Yes, they have done such studies, especially on fall chinook. It was like 75- 95% lost for those fish that were released after being gill netted.

Try this site out: http://wdfw.wa.gov/fish/commercial/selective/tangleprogress1.pdf
Or: http://www.psc.org/Pubs/CHK971.html

http://www.nwr.noaa.gov/1hydrop/hydroweb/minutes/it020502.pdf

Or just go to goggle and do a search on "Gill net studies for chinook"

Cowlitzfisherman