270 



Fishery Bulletin 97(2), 1999 



None Occasional Scattered Common Abundant 



Boulder abundance 



Figure 3 



Fwe levels of boulder abundance observed on the seafloor, and 

 the percent occurrence of habitat and percent occurrence of 

 shortraker and rougheye rockfish associated with each level. 

 Observations are from a submersible in the eastern Gulf of Alaska, 

 1992. 



their above-bottom distributions, grouping behavior, 

 and use of boulders. 



The 150 rockfish observed above bottom were at 

 nine different dive sites and all were descending, 

 indicating they were reacting to the submersible. 

 They were probably seeking the seafloor in response 

 to the submersible. A diving response to trawl and 

 vessel disturbances has been noted for other species 

 of offshore rockfish (Kieser et al., 1992). The protec- 

 tion provided by the seafloor may explain their re- 

 luctance to move when they are on the seafloor and 

 approached by the submersible. Assuming rockfish 

 were descending in response to the submersible, the 

 proportion of rockfish observed above bottom is prob- 

 ably a minimum estimate because some had prob- 

 ably reached the seafloor before they were viewed 

 from the submersible. The 80 rockfish above bottom 

 at site 5 indicate that a high percentage of rockfish 

 move above the seafloor, although the frequency and 

 duration of their movements are unknown. Rockfish 

 may move above bottom to capture prey such as the 

 squid and lantern fish (Myctophidae) on which they 

 are known to feed (Yang, 1993, 1996). About two- 

 thirds of the rockfish were in groups of 2-6 fish; only 

 two groups contained more than 12 fish. The reason 

 for the close pairing of a small and medium rockfish 

 is unknown; it is probably not related to mating be- 

 cause female shortraker and rougheye rockfish shorter 

 than 30 cm are not mature (McDermott, 1994). 



The spatial distribution of rockfish varied within 

 dive sites. This variability can be partially explained 



by their grouping behavior and by their habitat as- 

 sociations. Rockfish were associated with most of the 

 habitats encountered, but the greatest densities were 

 associated with soft substrates, frequent boulders, 

 and slopes greater than 20°. Their association with 

 soft substrates may be prey related. Pandalid shrimp 

 and hippolytid shrimp, which concentrate on soft 

 substrates, were the main prey of rougheye rockfish 

 examined from the Gulf of Alaska (Yang, 1993, 1996) 

 and from the Aleutian Islands (Yang, 1996). The as- 

 sociation of rockfish with boulders in this study and 

 on the continental shelf ( Krieger, 1992 ) indicates that 

 boulders are important for these species. Perhaps 

 boulders are a necessary habitat feature, because 

 shortraker and rougheye rockfish were absent at the 

 one site without boulders in this study and at the 

 three sites without boulders in a previous study 

 (Krieger, 1992). These species may use boulders as 

 territorial markers, to avoid currents, or to capture 

 prey. Rockfish were least abundant on shallow-slope 

 habitat (<5°) in this study, and Krieger (1992) ob- 

 served shortraker rockfish at three sites where the slope 

 was 3-12 " but none at six sites with slopes less than 

 2°. Steep slopes may provide relief fi-om currents. 



The mean observation rate from the submersible 

 was about twice the mean catch rate from bottom- 

 trawl surveys, probably because of the limited habi- 

 tats sampled during trawl surveys. Bottom trawling 

 may be effective for sampling shortraker and 

 rougheye rockfish on low-relief habitats because 

 these rockfish descended and remained on the sea- 



