Fish Nutrient Excretion Rates New York

Each year, millions
of hatchery raised fish are stocked annually into streams and rivers worldwide,
yet the effects of hatchery raised fish on stream nutrient cycles has seldom
been examined. We quantified the influence of supplemental non-native 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 (NH4+-N and soluble reactive phosphorus)
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 NH4+-N that could account for up to 85% of
demand for that nutrient. Stocked trout had minimal effects on in-stream
soluble reactive phosphorus (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.