Kaimmer: Hooking behavior of Hippoglossus stenolepis 



factor in attack rate, although approach direction was 

 highly significant; fish that approached the bait up- 

 stream bit almost twice as often as fish that ap- 

 proached from the side or downstream, and four 

 times as often as fish that approached during slack 

 current. It is likely that fish approaching upstream 

 were following a scent trail from the baits and had a 

 higher motivation to bite than fish that approached 

 from other directions. 



It is interesting to note that although halibut ap- 

 peared throughout the gear deployment, consistent 

 with the idea of being aroused by and following a 

 scent plume from different distances, once a fish ar- 

 rived at the area of the baited hooks, bait attacks 

 occurred quickly. Most attacks occurred within the 

 first minute after a halibut appeared, and less than 

 five percent of the bait attacks occurred more than 

 one and one-half minutes after the fish appeared. 



Postbiting behavior and hooking success 



In almost all cases, complete bites were followed by 

 rushing behavior, which resulted in about equal num- 

 bers of hooked fish and fishes that spat the hook and 

 then departed the observation area. Hooked fish 

 struggled violently for a short period, then went into 

 a series of resting and rushing behaviors which con- 

 tinued through the observation period, the duration 

 of rushes becoming gradually shorter. 



Size selectivity by hook-and-line gear has a num- 

 ber of components, including encounter rate, attack 

 rate, and hooking success. A higher encounter rate 

 has been shown for larger fish, which may have 

 greater foraging ranges and therefore a higher prob- 

 ability of encountering baited gear ( Lokkeborg and 

 Bjordal, 1992; Engas and L0kkeborg, 1994; Bjordal 

 and Lokkeborg, 1996). It is possible that the present 

 experimental arrangement was skewed in this way 

 toward catching larger halibut. Attack rate has been 

 related to bait size for some species; larger fish show a 

 preference, and therefore a higher attack rate, for larger 

 baits, and smaller fish showing a preference for smaller 

 baits (Bjordal and Lokkeborg, 1996). No difference in 

 attack rate by fish size was seen in the present study. 



Hooking success was found to be strongly depen- 

 dent on fish length, ranging from zero for fish less 

 than 62 cm, to 839. for fish 93-97 cm (Fig. 5). This 

 finding is consistent with and can explain much of 

 the selectivity estimated from representative IPHC 

 commercial and setline survey data (Clark^). A circle 



0% > 

 65 



70 75 80 85 90 95 



Fork Length (5-i:iii interval) 



• - Observed hooking success 



— Selectivity estimated from survey 



Figure 5 



Hooking success from direct observation and se- 

 lectivity estimated from IPHC survey data by 

 .5-cm length groups. 



' Clark, W. 1997. Coastwide distribution of exploitable bio- 

 mass according to 1997 setline surveys. Int. Pac. Halibut 

 • Comm. Rep. of Assessment and Research Activities, p. 161- 

 202. International Pacific Halibut Commission. P.O. Box 

 9.5009, Seattle, WA 9810.5-2009. Unpubl. manuscript. 



hook is designed so that it is pulled to the corner of 

 the mouth during rushing, with the point of the barb 

 in line with the pull of the gangion (Bjordal and 

 Lokkeborg, 1996). Hooking results from the orienta- 

 tion of the hook during the rush and the penetration 

 of the barb caused by the pull on the gangion during 

 that rush, the point of the hook circling the jawbone 

 and exiting through the cheek (Johannes, 1981). 

 Ninety to ninety-five percent of halibut caught on 

 circle hooks are hooked in this manner (Kaimmer 

 andTrumble, 1997). 



Hooking success has been related to hook size only 

 when the range of hook sizes is very great, on the 

 order of 2009^ or more (L0kkeborg and Bjordal, 1992) 

 and has been explained in terms of larger fish being 

 able to exert a stronger pull on the gangion during 

 rushing. This stronger pull generates the greater 

 force necessary to pull the point of a larger hook fully 

 into the tissue of the mouth cavity, resulting in a 

 higher rate of hooking success. A greater hooking 

 success as the result of a larger fish exerting a stron- 

 ger pull may be countered by weaker tissue in the 

 mouth of smaller fish, requiring less force for the hook 

 to penetrate (Bjordal and Lokkeborg, 1996). The 

 hooking success table (Table 4) in this study was con- 

 structed for fish 62-97 cm in length, the largest just 

 1.5 times the length of the smallest, and showed dra- 

 matic differences in hooking success over length 

 changes as small as 10-20 cm. Some mechanism be- 

 yond pull strength was probably responsible for these 

 differences. In an earlier study where the same hooks 



