Impacts of Fish Stocking on Willow Flycatchers - Really?

As opening day for trout fishing season approaches, stories in the local papers often dwell on impending issues that might affect the number of fish stocked and stocking locations. Last year the stories focused primarily on issues surrounding the court order that mandated that the California Department of Fish and Game (DFG) prepare an environmental impact report detailing the effects of its fish stocking program. This year the stories have focused almost exclusively on the concern that the presence of willow flycatchers could preclude fish stocking. All of the media stories that I've read on this issue have failed to provide much context for this issue, focusing instead on how outside environmental groups are to blame for the current "crisis". The following is my attempt at clarifying how the willow flycatcher got wrapped up in the fish stocking issue and what the current DFG plans are related to this species. 

When I read the DFG fish stocking EIR-EIS I was quite surprised to see the willow flycatcher listed as a "decision species" ("decision species" were defined as those species potentially affected by hatchery and stocking programs). My surprise stemmed from the fact that there were no published studies suggesting such impacts on willow flycatchers. Furthermore, the interim court order that was developed jointly by the Center for Biological Diversity, Pacific Rivers Council, and the DFG (the intent of which was to prevent harm to sensitive species while the EIR-EIS was being written) focused only on fish and amphibians (13 and 11 species, respectively). So, how did the willow flycatcher end up in the EIR-EIS as a decision species? 

It turns out that it was the DFG that added the willow flycatcher to the decision species list, and without any prompting from environmental groups or the public at large. The rationale was basically that trout and willow flycatchers both depend on insect prey for a large proportion of their diets. Because the introduction of trout into mountain lakes dramatically reduces the biomass of aquatic insect larvae and the subsequent emergence of adult forms of these aquatic insects, the DFG reasoned that this reduction in adult aquatic insects could reduce prey availability for willow flycatchers. The willow flycatcher was on the DFG "radar screen" because it is listed as "endangered" under the California Endangered Species Act. 

The possibility that trout stocked into aquatic ecosystems could affect adjacent terrestrial ecosystems is a fascinating area of ecological study, and recent research in the Sierra Nevada shows that stocked trout have in fact altered the distribution and foraging habits of the Gray-crowned Rosy-Finch, one of the few species that nests in the alpine zone. This soon-to-be-published study provides evidence that trout-containing lakes had 98% fewer mayflies than did neighboring fishless lakes, and that Rosy-Finches were six times more abundant at fishless lakes than at fish-containing lakes as a result (mayflies make up a significant fraction of Rosy-Finch diets during the emergence period). However, it is a stretch to extend these Rosy-Finch findings to willow flycatchers, but that is exactly what the DFG did. 

In the EIR-EIS the DFG suggested that impacts of trout on willow flycatchers were possible, and proposed to mitigate these impacts by conducting a "pre-stocking evaluation" at each mountain lake that receives hatchery trout and that lies within the range of the willow flycatcher. As stated in the EIR-EIS (page 4-100), "Under the protocol, each stocking location shall be evaluated in a stepwise fashion to determine whether interactions between stocked trout and willow flycatchers may occur and to evaluate the potential for trout stocking to result in an (sic) substantial effect on willow flycatchers. If such an impact is determined likely, then DFG shall either cease stocking at that location or develop and implement, prior to stocking at that location, an ABMP". (ABMP = Aquatic Biodiversity Management Plan).

With the EIR-EIS finalized in January of this year, the upcoming opening of the trout fishing season is the first in which all fish stocking is required to meet the guidelines specified in the EIR-EIS. The range of the willow flycatcher includes the eastern Sierra Nevada so technically no lakes can be stocked in preparation for the season opener unless a pre-stocking evaluation for willow flycatchers has been conducted, and until a few weeks ago no such evaluations had been conducted for stocked waters anywhere in California. And how can evaluations be conducted when most mountain lakes are still surrounded by snow and it will be weeks before willow flycatchers arrive to breed? The DFG has been scrambling to figure this out and the ensuing confusion has generated a slew of rumors, including how the Center for Biological Diversity forced the DFG to include the willow flycatcher in the EIR-EIS and how the willow flycatcher is being used as an excuse by bureaucrats in Sacramento (including DFG bureaucrats, in some version of the rumors) to shut down the fish stocking program. That these rumors have little basis in fact has not stopped the local media from reporting and elaborating on the rumors and innuendo. 

So, here is my take on all of this. While I applaud the DFG for acknowledging that fish stocking impacts likely extend beyond the water's edge and can in some cases impact songbirds and other terrestrial predators, it is ill-advised to change fish stocking management without a scientific rationale. There are no data suggesting that willow flycatchers are negatively impacted by fish stocking. The current lack of evidence certainly does not indicate that no such impacts are occurring, but the lack of any scientific evidence makes designing effective mitigation essentially impossible. I would have much preferred a more cautious approach in which the DFG acknowledged the potential for impacts of fish stocking on willow flycatchers and committed to studying the issue over the next year. Subsequent management actions and mitigation could then have been designed based on the results of the accumulated evidence.


I never thought I would criticize the DFG for being overly aggressive in developing mitigation strategies to reduce impacts of fish stocking. Go figure.

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