By Timo Turunen, Anne Kakela, Heikki Hyvarinen
Fisheries Research, Volume 19, Issues 1-2, February 1994, Pages 51-64, ISSN 0165-7836, DOI: 10.1016/0165-7836(94)90014-0.

Trawling stress and mortality in undersized brown trout (Salmo trutta L.) were examined at openwater seasons in 1989 and 1990 on the big lakes in eastern Finland. Large scale trawling on inland waters in this area is a new phenomenon and it has caused conflicts between different fishing groups. It has generally been assumed that undersized trout freed after being caught in trawls do not survive. Blood lactate and glucose concentrations and plasma chloride concentrations were measured from blood samples taken immediately after trawling. The recovery of the fish from trawling stress was also monitored by blood sampling and following the mortality of fish caged for 7 days after capture.
The trout were observed to become considerably stressed by trawling. Abundant catch and high water temperature increased stress of the trout. These variables and the duration of trawling haul as well as the cod end empting technique explained 72% of the increase of blood lactate.
The indicators of stress had not returned to control values by 4 h post-capture, but blood lactate showed an abrupt decrease over 2 h. Plasma chloride concentration was, however, still decreasing 4 h after the end of trawling.
The percentage of caged fish which survived a week was 85.5. Lifting of the cod end to the vessel clearly increased the mortality of trout. Without the extra stress connected with caging, the proportion of surviving fish would probably have been considerably greater. On the basis of the results it can be said that if undersized trout were freed immediately by emptying the cod end in small portions directly in the water, trawling would not cause them considerable mortality.

More details...