Rodgveiier et a!: Evidence of hook competition in longlme surveys 
373 
areas where sablefish catches were high, and vice 
versa, during the AFSC sablefish longline surveys. 
They concluded that because sablefish are more 
mobile and aggressive, they are likely out-com- 
peting other species for baited hooks. If competi- 
tion between less aggressive groundfish and more 
energetic predators like sablefish is occurring on 
the AFSC longline surveys, as our data indicate, 
catch rate trends may not be proportional to ac- 
tual trends in fish densities. 
Directed sablefish experiments have shown that 
the decrease in probability of catching a sable- 
fish as the number of baited hooks decreases is 
not linear. However, in existing models where 
longline catch rates are adjusted for competition, 
this decrease is assumed to be linear (Murphy, 
1960; Rothschild, 1967). In these models, when 
the number of recovered baited hooks is high, the 
magnitude of competition is low and model adjust- 
ments to catch rates are minimal. Conversely, as 
fewer baited hooks remain, competition increases, 
resulting in a greater need for model adjustments. 
Sigler (2000) documented the time until capture of 
sablefish on a hook-by-hook basis on longline gear 
and observed a nonlinear relationship between 
hooking probability and the number of baited hooks. 
The probability of hooking a fish remained constant 
until approximately half of the baited hooks were left; 
it then decreased steeply toward zero. This indicates 
that hook competition does not affect sablefish catch 
rates until fewer than 50% of the baited hooks remain, 
and that the decrease in hooking probability with a 
decrease in baited hooks is not linear. In another ex- 
periment, Sigler (2000) examined 12- to 42-m hook 
spacings that represented a condition of only 17% and 
5% baited hooks remaining, respectively, and found that 
sablefish catch per hook decreased only 8%. The hook- 
spacing experiment showed that there is little decrease 
in catch probability, indicating that competition affects 
sablefish catch rates very little and also indicates that 
the decrease in catch probability does not decrease lin- 
early to zero as the number of baited hooks decreases; 
it may decrease very slowly until very few baited hooks 
remain and then drop off quickly. If the effect of baited 
hooks on catch probability is not linear, as Sigler (2000) 
showed, results from the Murphy (1960) and Rothschild 
(1967) competition models used to adjust catch rates 
would be biased. Assuming a linear relationship, when 
it is in fact nonlinear, will underestimate relative abun- 
dance when fish densities are high, and will overesti- 
mate relative abundance when fish densities are low. 
Competition for hooks likely occurs during the AFSC 
longline surveys. Both Skud (1978) and Sigler (2000) 
found that groundfish catch rates can be affected by 
hook competition; therefore it is likely that the catch 
rates of more docile groundfish, such as grenadier and 
rock fish, would also be affected. Currently gear satu- 
ration effects are not taken into account when relative 
population sizes of groundfish are calculated. Hook 
timing studies, such as Sigler’s (2000), have not been 
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Sablefish Grenadier Shortraker Rougheye 
Figure 6 
Average total length (cm) and associated 95% confidence 
intervals for sablefish (Anoplopoma fimbria), giant grena- 
dier ( Albatrossia pectoralis ), shortraker rockfish ( Sebastes 
borealis), and rougheye rockfish ( Sebastes aleutianus) caught 
during the National Oceanic and Atmospheric Administration 
Alaska Fisheries Science Center longline and trawl surveys 
in 2003. 
directed at other species caught during the longline 
surveys. To accurately assess competition effects on 
grenadier and rockfish, directed experiments on hook 
spacing or with hook timers would be needed to develop 
alternate models of the relationship between catch prob- 
ability and number of baited hooks. 
Acknowledgments 
We thank M. Sigler, J. Heifetz, P. Rigby, and D. Han- 
selman from the National Oceanic and Atmospheric 
Administration, Alaska Fisheries Science Center for 
their helpful reviews with earlier versions of this manu- 
script. We also thank the anonymous reviewers for 
their insightful comments, which greatly improved this 
manuscript. 
Literature cited 
Britt, L. L., and M. H. Martin. 
2001. Data report: 1999 Gulf of Alaska bottom trawl 
survey. U.S. Dep. Commer., NOAA Tech. Memo. NMFS- 
AFSC-121, 249 p. 
Clark, W. G„ and S. R. Hare. 
2006. Assessment and management of Pacific halibut: 
data, methods, and policy. Int. Pac. Halibut Comm. 
Sci. Rep. 83, 104 p. 
Clausen, D. M. 
2006a. Grenadiers in the Gulf of Alaska, Bering Sea, and 
the Aleutian Islands, appendix F. In Stock assessment 
and fishery evaluation report for the groundfish fisher- 
ies of the Gulf of Alaska, p. 563-600. North Pacific 
Fishery Management Council, 605 W 4th Avenue, Suite 
306, Anchorage, AK 99501. 
