LB 2-14 - Socio-economic disturbance decouples habitat and life stage in a keystone species

Anthropogenic disturbance of stream ecosystems, often chronic in nature, has been studied extensively. However, when disturbance is driven by more than one resource policy over many decades, feedback between habitat evolution and biological adaptation can be disrupted and ecological function affected in unforeseen ways. We analyzed over 100 years of female Chinook Salmon length frequency trends associated with fisheries management and changes in available spawning substrate linked to flow regulation in a California river. Over time, lengths generally decreased, fluctuating with exploitation and hatchery production rates. Female size reduction, coupled with a degrading and coarsening channel, and perching peripheral habitat, indicates available spawning substrate may be too large to support the current salmon population. Assuming a salmon can move material ~ 10% of her body length, length frequency data and current substrate size distribution suggests that increasing salmon sizes to historic distributions could increase available spawning habitat by as much as 20%. Alternatively, decreasing substrate size within spawning riffles could support a greater proportion of the current population. To test the latter hypothesis and inform future management actions, we monitored two spawning riffles where large and small spawning gravel was placed on top of cobble. We observed immediate spawning activity increase that was more pronounced where smaller gravel was deposited. Following a decade of habitat decline, the two sites were both replenished with a medium gravel size. Elevated spawning use occurred immediately at both sites, concurrent with this intermediate size, further supporting our hypotheses. Sediment coarsening and habitat disconnect below dams, combined with reduced salmon size, suggest the natural spawning process may be decoupled from available habitat below dams in the foreseeable future without active and continuous intervention. Developing a coarse sediment budget for anadromous rivers below dams and actively managing salmonid population demographics through modified hatchery and size-selective harvest practices could produce immediate benefits for ecosystem services, including salmon populations, but will require continued maintenance and informed socio-economic goals to remain successful.