Association of brook trout andOncorhynchusspp. with large wood jams in a Lake Superior tributary in a northern old‐growth watershed

Abstract

AbstractWood in streams functions as fish habitat, but relationships between fish abundance (or size) and large wood in streams are not consistent. One possible reason for variable relationships between fish and wood in streams is that the association of fish with wood habitat may depend on ecological context such as large‐scale geomorphology. We studied the relationship between salmonid assemblages and large wood jams (LWJ) in four settings that differed geomorphically at the scale of the stream corridor along a tributary toLakeSuperior in old‐growth conifer–hardwood forest in northernMichigan. The focal fish species of this study were brook trout (Salvelinus fontinalis), which were wild in the stream. Relocation efforts for coaster brook trout (an adfluvial life history variant of brook trout) were ongoing in the study stream. We measured fish abundance and length in pairs of pools of similar size and substrate, but varying in the presence ofLWJ; this allowed us to evaluate associations of fish simply with the presence ofLWJrather than with other channel or flow‐shaping functions ofLWJ. The length ofOncorhynchusspp. and young introduced brook trout was not strongly correlated withLWJpresence; however, the presence ofLWJin pools was positively correlated with larger wild brook trout. We also found that the correspondence ofLWJwith the abundance of salmonids appears to be moderated by the presence of alternative habitat in this relatively natural, old‐growth forest stream.