O'Farrell and Satterthwaite: Inferred historical fishing mortality rates of Oncorhynchus tshawytschc 
349 
ed as threatened under the ESA in 1989. Fishing 
constraints included establishment of a closed area 
for the recreational sector near the mouth of San 
Francisco Bay and implementation of river fishery 
restrictions (PFMC 11 ). Since those initial SRWC 
conservation measures were implemented, addi- 
tional SRWC-focused fishery management mea- 
sures have been established, including truncation 
of commercial and recreational fishing seasons, 
increased minimum size limits, and most recently, 
impact-rate “caps” set annually by a control rule 
(see Appendix C in PFMC 12 ). Although we do not 
directly link changes in the impact rate to particu- 
lar management measures, the increased fishery 
constraints over the past 35 years have clearly in- 
fluenced the impact rate. 
Estimates of contact rates per unit of effort have 
a strong effect on the impact-rate forecasts made 
with the Winter Run Harvest Model (O’Farrell et 
ah, 2012b) and, therefore, on the hindcasts pre- 
sented here. We assumed that contact rates per 
unit of effort estimated with cohort reconstruction 
methods for the years 2000-2012 are representa- 
tive of contact rates per unit of effort in the years 
before 2000. Contact rates per unit of effort are a 
function of catchability and stock distribution, and, 
in the absence of large temporal changes in these 
components, assuming contemporary contact rates 
per unit of effort for past years without direct esti- 
mates is reasonable. Large changes in catchability 
in either the commercial or recreational salmon 
sectors would be unlikely because modes of fishing 
and associated fishing gear have changed little over the 
past 35 years. We would also not expect substantial 
shifts in SRWC ocean distribution over time because 
this population has been shown to have a relatively 
compact southerly distribution (O’Farrell et ah, 2012a; 
Satterthwaite et al., 2013). 
Nevertheless, there is likely to be nontrivial process 
and measurement error associated with the estimates 
of contact rates per unit of effort. Local conditions 
undoubtedly affect both the ability of fishing fleets to 
contact SRWC and the precise local concentration of 
SRWC cohorts. With regard to sampling error, esti- 
mates of contact rates per unit of effort are made by 
using expanded coded-wire tag data and estimates of 
fishing effort. SRWC coded-wire tag recoveries can be 
relatively rare for a variety of reasons. SRWC have 
a lower abundance than that of other Chinook stocks 
npFMC (Pacific Fishery Management Council). 1990. Pre- 
season report III: analysis of Council-adopted management 
measures for 1990 ocean salmon fisheries. Pacific Fishery 
Management Council, 24 p. Portland, OR. [Available at 
website.] 
12 PFMC (Pacific Fishery Management Council). 2013. Pre- 
season report I: tock abundance analysis and environmental 
assessment part 1 for 2013 ocean salmon fishery regulations, 
135 p. (Document prepared for the Council and its advisory 
entities.) Pacific Fishery Management Council, Portland, OR. 
[Available at website.] 
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SRFC harvest rate south of Point Arena 
Figure 6 
Sacramento River winter Chinook salmon (Oncorhynchus 
tshawytscha) (SRWC) hindcast median impact rates plot- 
ted as a function of Sacramento River fall Chinook salmon 
(SRFC) harvest rates for fisheries south of Point Arena, 
California. Numbers denote the calendar years 1983-2012. 
that contribute to California fisheries, and only the 
hatchery-origin component of the population is marked 
and tagged. Approximately 20% of the landed catch is 
sampled, and early-season fisheries with large mini- 
mum size limits would be expected to retain few SRWC 
because few fish would be greater than the minimum 
legal size. 
These sampling issues for rare stocks can read- 
ily lead not only to an estimate of zero contacts in a 
month, area, and sector stratum despite nonzero actual 
contacts but also to cases where a single tag recovery 
implies a very large number of contacts. In some stra- 
ta, estimates of SRWC contact rates have been based 
on a very small number of coded-wire tag recoveries 
(O’Farrell et al., 2012a). 
To infer values of potential contact rate per unit 
of effort for the recreational sector in February and 
March, we developed a method based on estimates 
of the monthly distribution of harvest when fisheries 
were open from February through November. The in- 
ferred recreational contact rates per unit of effort for 
February and March from this procedure were higher 
than estimates of contact rates per unit of effort from 
months after March, although such high values would 
be expected given the relatively large proportion of har- 
vest that was estimated for 1971-1972 and the small 
proportion of age-3 SRWC expected to be greater than 
the minimum legal size limit during those months. 
