What’s the effectiveness of stocking actions in small creeks? The role of water discharge behind hatchery trout downstream movement

Abstract

Fish stocking to enhance freshwater fisheries or to improve the conservation status of endangered fish species is a common practice in many countries. Little is known, however, of the effectiveness of these practices in spite of the high efforts and investments required. The movement of subadult/adult hatchery-released brown trout Salmo trutta L. was studied by passive telemetry in a small tributary of Lake Lugano (i.e., Laveggio Creek, Canton Ticino, Switzerland). Hatchery fish, together with some resident wild individuals sampled during electrofishing surveys, were tagged with Passive Integrated Transponders (PIT) tags. Hatchery fish were released upstream and downstream a submersible monitoring antenna, which was anchored to the streambed in a pass-over orientation. The number of hatchery fish detected daily by the antenna (divided between fish released upstream and downstream the antenna) was analyzed in relation to the daily water discharge, to search for similar patterns in their fluctuation over time. Only the movement of fish released upstream the antenna displayed a significant relationship with water discharge, with the highest number of fish detected during periods of high-water flow, occurring after heavy rains. High-water discharge events had a significant role in hatchery trout downstream movement in our study site, likely acting as a driver for the downstream migration to Lake Lugano. Such events contributed to the poor effectiveness of stocking actions in this small tributary, providing further evidence against stocking strategies based on subadult/adult fish.

Movement, not survival, differs between wild and hatchery‐reared imperiled desert fishes

“…This relationship of higher emigration rates from the stocking location was also observed in our longer‐term analysis examining apparent survival, which we attribute to the continued high emigration rates of hatchery fish. Surprisingly, and contrary to our predictions, high‐flow events did not have an observed effect on estimates of apparent survival for either wild or hatchery fish, a result that contrasts with other studies, especially in small streams (Vincenzi et al 2012; Brignone et al 2022). One potential explanation are these species ability to resist displacement by floods, as evidenced by a lack of long‐term population declines following large floods (Minckley and Meffe 1987; Ward et al 2003; Gido et al 2019; Hedden et al 2022).…”