Salmonid Field Protocols Handbook: Techniques for Assessing Status and Trends in Salmon and Trout

Tangle Nets

Charmane E. Ashbrook, Kyong W. Hassel, James F. Dixon, and Annette Hoffmann

doi: https://doi.org/10.47886/9781888569926.ch16

Tangle net (or tooth net) research first came about in the U.S. Pacific Northwest and Canada as a way to evaluate whether a conservation goal could be achieved for a salmon species or stock of concern. Specifically, researchers wanted to determine the survival rates of nontarget salmon captured and released from tangle nets. This knowledge—whether the survival rate is acceptable—enables fish managers to allow commercial “selective fishing” while protecting weak stocks. Commercial selective fishing is deemed successful if conservation goals are achieved for the species or stock of concern and if a harvest goal is met to make the fishery economically viable.

Work done by Vander Haegen et al. (2002, 2004) compared tangle nets with traditional gill nets and showed that the use of a tangle net, with careful handling techniques, can reduce spring chinook salmon Oncorhynchus tshawytscha mortality. The handling techniques included an abbreviated soak time, a shorter net, and meticulous removal of fish from the net. They also included the use of a revival box, which Farrell et al. (2001a) showed could reduce the physiologic stress on coho following their capture in a gill net. Research by Ashbrook et al. (2004a, 2004b) has provided further information for estimating survival of bycatch following their release from tangle nets. Consequently, Columbia River fishery managers have instituted selective tangle net fisheries for upriver spring chinook salmon. In addition to their use in selective fishing studies, applications for tangle nets include capturing fish to apply tags, and for broodstock and biosample collection.

Selective capture and subsequent release of nontarget bycatch is possible because the tangle net can efficiently capture salmonids in large rivers and estuaries in short time periods with low immediate mortality rates and relatively low postrelease mortality rates (Vander Haegen et al. 2002 and 2004; Ashbrook et al. 2004). Experienced gill-net fishermen can transition easily to tangle nets, which operate similarly. Tangle nets are visually comparable to gill nets (see Figure 1), and the two gears are fished in the same manner; however, the mesh of the tangle net is smaller than that of a conventional gill net, which results in the fish being caught by the snout or teeth. Ideally, the smaller mesh size increases the chance for fish to continue respiring in the net so they can be released live.

Although different species can be easily sorted, stock-selective fisheries in the U.S. Pacific Northwest and Canada rely on a physical mark—in most cases, adipose fin excision—that allows fishers to distinguish easily between stocks of hatchery-origin fish (which can be retained) and unmarked naturally spawning stocks (which must be released).