O'Farrell and Satterthwaite: Inferred historical fishing mortality rates of Oncorhynchus tshawytscha 
343 
1978 1982 1986 1990 1994 1998 2002 2006 2010 
Year 
Figure 2 
Estimates of commercial (black line) and recreational (gray line) 
sector fishing effort for the years 1978-2012 in ocean areas south 
of Point Arena, California. Direct estimates of the Sacramento 
River winter Chinook salmon ( Oncorhynchus tshawytscha) impact 
rate have been made with cohort reconstruction methods for the 
years to the right of the dashed line (post-2000). Note that fishing 
effort units differ between the commercial and recreational fisher- 
ies and are not directly comparable. 
fort have occurred since the late 1970s and 
the inferred fishing effects for recent SRWC 
cohorts may not be indicative of earlier ex- 
ploitation patterns. 
In this article, we describe a model for the 
hindcasting of fishery impact rates to assess 
the implications for SRWC fishing mortality 
resulting from changes in California salmon 
fisheries that have occurred over the past 35 
years. The model uses data on historical fish- 
ery regulations and estimates of fishing effort 
in California salmon fisheries in 1978-2012, 
and it couples these estimates with contact 
(i.e., fishery encounter) rates per unit of fish- 
ing effort estimated for recent years (2000- 
2012), to generate hindcasts of the impact 
rate for years when direct estimation is not 
possible. To fully parameterize the model, we 
developed a procedure to infer contact rates 
per unit of effort for months when no direct 
estimates exist because of the contraction 
of modern fisheries in relation to fisheries 
from the 1970s through the 1990s. The model 
structure is similar to the Winter Run Har- 
vest Model (WRHM; O’Farrell et al., 2012b), 
a tool used to forecast the impact rate during 
the annual PFMC salmon fishery planning 
process. Although we are not able to recon- 
struct the complete SRWC exploitation his- 
tory, the extension of the impact rate time 
series back to 1978 encompasses a dynamic 
period for California ocean salmon fisheries 
and provides more context for the current 
levels of ocean fishing mortality. 
Materials and methods 
Impact-rate model 
The data and model used for this analysis were strati- 
fied by year (y), month (t), management area (z), and 
fishery sector (x). Because harvest of SRWC is rare 
north of Point Arena, California (O’Farrell et al., 
2012a; Satterthwaite et al. 2013), the spatial extent of 
our model is limited to the 2 ocean management ar- 
eas south of Point Arena: San Francisco (SF) and Mon- 
terey (MO) (Fig. 1). The MO management area extends 
from Pigeon Point to the border of the United States 
and Mexico, but salmon harvest is generally small and 
more variable south of Point Sur. Fishery sectors in- 
clude commercial and recreational. 
Fishery impacts (I) included fish that died because 
they were retained as harvest iH), fish that were re- 
leased because they were smaller than the minimum 
size limit and died because of release mortality (S), and 
fish that died from “dropoff” mortality (D) that occurs 
when fish are encountered by fishing gear but not suc- 
cessfully retrieved. Mortality related to an encounter 
with gear can come from multiple sources, such as a 
hooking injury or predators (PFMC 4 , and the refer- 
ences therein). The impact rate is defined as the to- 
tal fishing mortality, acute and delayed, divided by the 
starting cohort abundance. 
The impact-rate model simulates the age-3 cohort 
abundance from the beginning of age 3, f=February of 
year by), to t=January of year y+1, 
N t+1 = (N t - h)( 1-v). ( 1 ) 
by deducting monthly impacts, / t = X z>x ^t,z,x’ and ac- 
counting for the natural mortality rate v. Application 
of Equation 1 over months t enables the computation of 
the annual impact rate, defined as the sum of monthly 
impacts divided by the initial abundance of age-3 fish: 
i = _ (2 ) 
February 
Impacts are computed from a string of equations 
initiated by an estimate of the contact rate c, which 
is formulated as the contact rate per unit of effort (3, 
multiplied by the amount of fishing effort f, 
4 PFMC (Pacific Fishery Management Council). 2000. STT 
recommendations for hooking mortality rates in 2000 rec- 
reational ocean Chinook and coho fisheries. STT Rep. B.2, 
18 p. Pacific Fishery Management Council, Portland, OR. 
[Available at website.] 
