Recovery of a headwater stream population of brown trout after a fish kill in southeastern Minnesota, USA

Abstract

After a complete fish kill on the 2-km-long, spring-fed headwater section of Garvin Brook, the brown trout (Salmo trutta) population was studied over the next 29 months to examine the relative roles of spawning migration of adults and natural reproduction in the population recovery process. Spawning redd counts during two spawning seasons post-kill, and repeated population surveys in kill and reference zones, were conducted to assess adult and total population changes and shifting age structures. One month after the kill, spawning adult trout were observed in the kill zone. An estimated 100 to 200 adult trout spawned in the kill zone during each of the two spawning seasons after the fish kill, 40% fewer than spawned in the reference reach. Total trout populations were similar between kill and reference zones 12 to 16 months after the kill due to proportionally (80% versus 40 to 60%) larger numbers of young trout spawned within the kill zone, but adult trout remained 4X more abundant within the reference reach than in the kill reach until 25 months after the kill. Age structures still differed between kill and reference zones 29 months after the kill, due to few large adults within the kill reach. Ultimately, natural recovery of the brown trout population in the headwaters of Garvin Brook was accomplished through a combination of spawning migrations of adult fish from downstream unimpacted reaches, reproduction producing large numbers of young fish, and subsequent recruitment of those young fish to the adult age classes.