Yes, if the mortality is halved – one would expect the escapement numbers to also double – if there is no density dependent interactions or stress-related mortalities (like from an introduced or existing disease vector). The actual numbers wrt marine mortality "limits" is dependent upon year, stock, species, area, and watershed and associated environmental conditions. Different impacts may operate and overlap at different scales. One would expect that the smaller juvenile salmon that spend extended time in the nearshore areas that contain fish farms to be most affected by those potential impacts.
From: p. 213 R. J. Beamish and C. Neville
CLIMATE-OCEAN CHANGES AND THE IMPACTS ON YOUNG SALMON IN THE STRAIT OF GEORGIA; IN: Emmett, R.L., and M.H. Schiewe (Editors). 1977.
Estuarine and Ocean Survival of Northeastern Pacific Salmon: Proceeding of the Workshop. U.S. Dept. Comm., NOAA Tech. Memo. NMFS-NWFSC-29, 313 p.
https://www.nwfsc.noaa.gov/assets/25/6622_08042010_100759_Emmett.and.Schiewe.1997-rev.pdf
“
Catches of chinook salmon in the Strait of Georgia began to decline in 1979 (Fig. 1). Synchronous with this decline was an abrupt and precipitous decrease in marine survival from an average of 4.8% from 1974 to 1977 (year to sea) to 0.7% from 1978 to 1988 (year to sea). If overfishing was the major cause of the decline, there would be a gradual reduction in smolt production at the time of the decrease in catch and marine survival. However, the total number of wild and hatchery reared chinook salmon smolts produced after the declines actually increased from a yearly average of about 30 x 106 from 1974-76 to about 82 x 106 in the early 1990s (Beamish et al. 1995).
There was a decline in coho salmon survival shortly after the decrease occurred for chinook salmon (Fig. 2). Although the average catch of coho salmon in the Strait of Georgia remained about the same, there was an increase in smolt production from about 15 x 106 before the change in survival to about 25 x 106 smolts in the early 1990s. Because the marine mortality for hatchery reared and wild coho salmon is about equal, it is clear that the addition of smolts to the Strait of Georgia did not increase the total abundance of post-juvenile fish over the pre-1977 levels. “
