WILKINS: ABUNDANCE OF WIDOW ROCKFISH 



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3dB ?°° 



Half-power point 



roc 



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Figure 13— The theoretical directivity pattern of one transducer element of the sector scanning sonar show- 

 ing side lobes which may interfere with the signals received by adjacent transducers. 



a more proportional measurement of school length 

 than the echo sounder-based lengths. The accuracy 

 of school dimension measurements could be im- 

 proved by using more sensitive and specialized sonar 

 equipment. 



These problems with the limitations of sector scan- 

 ning sonar should not be difficult to overcome More 

 sensitive quantitative sonar equipment is now avail- 

 able or relatively easy to develop. Lateral resolution 

 may remain a problem because of the difficulty and 

 expense of producing narrow-beam transducers, but 

 the errors it causes are relatively unimportant. 



The accuracy of mean school biomass estimates 

 would be improved by target strength studies specific 

 to widow rockfish. Calculation of average fish den- 

 sity within each school was relatively straight- 

 forward but involved assuming a target strength of 

 -35 dB/kg. Ideally, the target strength should be 

 calculated specifically for widow rockfish but such 

 specialized work was beyond the scope of this 

 study. 



The ability to distinguish widow rockfish schools 

 from those of other species using hydroacoustic 

 equipment is an important element of this tech- 

 nique Through these studies, our ability to correct- 

 ly identify widow rockfish echo sign has been im- 

 proved. The accuracy of species identification varies 

 depending on the nature of the species complex in 



the survey area. Where shortbelly and redstripe 

 rockfish are present, the potential for misidentifica- 

 tion increases. Technological improvements in sonar 

 equipment may help to reduce this problem. The 

 density of a school is an important criterion for 

 distinguishing widow rockfish from other species and 

 newer sonar equipment includes density-graded color 

 video displays. Other techniques, such as underwater 

 photography or remote video camera vehicles, might 

 also improve our ability to identify species. I believe, 

 however, that test fishing will always be a necessary 

 component of hydroacoustic resource assessment 

 surveys. 



Surveys of widow rockfish resources must be 

 designed with the behavior and distributional vari- 

 ability of the species in mind. The diel behavior of 

 the species indicates that the most effective sampling 

 period is at night, but even then unpredictable be- 

 havior places special demands on survey design. 

 Observations from hydroacoustic transects which 

 were replicated on several nights (Fig. 11) show that 

 long-term variability in abundance (eg, night-to- 

 night or week-to-week) is even more marked than 

 that over a shorter time These results are substan- 

 tiated by other surveys (see footnotes 5, 7, and 8) 

 and illustrate the difficulty of estimating widow 

 rockfish abundance Long-term variability is also a 

 factor in area-swept bottom trawl surveys. The 



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