Fishing / Shellfishing - Selective Fishing
Date Published: July 2007
Number of Pages: 78
Publication Number: FPT 06-06
Author(s): C.E. Ashbrook, J.F. Dixon, K. E. Ryding, K.W. Hassel, and E. A. Schwartz
In the presence of endangered and threatened species, the status of bycatch animals not harvested becomes just as or more important than the number harvested. One way to minimize bycatch mortality is through live capture and selective harvest methods, more commonly referred to as selective fishing. Selective fishing methods have been successful for spring Chinook (Oncorhynchus tshawytscha) captured in freshwater. However, their success for coho (O. kisutch) and fall Chinook in an estuarine environment was unknown. Survival differences between these two environments were expected because estuary water temperatures may be relatively warmer, salmonids are captured as they are undergoing physiologic transformation from salt to fresh water, coho and fall Chinook spawn soon after capture and release, and because of differences in species and race. The goals of this study included estimating survival, evaluating condition at capture, comparing fishing gears, testing a modified purse seine as a way to capture control fish, and evaluating reproductive success to learn if selective fishing is a reasonable management tool in an estuary setting.
During 2003, experienced commercial fishers used 8.9 cm tangle nets (3.5" mesh size, multifilament net) and 14.6 cm conventional gill nets (5.75" mesh size, monofilament net) combined with revival boxes, shorter soak periods, shorter nets, and careful handling techniques. Sport fishers used barbless hooks and careful handling techniques. Coho and Chinook salmon captured in these gears were tagged and released for later recovery in sport fisheries, commercial fisheries, at hatchery racks, and during spawning ground surveys. A modified purse seine was used to collect control fish to obtain actual (as opposed to relative) survival estimates. A comparison of the reproductive success of coho that had been captured and released from these gears with coho that had not been previously caught was performed at the Forks Creek Hatchery.
For coho, we found that the tangle net was as effective at capturing fish as the conventional gill nets but that the fish were somewhat smaller. Fish captured by the tangle net were in better condition than fish caught in traditional gill nets and had less injury to their bodies. The immediate survival (from capture to release from the boat) of adult coho salmon caught in the tangle net was not significantly different than for fish caught in the gill net. We were not able to detect differences in relative recovery rates, and hence long-term survival, between tagged adult coho released from the two net types. More non-target species were captured in the tangle net than the gill net. Egg-to-fry survival differences occurred by gear type, with the progeny of tangle net captured adults faring better. Too few fish were captured in the sport gear to evaluate the effects of a selective fishery for coho.
For every three coho captured in the commercial gears, one Chinook was captured. For Chinook salmon, the tangle net was as effective as the gill net at capturing fish, the fish were in better condition, had fewer bodily injuries, were somewhat smaller, and had higher immediate survival as compared to Chinook caught by the gill net. We could not detect clear differences in tag recovery rates, and subsequently long-term survival, between Chinook caught in tangle nets and those caught in gill nets. Although Chinook are larger than coho, the 8.9 cm net acts as a tangle net and the 14.6 cm net acts as a gill net for this salmon species. Too few fish were captured in the sport gear to evaluate the effects of this type of selective fishery for Chinook.
Further research is needed to provide managers with survival estimates for bycatch salmon released during an estuarine commercial selective fishery. Although our study developed methods to estimate survival, and control fish captured using a modified purse seine method were in excellent condition, the modified purse seine did not provide a suitable control for Willapa Bay because this gear could only be used in a few locations. All salmon captured using the modified purse seine were in excellent condition, hence we believe this gear should be more completely evaluated in the future.
To estimate long term survival, we recommend further studies using tags with higher detection probabilities. We observed that many coho and Chinook captured in tangle nets and gill nets in this environment were lethargic and needed to be revived. This differs from similar studies in the Columbia River, where most spring Chinook and steelhead were in vigorous condition at capture. The reason for this difference is unknown and could be from the following factors: low dissolved oxygen from warm water temperatures, the immediacy of physiologic transformation, or the immediacy of spawning for coho and fall Chinook. Because many fish needed to be revived, we expect that successful commercial selective fisheries in this setting will require a high ratio of marked hatchery to wild fish.