The North Fork of the Tuolumne is a quaint little stream, with overhanging riparian vegetation, warm summer temperatures, and a low gradient. The North Fork is an angler’s paradise, with aggressive rainbows holding in the bubble curtains, and fired up smallmouth bass in the pools. Few of the annoying ‘squawfish’ that inhabit the rest of the Tuolumne occur in this little stream.
To anyone a passion for native California fishes, the stream above does sound quite pleasant (images of roach, hardhead, and pikeminnow should be swimming around in your skull). Until, that is, you read about the smallmouth bass. Smallmouth are native a minnow’s nightmare. Smallmouth typically displace hardhead and California roach completely where the two species co-occur.
Because of the effect that smallmouth have in the stream ecology, a major question going into the survey was simply: where did these guys come from? The North Fork is a free flowing river, with no reservoir upstream in which the smallmouth could obviously originate.
There are still two possible directions of dispersal possible for the smallmouth, upstream and downstream. If the bass are originating from the upper reaches of the North Fork, our surveys would show bass above and below fish barriers. Initial reconnaissance using satellite imaging showed farm ponds adjacent to the stream in the upper reaches. This provided a possible upstream source for the bass. If the bass had originated from the river, (the invasion direction would thus be upstream) we would expect the distribution of bass in the North Fork to closely mirror rainbow trout, which almost certainly do migrate up stream from the mainstem. The largest uncertainty in the upstream theory involved the bass’ ability to migrate up through the cold waters of the mainstem Tuolumne to the warm water haven in the North Fork.
The downstream dispersion hypothesis was the slight favorite going into the surveys on the river, however our surveys found that the smallmouth and rainbow trout we both limited by a large barrier falls about a mile upstream of the mainstem (see graphs below). Based on our data, the smallmouths are invading the North Fork in an upstream direction, from the mainstem.
In the absence of smallmouth and rainbow trout above the falls, native minnows dominated the assemblage. The only hardhead in the system we observed above the falls. As well as the only live California roach. The clear dichotomy between the upper and lower North Fork show the devastating effect that the smallmouth have on a stream ecosystem, and that these feisty little bass are not entirely all they are cracked up to be.
