O'Farrell and Satterthwaite: Inferred historical fishing mortality rates of Oncorhynchus tshawytscha 
347 
ment area, and, therefore, there were larger differences 
between the median and mean estimates of the con- 
tact rate per unit of effort in MO in relation to SF. In 
many cases, the median contact rates per unit of effort 
were zero, particularly in the commercial sector. For 
recreational fisheries in February and March, inferred 
contact rates per unit of effort were much higher than 
estimates for other months. These values were higher 
than all estimates of contact rates per unit of effort in 
the SF management area and were among the highest 
values observed in the MO area. These high contact 
rates per unit of effort were not unexpected given the 
high proportion of the total harvest that occurred in 
February and March, according to estimates derived 
from the data for 1971-1972 (0]=O.33), and given the 
low estimates of the proportion of legal-size fish for 
these months (Table 1). High levels of contact rates 
per unit of effort in February and March would be 
required to result in the substantial harvest propor- 
tions in those months and, therefore, to approximate 
the monthly harvest distributions from the data for 
1971-1972. 
To evaluate whether the impact-rate hindcasts had 
a pattern similar to measures of fishing mortality for 
other stocks subjected to a common ocean fishery, we 
compared the median SRWC impact-rate hindcasts 
with Sacramento River fall Chinook salmon (SRFC) 
harvest rates directly estimated from coded-wire tag 
data. The SRFC stock is the largest contributor to 
ocean salmon fisheries south of Point Arena. Harvest 
rates were estimated by computing the ratio of SRFC 
adult (ages 3-5) harvest south of Point Arena to the 
Sacramento Index (the Sacramento Index is the sum 
of ocean harvest south of Cape Falcon, Oregon; river 
harvest; and spawner escapement; see O’Farrell et 
al., 2013, for a detailed description). The impact-rate 
hindcasts for SRWC were highly correlated to SRFC 
harvest rates in 1983-2012, the years for which the 
SRFC harvest rate is estimable (correlation coefficient 
[/■] =0.827, PcO.OOl) (Fig. 6). 
Discussion 
The large changes in California ocean salmon fisher- 
ies over the past 35 years have resulted in substantial 
changes to the levels of fishing mortality experienced 
by SRWC. Impact-rate hindcasts indicate lower rates 
in the 2000s than those in the mid-1980s through the 
mid-1990s. A decline in the impact rate was evident for 
the recreational sector following a peak in the 1980s and 
1990s, and a decline has been observed for the commer- 
cial sector throughout the entire period. The trajectories 
of the sector-specific impact rates (Fig. 5A) reflect the 
observed changes in fishing effort (Fig. 2). The strong 
correlation between harvest rates for SRFC in the re- 
gion south of Point Arena and the impact-rate hindcasts 
for SRWC, where both populations have been subjected 
to the same fisheries, supports the hindcasting methods 
for calculating impact rates used in this study. 
In their analysis of the brood years 1998-2007, 
O’Farrell et al. (2012a) reported that exploitation rates 
experienced by SRWC averaged approximately 20% and 
that the bulk of the ocean impacts were the result of 
the recreational sector. Although the results from our 
study are consistent with O’Farrell et al. (2012a) over 
the common time period, they indicate that recent esti- 
mates of the impact rate are unlikely to represent the 
