328 



Fishery Bulletin 97(2), 1999 



from Aglen's formula whereas the variance for the 

 combinecd area of B5 and B6 was higher (Table 8). In 

 both cases, the observed variance among surveys was 

 typical of acoustic surveys. 



Discussion 



We have assumed that the dominant part of the 

 acoustic signal selected from the short transects rep- 

 resented yellowtail rockfish, a conclusion supported 

 by fishing results and the continuity on the echo- 

 grams between diurnal and nocturnal distribution. The 

 longer sections of the transects, which indicated about 

 16% (5-30%) more total biomass, probably included 

 other species of deeper-dwelling rockfishes. 



Yellowtail rockfish share with other species a diel 

 pattern of diurnal schooling and nocturnal dispersal 

 (Simmonds et al., 1992). We observed that the schools 

 reform at dawn near a minor but nearly continuous 

 topographic feature that crosses the study area on a 

 north-south axis. The strong and persistent affilia- 

 tion of yellowtail rockfish with this small cliff is con- 

 sistent with long-term commercial fishing records at 

 this location. The diurnal schools are near the bot- 

 tom at dawn then rise slowly during the day, dis- 

 persing at dusk. This is congruent with fishermen's 

 observations that early morning is the most produc- 

 tive time for bottom trawling for this species. The 

 fidelity to topographic features is consistent with 

 reports of Pearcy (1992) and Carlson and Haight 

 ( 1972). Pearcy's reports on acoustically tagged speci- 

 mens includes an example of "homing" near to the 

 original capture sites, even as far as 2 nmi overnight. 



Because we conducted the study at only one time 

 of year, we could not examine how diel behavior would 

 vary with season. Fishermen report that yellowtail 

 rockfish occupy the "cliffs" at all times of year. They 

 also report that the dawn tows are the best fishing 

 on a year-round basis. We would suggest that if diel 

 vertical movement is triggered by or at least associ- 

 ated with light intensity at depth, then the effect would 

 be more obvious during summer months when light 



intensity increases and the twilight period shortens. It 

 might also be more dramatic on brighter days. 



The diversity of species, especially near the shelf 

 edge, illustrates that a difficult issue facing acoustic 

 assessment of rockfish will be species identification. 

 This process will be complicated by the difficulty of 

 trawling much of the habitat. Our study was based 

 on single-beam and single-frequency acoustic obser- 

 vations. Encouraging results on species identifica- 

 tion have been obtained by more advanced acoustic 

 analysis of individual fish schools (Kieser and 

 Langford, 1991; Scalabrin and Masse, 1993), com- 

 parison of day and night survey data ( Gerlotto, 1993 ), 

 and multifrequency and wide band observations 

 (Simmonds and Armstrong, 1990; Zakharia, 1990). 

 In this context, innovative transducer design and 

 deployment will be important to obtain more detailed 

 acoustic data from individual fish and schools near 

 the bottom.'^ Supplemental information on habitat 

 and depth preference by species, perhaps developed 

 from simultaneous use of submersible devices or side- 

 scan bathymetry, may help to estimate species' pro- 

 portions (Richards et al., 1991). 



Diurnal versus nocturnal density and biomass 

 estimates 



Although the echograms showed differences in diel 

 distribution for yellowtail rockfish, they did not in- 

 dicate the extreme densities which lead to acoustic 

 shadowing (Foote, 1990). In contrast to other stud- 

 ies (summarized in Appenzeller and Leggett, 1992), 

 which report higher density estimates from night 

 obsei-vations, our day and night estimates were simi- 

 lar. We conclude that, for yellowtail rockfish, diel be- 

 havior patterns do not bias hydroacoustic biomass 

 estimates. It remains possible, however, that the 

 similarity between diel periods is purely fortuitous 

 in that the various factors that could affect estima- 

 tion, such as movement in and out of the study area 



- Dalen, J., and H. Bodholt. 1991. Deep towed vehicle for fish 

 abundance estimation, concept and testing. ICES Council 

 Meeting (CM) 1991/B:53. 13 p. [Mimeo.l 



