873 



Abstract.— Direct observation of a 

 6-hook model setline off Canada's 

 northwest coast with an underwater 

 camera allowed estimation of approach 

 direction, attack rate, and hooking suc- 

 cess for Pacific halibut iHippoglossiis 

 stenolepis). In observations of 129 hali- 

 but approaches, 29 halibut simply con- 

 tinued swimming past the gear and 

 away. The remaining 100 halibut re- 

 sponded in some way to the gear, ex- 

 hibiting behaviors ranging from a 

 simple change in swimming direction 

 toward the gear, to bites on the baited 

 hooks. Behavior transitions are de- 

 scribed both leading up to and follow- 

 ing 57 observed bites. Halibut dis- 

 played a positive rheotactic response; 

 74'''f approached upstream toward the 

 bait. A higher proportion of upstream 

 approaches resulted in bites than ap- 

 proaches from other directions or 

 approaches that occurred during slack 

 current. Most bites were associated 

 with a vigorous rushing behavior, 

 which often led to hooking. Hooking 

 success was demonstrated to be length- 

 dependent, ranging from zero for the 

 smallest fish to approaching one for the 

 largest sizes caught. Very low rates of 

 hooking success were observed for some 

 rockfish, which in some areas are im- 

 portant bycatch species of the halibut 

 fishery. 



Direct observations on the hooking behavior 

 of Pacific halibut, Hippoglossus stenolepis 



Stephen M. Kaimmer 



International Pacific Halibut Commission 



PO. Box 95009 



Seattle, Washington 98145 



E-mail address stevek g iphc Washington edu 



Manuscript accepted 1 December 1998. 

 Fish. Bull. 97: 873-883 ( 1999). 



The International Pacific Halibut 

 Commission (IPHC) sets harvest 

 quotas for the stock of Pacific hali- 

 but (Hippoglossus stenolepis ) off the 

 Pacific shores of the United States 

 and Canada. An important tool in 

 setting these quotas is the catch per 

 unit of effort (CPUE) both of the di- 

 rected commercial fisheries of these 

 countries and of setline surveys con- 

 ducted annually by the IPHC. The 

 trends in these CPUE indexes, both 

 among areas and over years, are 

 important to the IPHC stock-assess- 

 ment model. Over the years, the 

 IPHC has conducted fishing experi- 

 ments in order to understand dif- 

 ferent aspects of CPUE. A selection 

 curve for the size 16/0 J-style hooks 

 used in the fishery prior to 1984, dem- 

 onstrated by Myhre (1969), ranged 

 from zero for fish around 40 cm, to 

 one for fish around 80 cm. Tlie mecha- 

 nism for size selection in halibut has 

 not been demonstrated. Skud ( 1978) 

 reported that halibut was a dominant 

 predator, probably more successful 

 than other species in competing for 

 setline baits, and concluded that al- 

 though other fish and invertebrates, 

 even if not caught, could cause bait 

 loss, competition by these species was 

 unlikely to affect the CPUE of hali- 

 but. Later studies with circle hooks 

 (Hoag et al., 1984; Kaimmer and St- 

 Pierre, 1993) demonstrated effective 

 competition by a decreased catcha- 

 bility of halibut associated with the 

 presence of large numbers of spiny 

 dogfish (Squaliis aca?ithias). 



Attack rate (bites/fish) and hook- 

 ing success ( hookings/bite ) by fish 



length are important parameters of 

 most hook-catch models ( Deriso and 

 Parma, 1987), although these pa- 

 rameters often must be estimated 

 from either a theoretical model or 

 observations of captive fish. This 

 paper presents results from an 

 IPHC study designed to estimate 

 these parameters directly from ob- 

 servations of halibut approaches 

 and of bites on the circle hooks used 

 in the present-day fishery. These 

 behavioral studies conducted in the 

 field, rather than through observa- 

 tion of captive fish, should yield re- 

 sults most applicable to and repre- 

 sentative of setline catches of wild 

 fish (Lokkeborg et al., 1993). 



Materials and methods 



Vessel and area 



A 12-day experiment was conducted 

 11-20 June 1994 from a 15.5-m char- 

 tered vessel. An underwater camera 

 permitted a view of a model setline 

 consisting of six baited fishing hooks. 

 The gear was placed at various hali- 

 but habitats in the Queen Charlotte 

 Islands of British Columbia, Canada. 

 This region was chosen to provide an 

 opportunity to view a variety of hali- 

 but habitats, in terms of depth and 

 bottom type, with a likelihood of rela- 

 tively good water clarity. The mid- 

 summer timing allowed for the pres- 

 ence of halibut on their midyear feed- 

 ing grounds, long working days, the 

 opportunity of fair weather, and a 

 window between expected plankton 



