Changes in aquatic insect emergence in response to whole‐lake experimental manipulations of introduced trout

Abstract

1. Insects emerging from mountain lakes provide an important food source for many terrestrial predators. The amount of insect subsidy that emerges from lakes is influenced by predator composition, but predator effects could be ameliorated by greater habitat complexity. We conducted a replicated whole-lake experiment to test the effects of introduced fish predators on the abundance and composition of aquatic insects within and emerging from the littoral zone of 16 mountain lakes in the Trinity Alps Wilderness in northwestern California. 2. Study treatments matched the fisheries management options being implemented in California's wilderness areas: (i) continued stocking with non-native trout, (ii) suspension of stocking, and (iii) suspension of stocking and removal of fish. We also included four naturally fishless 'reference' lakes. We compared abundances and biomass of emerging aquatic insects before treatments were initiated and for 3 years following their establishment. Abundances of benthic insects were also compared in the third year post-treatment. 3. Trout removal rapidly increased abundances of mayflies, caddisflies, and insect predators, and overall insect biomass emerging from lakes. No significant differences were found between the suspension of stocking lakes and continued stocking lakes. Fish density was a more important predictor of aquatic insect emergence than habitat complexity. 4. Mayfly larvae responded positively to fish removal and caddisfly larvae tended to be more abundant in lakes without fish, but we did not detect effects on abundance of predatory insects. However, we found large insect predators in shallower water in lakes with fish compared to fish removal or fish-free reference lakes. 5. These results provide insights into the continuing effects of past and current fish stocking practices on the flow of insect prey from mountain lakes into the neighbouring terrestrial environment. We also show that these consequences can rapidly be reversed by removing non-native fishes.

Semidiscrete biomass dynamic modeling: an improved approach for assessing fish stock responses to pulsed harvest events

“…This discrete input of fish biomass can have major food web impacts through an instantaneous increase in the consumption required to support these newly stocked fish. For example, Pope et al (2009) found that trout stocked in alpine lakes altered the alpine lake ecosystem by preying on emerging insects that would otherwise be a terrestrial subsidy. SDBDMs could be used to more realistically explore the consequences of stocking timing and amount on prey dynamics in food web and ecosystem models.…”

Variation in reciprocal subsidies between lakes and land: perspectives from the mountains of California

“…In contrast to larger bodied taxa, the effects of introduced trout on smaller bodied Diptera are mixed. In the Sierra Nevada, the total abundance of dipteran larvae tends to be reduced in lakes with trout, an effect that is strongest at lower elevations (R.A. Knapp, unpublished data), and chironomids tended to emerge in greater numbers from lakes in the Klamath Mountains where fish were removed (Pope et al 2009). However, a study of high-elevation lakes in the southern Sierra Nevada found that mosquitos (Culex spp.)…”

“…The larvae of most large-bodied aquatic insect taxa, including Odonata, Ephemeroptera, and Trichoptera, are negatively affected by trout in both the Sierra Nevada (Knapp et al 2001b and the Klamath (Pope et al 2009;Pope and Hannelly 2013) mountains. This translates into reductions in the biomass of aquatic insects emerging from trout-containing lakes (Finlay and Vredenburg 2007;Pope et al 2009;Epanchin et al 2010). Importantly, the effect of trout on insect abundance varies with elevation for some taxa.…”

“…However, particular taxa that are rare at lower elevation lakes are among the dominant taxa at high-elevation lakes (e.g., Ameletus mayflies and Desmona caddisflies). While small-bodied Diptera, especially Chironomidae, tend to be the most numerically abundant aquatic insects in California's mountain lakes, the total biomass of emerging aquatic insects is often more closely related to the abundance of larger bodied insects, as they have a higher biomass per individual than chironomids (Finlay and Vredenburg 2007;Pope et al 2009). Total emerging biomass is higher in littoral zones than in profundal zones (S. Chandra, unpublished data) and peaks in early to midsummer (K.L.…”

“…This creates a patchwork in which most lakes are now occupied by an introduced top predator, while a smaller number exist in their naturally fishless state. Trout have dramatic effects on aquatic communities in California's mountain lakes, and these effects tend to be at least as important as the effects of abiotic environmental variables, if not more so Pope et al 2009). Because introduced trout have dramatic and robust impacts on factors related to reciprocal lake-land subsidies, we also include fish in our conceptual model.…”