Bass and salmon: who’s for dinner?

I went to an interesting lecture this week at the University of Washington Fisheries department on the potential effects of climate change on the interactions between Chinook salmon and smallmouth bass. 

Smallmouth bass

Juvenile Chinook salmon

Smallmouth bass are native to the Northeast and upper Midwest, but like many other fish species, they were spread across the United States by avid fishermen during the 20^th century. One method of spreading bass was to put them in large milk containers on trans-continental trains, and then stop and dump fish in every body of water they passed along the way. By the second half of the 20^th century, stocking fish became even easier, with the invention of planes.

Fish being released from a plane to stock a lake for fishing

As with many of the things we did to nature in the 1900s, we are now beginning to understand the consequences of planting bass. In western rivers, one major impact of non-native bass on Pacific salmon is predation. Bass eat juvenile salmon, and in rivers where they co-occur, they form a predatory gauntlet for the juvenile salmon migrating out to the ocean each year. As one fishing website declares, “smallmouth bass are aggressive freshwater fish that will readily engulf nearly anything that they can fit in their mouths.” And juvenile Chinook certainly fit that description.

The fact that Pacific salmon populations have crashed over the past half century as a result of climate change (hotter river temperatures), freshwater habitat loss (dams etc.), and overfishing, is well-known. There are lots of studies showing that rising river temperatures negatively affect salmon, but what I found really interesting about this lecture was the discussion of how climate change (in the form of hotter rivers) could affect the interaction between bass and juvenile Chinook salmon. Salmon are cold-water fish and can only tolerate water up to about 75F, so as rivers warm, juvenile salmon are forced higher up into watersheds to find suitably cold water to rear in. At the same time, small-mouth bass are limited in the opposite direction – if the water is too cold, they can’t spawn. So as rivers warm, they are able to move further and further up watersheds. In the John Day River in Oregon, smallmouth bass and juvenile Chinook rearing habitat now overlap. This range shift and overlapping habitat lead to new questions: will bass presence negatively affect juvenile salmon in other ways than direct predation? It’s no longer just a predatory gauntlet, a one-time-only deal that the juvenile Chinook have to face as they out-migrate. Now their daily interactions and behavior, and possibly their growth potential, could change as a result of the encroaching bass. On a much larger scale, it is these kind of unforeseen effects of climate change that make it so hard to predict.