Stachura et al Estimation of discard mortality of Anoplopoma fimbria in Alaska longline fisheries 
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In our study, the location of hooking injury did not 
significantly affect recapture rates. However, a large 
portion of injuries occurred on the cheek and upper 
and lower jaws — locations that are typically affected by 
circle hooks. We likely did not have enough samples of 
fish with injuries in other locations to detect the effects 
of those injuries. Unlike our results, results from stud- 
ies of catch-and-release of recreationaliy caught species 
have indicated that hooking location was the most sig- 
nificant factor in estimating mortality (reviewed in Bar- 
tholomew and Bohnsack, 2005). Deep-hooking injuries 
in critical locations such as the esophagus, stomach, 
gills, eyes, and brain significantly increase mortality 
in many species (e.g., Muoneke and Childress, 1994; 
Palsson et al., 2003; Aalbers et al., 2004; Aids et al., 
2009). The circle hooks that are used in Alaska longline 
fisheries usually hook fish in the mouth and injuries in 
critical locations are not common (reviewed in Trumble 
et al., 2000). Capture with other hook types or fishing 
gears, such as trawl gear, would likely produce injuries 
on other locations of the body. 
Depth of capture significantly affected the recapture 
rate of sablefish, which is common for other fish species. 
We found a positive relationship between depth of cap- 
ture and assumed survival (i.e., fish caught at shallow 
depths were less likely to be recaptured). The sablefish 
fishery extends to at least 800 m in many areas and 
so the effect of depth on recapture rates may be even 
more pronounced at depths greater than 419 m, the 
maximum sampling depth in our study. The opposite 
has been observed in physoclistous species due to baro- 
trauma, because of organ damage caused by gas expan- 
sion in the body cavity during capture (e.g., Gitschlag 
and Renaud, 1994; Wilson and Burns, 1996; Collins et 
al., 1999; St. John and Syers, 2005). Sablefish lack a 
swim bladder, thus no correlation between mortality 
and depth of capture was expected. Deeper-dwelling 
fish can also have increased injuries with greater cap- 
ture depths, indicating that injuries are inflicted while 
fish struggle during hauling (Atlantic cod; Palsson et 
al., 2003). 
There are some potential explanations for why fish 
caught at shallow depths had lower recapture rates. 
First, sablefish caught at deeper depths (320-419 m) 
could be less vigorous because of the longer retrieval 
time and the increased time spent fighting the line 
during retrieval and therefore they are less likely to 
become injured during the landing process when out of 
the water and onboard the fishing vessel. Differential 
predation in the depth categories may also affect the 
mortality of released sablefish, if they return to their 
previous depths after release. Two major predators 
of sablefish have greater concentration at shallower 
depths, Pacific halibut (27-274 m; IPHC, 1998) and 
Pacific sleeper sharks ( Somniosus pacificus) (Yano et 
al., 2007). Second, fishing effort likely differs by depth 
and therefore may affect depth-related recapture rates 
of tagged fish. Data on fishing effort by depth were 
not available from the Pacific halibut fishery or the 
southeast Alaska state sablefish fishery and there- 
fore a full examination of this supposition was not 
possible. 
In our study, amphipod predation was related to the 
recapture rate of sablefish and was prevalent for fish 
that were too severely injured to tag. Similarly, Pacific 
halibut that were tethered to longlines for extended 
periods suffered from amphipod predation and had a 
low survival rate (Trumble et al., 2000). Fishery-specific 
amphipod predation rates would need to be investigated 
to accurately assess this effect on the discard mortality 
of sablefish. 
The year of capture significantly affected the re- 
capture rate of sablefish. A greater recapture rate 
was found for fish tagged in Clarence Strait in 1990 
and several factors likely contributed to this differ- 
ence. First, a greater proportion of fish tagged in 1990 
(18.57%) were recaptured within 60 days of tagging 
compared to those tagged in 1989 (7.42%). This is 
likely explained by the occurrence of an Alaska De- 
partment of Fish and Game sablefish survey and the 
southern southeast Alaska directed sablefish fishery 
both occurring within 60 days of the initial tagging 
effort. Tagging conducted in Chatham Strait in 1989 
occurred after both the state survey and fishery period 
and therefore the grounds were not fished for nearly a 
year after the tagging effort. A minimum time at liber- 
ty was not used in our study because the year or loca- 
tion of tagging was secondary to our primary objective 
of determining the factors related to discard mortality 
and estimating absolute discard mortality based on 
the severity of injuries to sablefish. Second, longline 
fishing is permitted in the Chatham Strait fishery, 
and in Clarence Strait both longline and pot gear are 
allowed. Animals can exhibit varying levels of “trap 
addiction” (attraction to fishing gear) or “trap shyness” 
(an aversion to the gear) depending on the gear type 
(Seber, 1982). Previous tagging analyses have shown 
that sablefish may be trap shy towards longline gear 
within the first year after capture, likely because of 
the stress incurred during the initial capture (Carlile, 
et al. 3 ). Because many of our fish were caught soon 
after capture in the fishery, some of the difference in 
recapture rate that we saw may be explained by the 
differential recapture catch rates between pot and 
longline gear types. Finally, amphipod predation was 
significantly higher in 1989 (12.45%) than in 1990 
(3.86%) indicating that Chatham Strait may have a 
higher incidence of amphipods, which we found to be 
related to a decreased recapture rate. 
We calculated an absolute mortality rate for each 
level of severity of hook injury. The overall mortality 
rate of 11.71% is substantially lower than the 25% 
mortality rate assumed for sablefish discarded in the 
Pacific halibut fishery in state waters (i.e., Chatham 
3 Carlile, D., B. Richardson, M. Cartwright, and V. M. 
O’Connell. 2002. Southeast Alaska sablefish stock as- 
sessment activities 1998-2001. Regional Information Report 
IJ02-02, 86 p. Alaska Dep. Fish and Game, Douglas, 
AK. 
