been able to catch them on 

 or near the bottom during the day. Thus the distribu- 

 tion of widow rockfish relative to the sea bottom is 

 quite unpredictable during the daytime These 

 schools are also not as large as those that occur at 

 night. The appropriate time to survey this resource 

 thus appeared to be at night. The line transect survey 

 method, adapted for use with sector scanning sonar 

 and echo integration equipment, was chosen over 

 conventional echo integration and the line intercept 

 method because of its ability to survey areas more 

 quickly and thoroughly. 



Application of the method exposed several 

 problems affecting the precision and accuracy of the 

 abundance estimates. The estimation of school abun- 

 dance was hampered primarily by limitations of the 

 sonar equipment and by small samples. We were not 



'"Observations of midwater echosign and landing information 

 from commercial vessels fishing in the area confirmed that the 

 usual dense midwater widow rockfish aggregations were present 

 in the area at night during the 1981 bottom trawl survey. 



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