Hatchery rearing density affects poststocking survival

ABSTRACT Objective Research has indicated that salmonids raised in a hatchery and stocked into the wild have lower future survival and reproductive success than wild fish. Understanding whether hatchery rearing practices affect populations after stocking can help to maximize the quality, quantity, and economic value of fish stocked for conservation and recreational purposes. The goal of our study was to test whether and to what extent rearing density affects poststocking survival of Rainbow Trout Oncorhynchus mykiss when stocked into a put-grow-and-take fishery. Methods Rainbow Trout were raised for 3 months in the hatchery at two densities (high = 51,948–13,072 fish/m3, rearing index 2.0, 1,260 metric units; low = 12,987–3,268 fish/m3, rearing index 0.5, 315 metric units). A subset of the fish were PIT-tagged and stocked into Parvin Lake, Red Feather Lakes, Colorado. Recaptures of tagged fish occurred every 2 weeks for the first 2 months poststocking and at 7 and 12 months poststocking. Results At the time of stocking, there were no statistical differences in Fulton’s condition factor, total length, weight, or hepatosomatic index for each treatment. Recapture data indicated that there was no difference in Fulton’s condition factor and hepatosomatic index after stocking. Apparent poststocking survival was higher for fish raised at low density in the hatchery compared with those raised at high density. Conclusions Our study suggests that rearing density affects fingerling Rainbow Trout poststocking survival. Stocking more fish due to fish being reared at higher densities within the hatchery did not result in more total fish remaining after a year in the lake compared with the low-density treatment. Rearing fish at high densities does not provide a benefit of more fish after stocking and may result in more disease risk in the hatchery.