Starr et al.: Submersible-survey and acoustic-survey estimates of fish density 



Table 2 



Mean length (cm) of fishes observed from the submersible (sub), target strength (TS in dB) calculated from mean length of 

 observed fishes, and target strengths acoustically measured at Stonewall Bank, Oregon, from September through October 1991. 



' Estimated by using Love's equation (Love. 1971). 



2 Calculated by inserting dual-beam TS into Love's equation I Love, 197 1 1. 



3 Mean length offish estimated from the submersible at station 2 is 11.8 cm if juvenile rockfish from dive 2604 are included in the analysis. 

 J TS equals -^13. 4 dB if juvenile rockfish from dive 2604 are included in the analysis. 



Fish densities estimated acoustically in the "above 

 sub" stratum were on average 15.6 times higher than 

 those in the "below sub" stratum (Table 3). However, 

 submersible surveys indicated that fish density near 

 the bottom was greater than that indicated by acous- 

 tic measurements. For the stratum near the bottom, 

 submersible estimates of areal fish density were sig- 

 nificantly greater than acoustic estimates of fish 

 density (Wilcoxon paired sample test, ra=9, 2=-2.67, 

 P=0.008). Fish density estimated from submersible 

 operations was higher in each case than correspond- 

 ing acoustic estimates (Fig. 3); fish density estimated 

 from submersible transects averaged 6.7 times higher 

 than acoustic density estimates (Table 3). 



Fish distribution 



At all three stations on Stonewall Bank, more than 

 75% of the fishes insonified on acoustic surveys were 



located in the lower third of the water column (Fig. 

 4). There was no significant difference in depth dis- 

 tribution of fishes between stations (Kruskal Wallis 

 test, df=2, A=0.089, P=0.96). Station 1 was located 

 on the top of Stonewall Bank. The bottom at station 

 1 was relatively flat bedrock in primarily 50-55 m of 

 water; it contained low relief and few fish. Submers- 

 ible and acoustic surveys at station 1 exhibited the 

 lowest estimate of fish density generated from the 

 cruise (Table 3). Schools were sparsely distributed 

 (Fig. 5), averaged 11.1 m (SE=1.6) wide, and had a 

 mean vertical thickness of 2.6 m (SE=0.1). Of the 

 fish observed acoustically, 78% were located in the 

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



Station 2 was located near the north edge of Stone- 

 wall Bank, primarily in 60-70 m of water. The bot- 

 tom at station 2 contained smooth ridges of rock, with 

 occasional scarps 2-3 m high. Submersible observ- 

 ers at station 2 saw few fish, except for occasional 



