Keeping the wolf at the gate and the bass in the bag

Smallmouth bass make for a great fight on the line, and long ago home-sick easterners, reminiscent for their favorite sport fish, set out to introduce them in their new homes of the Pacific Northwest. The introduction of smallmouth bass sparked controversy even in the late 19th century. Archives from the Oregonian, the Pacific Northwest’s longest running newspaper, capture both sides of this debate, where some warned that “this predacious fish seems the natural enemy of all young fish” (1898), while others claimed “the bass will prove himself, if given an opportunity, the best friend of … our salmon and trout” (1893). ‘Frienemy’ they have remained. Fast forward to the present and fish and wildlife agencies in the states of Washington and Oregon continue to be tangled in a management dilemma surrounding smallmouth bass. These agencies are simultaneously charged with providing a thriving recreational bass fishery, while preventing their impacts to the economically valuable, and critically endangered salmon they, at times, feed on.

The impacts of bass on salmon can be drama tic, reducing individual runs by up to 35%, where bass and juvenile salmon occur at the same place, at the same time. Even if salmon don’t end up in a bass belly, bass may increase the stress on young salmon, and reduce their growth, as evidenced by experiments performed by Lauren Kuehne, a recent graduate of the Olden lab.

Bass continue to move in the Pacific Northwest, with and without additional human assistance. In many river systems bass were introduced in downstream areas where warm waters enhance their growth. Most managers that introduced them did not anticipate that bass would colonize upstream areas. And therein lies the problem; these cooler upstream areas also provide nursery habitat for some salmon species, including spring-run Chinook salmon that spend a full year in the river before migrating to the sea. In a recently published article in Freshwater Biology, myself and colleagues documented the upstream intrusion of bass into salmon rearing grounds in the John Day River, a river system in northeastern Oregon. Bass were originally introduced to the lower John Day River to enhance recreational fishing opportunities, but bass didn’t stay put. Since their introduction in the 1970’s, bass have moved further and further upstream, and they now co-occur with juvenile Chinook salmon during their first summer. This could be especially problematic because juvenile Chinook don’t recognize bass as predators, an oversight likely resulting from an unshared evolutionary history between predator and prey (see paper).

All climate forecasts for the Pacific Northwest point to warmer streams, which both enable the upstream expansion of bass in river systems there, while reducing quality habitat for young salmon. My current dissertation research within the Olden lab is focused on predicting how far upstream bass will move and how warming stream temperatures will alter the overlap between predatory bass and their potential salmon prey. But all is not lost. There are strategies available to stem the upstream tide of bass, to keep the wolf at the gate. One that we are currently investigating is the potential to reduce bass upstream movement through riparian restoration, encouraging the re-growth of stream side vegetation that has been removed from the land or overgrazed by cattle. These plants provide critical shade by intercepting the warming rays of the sun. The conservation of intact riparian areas and the re-planting of heavily grazed areas could provide a dual benefit of preventing bass from moving further into salmon rearing grounds while helping to maintain cold temperatures that young salmon need to grow adequately before their sojourn to the sea. This work relies on climate forecasts derived for the Pacific Northwest, paired with stream temperature models that translate predicted changes in air temperature and precipitation into forecasted stream temperatures. In turn, stream temperatures can be used to predict bass occurrence, because their upstream movement is strongly limited by cold water temperatures. We are now in the process of using these predictions to strategically prioritize restoration efforts to limit the further upstream colonization by bass. We anticipate these results will be available early next year.

In the meantime, managers in Washington are proposing a strategy of their own to limit the impact of bass on young salmon. They would like to raise the bag limit on bass, that is, the number of bass that can be kept by fisherman on a given fishing day within parts of the Columbia and Snake Rivers, and their tributaries (WDFW rule change). Similar proposals are under consideration by fishing managers within Oregon. This potential rule change may meet opposition from a warm water fishing community that prizes their favorite fighter, both intrinsically and in the form of millions of dollars in annual revenue resulting from the fishery. Managers will have to tread the difficult ground of balancing the value of this fishery against its cost to salmon recovery efforts, as tribal, local, state and federal government agencies seek to save marginal salmon runs and boost salmon production in others.

- Dave Lawrence