Evaluate Live Capture Selective Harvest Methods - February 2002

Selective fishing is the ability of a fishing operation to avoid non-target species or stocks, or when encountered, to capture and release them in a manner that minimizes mortality. Two gears, the tangle net and a floating trap net were tested on the lower Columbia River to selectively harvest adult spring Chinook salmon (Oncorhynchus tschawytscha) and coho salmon (O. kisutch). Experienced gill netters simultaneously fished tangle nets (3.5" and 4.5" mesh size) and conventional gill nets (8" mesh size) on the Columbia River to evaluate their effectiveness for live release of non-target stocks of spring Chinook salmon. Live fish were tagged and released for recovery in sport fisheries, commercial fisheries, at hatchery racks and traps, and during spawning ground surveys. Control fish that had not been captured in the test gears were tagged and released from an adult trap in Bonneville Dam, just upstream of the fishing area. The 4.5" tangle net was as effective for capturing spring Chinook salmon as the conventional gill net, but the 3.5" net caught significantly fewer spring Chinook salmon than the 8" gill net. Fish were generally captured in good condition. The immediate survival (from capture to release from the boat) of adult spring Chinook salmon captured in the 8" gill net was 99%, compared to 96% from the 3.5" tangle net, and 97% from the 4.5" tangle net. However, spring Chinook salmon released from the tangle nets were recovered at about 91% of the rate of controls, while spring Chinook salmon released from the conventional gill net were recovered at about 50% of the rate of the controls. These tests showed that using conventional gear with short soaks and careful fish handling is not enough to ensure the survival of released spring Chinook salmon. However, switching to the 4.5" or 3.5" tangle net, coupled with short soaks and appropriate fish handling is a viable selective harvest gear for the commercial gill net fleet fishing for spring Chinook salmon on the Lower Columbia River because the post-release mortality on non-target stocks can be greatly reduced compared to a conventional gill net, without sacrificing catch efficiency.

We fished a 5" gill net in tandem with the 8" gill net on four occasions on the lower Columbia River near Camas, Washington to evaluate its potential for selective harvest of spring Chinook salmon. During this short test, the immediate mortality of adult spring Chinook salmon rose to 10%, compared to 0% in the 8" gill net during the same period. This increased mortality was likely caused by an increase in capture by mouth clamping in the 5" gill net rather than by tangling or by the body as in the 8" gill net.

In fall, 2001, we evaluated the feasibility of using the tangle net to capture marked coho salmon while releasing unmarked coho salmon near the mouth of the Columbia River. A variety of tangle net configurations were used and showed that this fishing method warrants further consideration if the mark rate is high. Immediate mortality of unmarked coho salmon was 17% but because 84% of the coho salmon were marked, relatively few unmarked coho salmon were killed.

In spring and fall, 2001, we tested the feasibility of using a floating trap net near the mouth of the Columbia River to capture marked salmon live and allow the release of non-target species and stocks. The trap net was ineffective at capturing fish.