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Fishery Bulletin 106(3) 
Figure 5 
Scatterplot representing the fit of observed and predicted availability of 
sablefish ( Anoplopoma fimbria) to the fishery by age. Deviance is a function 
of the difference between observed and predicted values; a smaller deviance 
indicates a better fit of the exponential-logistic model to the observation. 
Figure 6 
Estimated availability (fraction of the total population) of sablefish (Ano- 
plopoma fimbria) to the fishery by age (years) for the open-access (1985-94, 
) and individual fishing quota (IFQ) (1995-2005, — ) fisheries. Param- 
eter estimates and 95% confidence intervals for 1985-94 are a = 1.50 (1.26, 
1.87), p = 3.33 (3.04, 3.69), y = 0.050 (0.023, 0.092), and A = 0.23 (0.22, 0.25) 
and for 1995-2005 are a = 1.14 (1.05, 1.24), j3 = 4.76 (4.53, 5.01), y = 0.18 
(0.15, 0.21), and A = 0.38 (0.36, 0.39). 
age distribution by area, corroborat- 
ed the counterclockwise pattern of 
sablefish migration in the northeast 
Pacific Ocean. 
Availability to the fishery by age 
The pattern of movement from shal- 
low to deep water with age (Fig. 2) 
results in increased availability to 
the fishery as sablefish grow older 
(Fig. 6). Estimates of availability at 
age have shown that about half of 
sablefish are available to the fish- 
ery by age 3 or 4, depending on the 
fishery management system, and 
that most are available by age 5 or 
6. The later availability of fish (at 
ages 5 or 6) under the IFQ manage- 
ment system (compared to earlier 
availability of younger fish under the 
open access system) also was also 
found to be the case in an analysis of 
length-frequency data from the two 
fisheries, and this pattern of avail- 
ability was likely created because the 
crowding of fishing vessels during 
the open access fishery pushed fish- 
ermen into areas and depths where 
there were smaller fish (Sigler and 
Lunsford, 2001). The IFQ fishery, 
with a longer season and fewer ves- 
sels, reduced crowding so that fish- 
ermen were able to avoid shallower 
depths with smaller, younger fish. 
Unlike previous analyses, where 
availability was assumed not to de- 
crease with age (e.g., Sigler, 1999), 
our analysis of known-age tag re- 
coveries showed that sablefish avail- 
ability decreases with age. An al- 
ternate explanation is that tag loss 
increased with time. However, dou- 
ble-tagging experiments have shown 
that the rate of tag loss is constant 
with time (Beamish and McFarlane, 
1988; Lenarz and Shaw, 1997). De- 
creased availability with age may 
occur because of reduced fishing ef- 
fort for older age fish. Fishing ef- 
fort is concentrated at intermediate 
depths (e.g., half of the recoveries 
occurred at depths 501-700 m [Table 
2] ) but fish exit these depths as they 
age and move deeper. In addition, 
older fish were less available for IFQ 
management compared to open ac- 
cess management, which also may 
have been due to reduced crowding 
of fishing grounds during IFQ man- 
