341 
NOAA 
National Marine 
Fisheries Service 
Abstract— The time series of esti- 
mated fishery exploitation rates 
for endangered Sacramento River 
winter Chinook salmon ( Oncorhyn - 
chus tshawytscha) is confined to a 
relatively recent period for which 
coded-wire tag data have been avail- 
able. However, the nature of ocean 
salmon fisheries before this period 
was substantially different, and it is 
likely that recent exploitation rates 
do not represent the level of fish- 
ing mortality experienced by these 
Chinook salmon in earlier years. To 
infer historical exploitation rates, a 
model was developed to hindcast the 
impact rate for age-3 winter Chinook 
salmon (an approximation of the ex- 
ploitation rate) by using 35 years of 
fishing effort estimates coupled with 
contemporary estimates of fishery 
encounter rates. The impact-rate 
hindcasts were highest during a pe- 
riod from the mid-1980s through the 
mid-1990s. Over time, the proportion 
of the impact rate attributed to com- 
mercial and recreational fisheries 
diverged from approximately equal 
shares early in the time series to an 
impact rate mostly composed of rec- 
reational fishery-induced mortality 
in more recent years. The inferred 
exploitation rates provide context 
for the fishing-induced mortality ex- 
perienced by winter Chinook salmon 
both before and after the time of the 
initial inclusion of this species on 
the Endangered Species Act (ESA) 
list in 1989 and through a dynamic 
period for ocean salmon fisheries in 
California. 
Manuscript submitted 1 October 2014. 
Manuscript accepted 2 June 2015. 
Fish. Bull. 113:341-351 (2015). 
Online publication date: 16 June 2015. 
doi: 10.7755/FB. 113.3.9 
The views and opinions expressed or 
implied in this article are those of the 
author (or authors) and do not necessarily 
reflect the position of the National 
Marine Fisheries Service, NOAA. 
Fishery Bulletin 
& established 1881 ■<?. 
Spencer F. Baird 
First U S. Commissioner 
of Fisheries and founder 
of Fishery Bulletin 
Inferred historical fishing mortality rates for 
an endangered population of Chinook salmon 
C Oacorhynchus tshawytscha ) 
Michael R. O'Farrel! (contact author) 
William H. Satterthwaite 
Email address for contact author: michael.ofarrell@noaa.gov 
Fisheries Ecology Division 
Southwest Fisheries Science Center 
National Marine Fisheries Service, NOAA 
1 10 Shaffer Road 
Santa Cruz, California 95060 
Determining the status of a stock 
often entails estimation of the popu- 
lation state or exploitation rate in 
relation to a baseline period before, 
or in the early stages of, a developing 
fishery. For example, determinations 
of an overfished status for many 
fish populations rely on estimates 
of depletion: the spawning stock bio- 
mass expressed as a fraction of its 
unfished level (Restrepo and Powers, 
1999; Haltuch et ah, 2008). The de- 
termination of Endangered Species 
Act (ESA) status for Pacific salmon 
(Oncorhynchus spp. ) populations in- 
clude analysis of current abundance 
levels in relation to the habitat car- 
rying capacity and historical popula- 
tion abundance (Myers et ah, 1998). 
The effect of fisheries (e.g., overfish- 
ing) generally is inferred from a time 
series of exploitation rate estimates 
that are compared with benchmarks 
of sustainable or optimal fishing-in- 
duced mortality rates. Estimation of 
historical fishing mortality rates is a 
focus of stock assessment, and such 
estimates are necessary inputs into 
life cycle models that allow infer- 
ences to be drawn regarding popula- 
tion dynamics in the face of compet- 
ing sources of mortality (Hendrix et 
al., 2014). Because there are often 
only short data series available for 
many stocks of conservation concern 
to allow evaluation of their status, 
development of approaches that can 
extend estimated abundance or ex- 
ploitation rates retrospectively can 
improve our historical understanding 
of a stock’s dynamics. 
For Pacific salmon, the time series 
of estimated exploitation rates can 
often be quite short because stock- 
specific catch data must be garnered 
from mixed-stock ocean fisheries, 
where the stock of origin cannot be 
determined visually, and therefore 
the estimate of exploitation rates 
requires tagging data (most com- 
monly with a coded-wire tag; John- 
son, 1990; Lapi et al., 1990; Nandor 
et al., 2010) or potentially genetic 
data (Milner et al., 1985; PSC 1 ; Sat- 
terthwaite et al., 2014). Estimates of 
exploitation rates are frequently con- 
fined to a period of time after major 
changes in the fishing capacity of 
fleets, allowing for only a glimpse of 
1 PSC (Pacific Salmon Commission). 2008. 
Recommendations for application of ge- 
netic stock identification (GSI) methods 
to management of ocean salmon fisher- 
ies. Special report of the GSI steering 
committee and the Pacific Salmon Com- 
mission’s committee on scientific cooper- 
ation. Pacific Salmon Comm. Tech. Rep. 
23, 35 p. [Available at website.] 
