Else et a\ Abundance of Sebastolobus alascanus in the Gulf of Alaska 



197 



due to gear avoidance or overestimation due to attraction 

 of fish to the submersible. The behavior of shortspine 

 thornyheads obsei^ed in the videotapes from our study 

 indicated that they were not affected by the submersible. 

 They typically remained relatively motionless unless the 

 submersible came very close (almost touching the fish). 

 It is also unlikely that they had moved away, or toward, 

 the submersible, before coming into the camera field. On 

 all dives there was an observer and another video camera 

 recording a broader area, and there was no indication 

 that shortspine thornyheads were responding to the sub- 

 marine. The passive behavior of shortspine thornyheads 

 in response to submersibles has been noted previously 

 (Krieger, 1992). 



In transect assessments, the probability of detecting or- 

 ganisms typically is not equal over the entire field of view. 

 Detection is affected by such factors as lighting, orienta- 

 tion of the organism and its reflectivity, sea floor relief. 



suspended particles, and size of the organism. Butler et 

 al.-^ examined the detection functions for three types of 

 fish (flatfish, hagfish, and thornyhead) from data collected 

 off California. For thornyheads, the probability of detec- 

 tion was relatively constant to a distance of about 180 cm. 

 This distance is larger than the width of the transects in 

 our study, indicating that our use of a constant detection 

 function (probability of 1.0) was appropriate. 



We found that shortspine thornyheads preferred habi- 

 tat with hard substrate. Submersible transects have been 

 used to identify habitat used by rockfishes and thorny- 

 heads in the northeast Pacific Ocean (Richards, 1986; 

 Pearcy et al., 1989; Stein et al., 1992; O'Connell and Car- 

 lile, 1993; Ki'ieger and Ito, 1999). Off Oregon, thornyheads 

 were included in an assemblage of fishes associated with 

 mud bottom (Stein et al., 1992), and Pearcy et al. (1989) 

 observed that in deep water they occurred on mud bot- 

 tom, but in shallower water were found over both rock and 

 mud. We have no explanation for the differences in hab- 

 itat association between Oregon and southeast Alaska. 



