The effect of waterfalls in streams on structuring salmonid fish populations and alternative life histories

Background/Question/Methods

Understanding how landscape features structure biodiversity has both ecological and management implications. Within stream systems, barriers such as waterfalls can separate species, populations, and even life history forms. Here I explore how waterfalls can structure fish populations of brook trout (Salvelinus fontinalis) and separate alternative life histories of rainbow/steelhead trout (Oncorhynchus mykiss). For the first project, I characterized genetic structure of brook trout sampled from 6 tributaries of the Swift and Diamond Rivers in New Hampshire using 12 microsatellite loci. Using genetic clustering methods, I related population structure to natural barriers such as waterfalls as well as corridors for movement within the river system. More recently, I am exploring how waterfalls can separate life history forms within species and the potential consequences for local stream ecology. Within tributaries of the South Fork Eel River in Northern California, both the resident and ocean-migrating form of Oncorhynchus mykiss occur. We hypothesized that waterfalls separate the two life history forms. To explore this hypothesis, we are using SNPs to assign a genotype, resident or migratory, to O. mykissbelow and above replicate waterfalls. 

Results/Conclusions

I observed a complex metapopulation of brook trout among six tributaries and two mainstem rivers in New Hampshire. I found two isolated populations above waterfalls that exhibited low levels of genetic diversity (H_O = 0.43±0.25, 0.30±0.29). The other tributaries and rivers were clustered together genetically as a metapopulation with a high level of genetic diversity (H_O = 0.62±0.11). We assigned migratory fish from mainstem rivers back to the metapopulation; the mobility of these individuals is likely a source of gene flow. This complex structure of brook trout highlights the importance of waterfalls and corridors in structuring headwater populations. My most recent research suggests that evolutionary change can also be exhibited above vs. below waterfalls in other salmonid fishes. In Northern California, we have characterized how small cascades in headwater streams influence life history distribution of O. mykiss: above cascades we find a higher proportion of resident alleles, and below cascades we find a higher proportion of migratory alleles. Ongoing work is exploring how life history correlates with feeding morphologies and the potential differences in food webs above vs. below waterfalls.