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Fishery Bulletin 1 10(2) 
federal waters off Alaska is managed by a catch shares 
program, where annual individual fishing quota (IFQ) 
shares are allocated to fishermen, for fish that can be 
caught anytime during the eight and a half month sea- 
son. For fishermen with IFQs, full retention of all sable- 
fish caught is required. However, sablefish are often 
legally discarded in other commercial longline fisheries, 
primarily in those targeting Pacific halibut and Pacific 
cod (Gadus macrocephalus). In the sablefish fishery, the 
practice of releasing small sablefish and retaining only 
the larger fish because of the greater value per pound 
of larger fish (a technique known as “highgrading” 
[Davis, 2002] ) is illegal. However, because there is an 
incentive to retain larger fish and not all fishing trips 
are monitored, highgrading may occur. 
Factors affecting discard mortality likely vary by spe- 
cies, gear type, depth, and other environmental factors. 
Injury location on fish has proven to be an indicator 
of short and long-term discard mortality (e.g., Bar- 
tholomew and Bohnsack, 2005). In Alaska, sablefish 
inhabit a wide range of depths and are caught primar- 
ily on longline gear, which can cause external injuries 
to different areas of the body. Fish tethered to longline 
gear for extended periods are subject to predation by 
parasitic amphipod crustaceans. Also fish size may 
affect mortality of discarded fish. The objective of our 
study is to determine if the location and severity of the 
hook injury, line and roller gear injury, water depth, 
fish size, and the level of amphipod predation affect 
the discard mortality rate in Alaskan longline fisher- 
ies. To answer these questions, the recapture rates of 
fish tagged and released in the marine environment 
were related to each factor. In addition, an absolute 
discard mortality rate was computed on the basis of the 
observed severity of hooking injuries. 
Materials and methods 
Tagging and data collection 
In 1989 and 1990, research surveys were conducted 
by the National Marine Fisheries Service (NMFS), 
Alaska Fisheries Science Center (AFSC) in Southeast 
Alaska. In 1989, sablefish were tagged during August 
and September in Chatham Strait; in 1990, sablefish 
were tagged during April and May in Clarence Strait 
(Fig. 1). Longline gear was fished on the bottom at 
depths from 210 to 419 m with a minimum 3-hour soak 
time. Gear configuration consisted of size 13/0 circle 
hooks baited with squid attached to 38-cm gangions 
that were secured to beckets tied in a 9.5-mm (3/8 in) 
groundline at 2-m intervals. This gear configuration is 
similar to that used in the commercial sablefish fishery 
in Alaska. However, the Pacific halibut fishery typically 
uses larger hooks (16/0). All sablefish, except those 
with extremely severe injuries, were tagged with plastic 
T-bar style anchor tags, and injuries were classified by 
the following 4 variables: location of hook injury, sever- 
ity of hook injury, severity of injuries due to amphipod 
predation, and the presence of injury sustained on fins 
or body from line and roller gears. Within each variable, 
a categorical condition code describing the injury was 
recorded (Table 1). The date of capture, capture location, 
and depth of capture were also documented. Fish were 
promptly released after they were measured (fork length, 
nearest mm) and tagged. 
To determine recapture rates of fish within each cat- 
egory, tagged fish were recovered in commercial fisher- 
ies and tags were returned to the AFSC for a reward 
(Maloney 2 ). Tags were also recovered during subsequent 
research studies. Data for fish recaptured from the time 
of tagging to June 2009 were used in our analysis (up 
to 19 years at liberty). 
Analysis 
A logistic regression model was constructed to determine 
which factors were related to significant differences in 
recapture rates. The relationship between the binary, 
dependent variable, Tj-, which represents whether a fish 
was recaptured or not, and seven independent explana- 
tory variables was estimated with the following full 
model, 
Logit(Yj) = a + bYr, + cL, + dD. + 
eHL i + ftiS. + gA t + hG ( , (1) 
where a 
Yn 
L, 
D i 
HL. 
HS t 
A, 
G ( 
the intercept, and b to h are estimated 
model coefficients; 
year of tagging (1989, 1990); 
fish length at capture; 
capture depth group (210-269, 270-319, 
320-419 m); 
location of the hook injury (cheek, upper jaw, 
lower jaw, nose, throat, eye, gill); 
severity of the hook injury (minor, moderate, 
severe); 
severity of amphipod predation injury (no 
injury, <10% scale loss, >10% scale loss); 
and 
type of injury sustained on fins or body from 
line and roller gears (no injury, fin damage, 
lacerations) for fish i (Table 1). 
Year can also be considered to be the effect of location 
because in each year fish were tagged at different loca- 
tions. All independent variables were treated as categori- 
cal except for length, which was continuous. Interaction 
terms were not included in the model because of the 
small sample sizes available across multiple categori- 
cal variables, which resulted in an inability to estimate 
these interaction parameters. 
Forward-stepwise model selection was performed to 
simplify the model to factors that significantly improved 
2 Maloney, N. E. 2002. Report to industry on the Alaska 
sablefish tag program, 1972-2001. AFSC Processed Rep. 
2002-01, 44 p. Alike Bay Laboratory, NMFS, NOAA, 11305 
Glacier Highway, Juneau, AK 99801. 
