Age-dependent juvenile mortality explains delayed smolting in a declining steelhead population

Fish populations in the Laurentian Great Lakes have undergone major changes over recent decades. Demographic changes in fish populations are often accompanied by changes in life history strategies that reflect variation in mortality applied to different life stages. We examined the steelhead (Oncorhynchus mykiss) population in Black Bay, Lake Superior, where a naturalized steelhead population experienced significant demographic changes over three decades. Initial increases in density in Portage Creek, Black Bay (1995–2007) were associated with reductions in angler mortality (applied to adult steelhead), but the population declined (2007–2018) despite no further changes in angler mortality. Simultaneously, the dominant life history among several Black Bay tributaries changed between 2013 and 2017, with returning spawners becoming primarily represented by individuals who smolted at age 2 (a more common pattern across other Lake Superior populations) from those who primarily smolted at age 1. To assess whether changes in juvenile mortality could explain observed life history shifts in surviving spawners and recent steelhead population declines, we constructed Leslie matrices with differential mortality applied to observed demographics from Portage Creek steelhead to evaluate scenarios representing increased parr (in-stream) mortality and increased smolt (in-lake) mortality. The observed demographic changes in Portage Creek (i.e., shift from 1 to 2 year smolts and associated population declines) were predicted by a model applying size-dependent smolt survivorship to female steelhead in a fashion consistent with increased in-lake mortality of age 1 smolts. This study provides an example of population-level responses to increased stage-specific mortality and offers an example of how in-lake conditions can influence potamodromous salmonids in the Laurentian Great Lakes.