Nonnative fish stocking alters stream ecosystem nutrient dynamics

Abstract

Each year, millions of hatchery-raised fish are stocked into streams and rivers worldwide, yet the effects of hatchery-raised fish on stream nutrient cycles have seldom been examined. We quantified the influence of supplemental nonnative fish stocking, a widespread recreational fishery management practice, on in-stream nutrient storage and cycling. We predicted that supplemental, hatchery-raised brown trout (Salmo trutta) stocking would result in increased N and P supply relative to in-stream biotic demand for those nutrients and that stocked fishes would remineralize and store a significantly greater amount of N and P than the native fish community, due to higher areal biomass. To test these predictions, we measured the biomass, nutrient (NH -N and soluble reactive phosphorus [SRP]) remineralization rates, and body carbon, nitrogen, and phosphorus content of the native fish community and trout stocked into four study streams. We then estimated fish growth rates to determine species-specific nutrient sequestration rates in body tissues for both stocked and native fish and measured ammonium and phosphorus uptake rates to determine the relative influence of net fish nutrient remineralization on stream nutrient cycles. When brown trout were stocked in these systems at density levels that were orders of magnitude higher than ambient native fish density, they provided a sizeable source of NH -N that could account for up to 85% of demand for that nutrient. Stocked trout had minimal effects on in-stream SRP cycles even at high release densities, likely due to low per capita SRP excretion rates. A unique feature of our study was that we evaluated the temporal component of the stocked trout nutrient subsidy by estimating the number of fish removed from the system through natural mortality and angler harvest, which indicated that the stocked trout subsidy lasted approximately 6-8 weeks after stocking. By combining population models with areal nutrient excretion rates and estimates of biotic nutrient uptake, we showed that trout stocking provided a strong pulsed nutrient subsidy.

Estimating the effects of non‐native species on nutrient recycling using species‐specific and general allometric models

“…Stream temperatures were recorded during sampling. Two sites dominated by stocked salmonids were removed from further analyses because the transient effect of put-and-take stocking efforts on ecosystem functioning was not the focus of this project (see Alexiades, Flecker, & Kraft, 2017).…”

“…Two sites dominated by stocked salmonids were removed from further analyses because the transient effect of put-and-take stocking efforts on ecosystem functioning was not the focus of this project (seeAlexiades, Flecker, & Kraft, 2017). Two sites dominated by stocked salmonids were removed from further analyses because the transient effect of put-and-take stocking efforts on ecosystem functioning was not the focus of this project (seeAlexiades, Flecker, & Kraft, 2017).…”

Do alpine macroinvertebrates recover differently in lakes and rivers after alien fish eradication?

“…Their response to fish eradication is clearly linked to the community recovery from a series of direct and indirect impacts involving the entire lake ecosystem (Knapp et al, 2001;Tiberti et al, 2019a). Although macroinvertebrates are highly resistant to fish introduction in streams (present study; Wooster 1994), fish are known to affect the integrity of alpine rivers in a variety of other ways, e.g., affecting native semiaquatic vertebrates (Bosch et al, 2006(Bosch et al, , 2019 and fish populations (Ross, 1991;Dunham et al, 2002), nutrient cycles and ecological connection with riparian habitats (Alexiades et al, 2017). Macroinvertebrate recruitments and the availability of spatial refugia could provide potential explanations for the high resistance of stream macroinvertebrates, which is consistently high, even when drift is low (i.e., outflowing river).…”

Taxonomic and geographic gaps in understanding the functional effects of imperilled fishes on freshwater ecosystems

“…This focus is likely in part because the migratory patterns of salmonids from marine to freshwater ecosystems make them an ideal study system to investigate resource flows between environments using before–after comparisons (Levi et al, ). Further, salmonids that have been stocked to promote fisheries create a convenient system to investigate the effects of species additions on ecosystem function (Alexiades, Flecker, & Kraft, ). Salmonids are relatively charismatic fishes which are sought in both recreational and commercial fisheries, which may be why they are so heavily studied relative to other imperilled species.…”