193 



Abstract— Short.spino thoniyhead (St-- 

 Ixi.stolohii.-i ulaacanus) abundance was es- 

 timated from 107 video transects at 27 

 stations recorded from a research sub- 

 mersible in 1991 off southeast Alaska at 

 depths rantrinf; from 165 to 355 m. Num- 

 bers of invertebrates in seven major taxa 

 were estimated, as was substrate type. 

 Thornyhead abundance ranged from 

 to 7.5/100 m-, with a mean of 1.22/100 

 m'-, and was positively correlated with 

 depth and amount of hard substrate. 

 Invertebrate abundances were not sig- 

 nificantly correlated with numbers of 

 thornyheads. Shortspine thornyhead 

 abundance estimates from this study 

 were several times higher than esti- 

 mates produced by bottom trawl sur- 

 veys off southeast Alaska in 1990 and 

 1993, the two years of survey that 

 encompassed the submersible tran- 

 sects: however, the trend of increasing 

 abundance with depth was similar in 

 the trawl surveys and in the submers- 

 ible transects, suggesting that trawl 

 surveys systematically underestimate 

 abundance of shortspine thornyheads. 



Shortspine thornyhead iSebastolobus alascanus) 

 abundance and habitat associations 

 in the Gulf of Alaska 



Page Else 



Lewis Haldorson 



School of Fisheries and Ocean Sciences 



University of Alaska 



11120 Glacier Highway 



Juneau, Alaska 99801 



E-mail address (for L Haldorson, coniaci author) lewhaldorsoniwuafedu 



Kenneth Krieger 



Auke Bay Laboratory 



National Manne Fishenes Service 



Juneau, Alaska 99802 



Manuscript accepted 24 August 2001. 

 Fish. Bull 100:193-199 (2002). 



Fish distributions are affected by physi- 

 cal conditions such as depth, tempera- 

 ture and substrate type and by biotic 

 variables such as prey distribution, pred- 

 ator presence, and habitat features (e.g. 

 kelp forests). Patterns of habitat use 

 are important factors in resource assess- 

 ments, and stratification based on habi- 

 tat characteristics are common features 

 of survey design. For example, trawl sur- 

 veys in the Gulf of Alaska are stratified 

 by depth and general bottom type (flats, 

 gullies, shelf break, slope) (Stark and 

 Clausen, 1995). Assessment offish popu- 

 lations based on traditional fishing gear, 

 such as trawls or longlines, provide rel- 

 atively economical surveys with broad 

 geographic coverage; however, they pro- 

 vide limited detailed information on spe- 

 cies associations or habitat preferences 

 (Matlock etal., 1991). 



Shortspine thornyhead (Sebastolobus 

 alascanus) is a member of the family 

 Scorpaenidae, which includes the rock- 

 fishes {Sebastes — over 60 species in the 

 northeast Pacific) and three species of 

 thornyheads {Sebastolobus). Shortspine 

 thornyhead range from Baja California, 

 Mexico, to the Bering Sea and are found 

 at depths to 1500 m (Moser, 1974). Off 

 southeast Alaska, most fish sampled 

 from 200-310 m depths were between 

 15 and 30 years old and had lengths 

 of 25-35 cm (Miller, 1985). The maxi- 

 mum age observed by Miller ( 1985 ) was 

 62 years, and age at 509!^ maturity was 

 12 years for both sexes. These life-histo- 



ry features are similar to those of rock- 

 fishes, and such long-lived fishes are 

 difficult to manage because they are 

 easily overexploited and recover slowly 

 from overfishing (Adams, 1980). Short- 

 spine thornyhead have been commer- 

 cially valuable in the Gulf of Alaska, 

 where catch (including discards) ranged 

 from 1298 to 2020 metric tons from 

 1991 tol996 (lanelli and ItoM. Bycatch 

 of thornyhead has the potential of forc- 

 ing closure of other high-value fisher- 

 ies (lanelli and Ito^). Estimates of their 

 abundance and size distribution are 

 currently based on bottom trawl and 

 longline surveys (lanelli and Ito^); how- 

 ever, direct observations from submers- 

 ibles can provide an alternate method 

 for assessing abundance ( Krieger, 1993 ). 

 Assessments of thornyheads and rock- 

 fishes often result in population esti- 

 mates with high variances; resulting in 

 considerable uncertainty for assigning 



' lanelli.J. N.andD. H. Ito. 1998. Status 

 of Gulf of Alaska thornyheads iSebastolo- 

 biis sp. ) in 1998. In Stock assessment and 

 fishery evaluation report for the ground- 

 fish resources of the Gulf of Alaska, p. 

 371-402. [Available from North Pacific 

 Fishery Management Council, 605 West 

 4'*' St., Anchorage, Alaska 99501.] 



- lanelli,.:. N., and D.H. Ito. 1994. Thorny- 

 heads. In Stock assessment and fishery 

 evaluation report for the gi-oundfish re- 

 sources of the Gulf of Alaska for 1995. 

 (Available from North Pacific Fishery Man- 

 agement Council. 605 4"' St., Anchorage, 

 Alaska 99501.1 



