! If 



Fishery Bulletin 94(1), 1996 



121)0 



□ Submersible 

 n Acoustic 



J-L 



Station 



Station 2 



Station 3 



Figure 3 



Comparison of submersible estimates offish density near 

 the bottom with acoustic estimates of fish density from 4 

 m above the bottom to the bottom for surveys conducted 

 from September through October 1991, off Newport, Oregon. 



very large schools of juvenile rockfish. The large 

 schools of juvenile rockfish observed at station 2 cre- 

 ated an anomalously high density estimate derived 

 from one submersible transect (Table 3). Similarly, 

 acoustic surveys showed few fish, except for occasional 

 large schools that were situated near the bottom ( Fig. 



5). Schools were on average 32.2 m (SE=1.8) wide, and 

 had an average vertical thickness of 3.8 m (SE=0.2). Of 

 the fish observed acoustically, 93% were located in the 

 bottom third of the water column (Fig. 4). 



Station 3 was located near the west edge of Stone- 

 wall Bank in primarily 50-60 m of water. The bot- 

 tom at station 3 contained more vertical relief than 

 other stations. It comprised numerous rock ridges 

 with 3-4 m high scarps. Submersible observers saw 

 small schools of unidentified juvenile rockfish, and 

 large schools of blue rockfish, yellowtail rockfish, 

 canary rockfish (Sebastes pinniger), and widow rock- 

 fish. Acoustic transects also recorded more fish at 

 station 3 than at other stations. Schools were on av- 

 erage 34.5 m (SE=1.1) wide, and had an average 

 thickness of 4.1 m (SE=0.2) (Fig. 5). Of the fish ob- 

 served acoustically, 79% were located in the bottom 

 third of the water column (Fig. 4). 



Target strength estimates 



Target strength estimated from Love's equation 

 ranged from -38.6 to -43.4 decibels IdB) (Table 2). 

 Target strength estimated from dual-beam analysis 

 ranged from -36.7 to -39.1 dB. Back-scattering cross 

 section estimated from dual-beam analysis were 1.1 

 and 1.0 times, respectively, that estimated from Love's 

 equation at stations 1 and 3 (Table 4). At station 2, 

 dual-beam methods provided a back-scattering cross 

 section that was 4.8 times higher than the back-scat- 

 tering cross section estimated from Love's equation. 



