Winter Distribution of Juvenile Coho Salmon (Oncorhynchus kisutch) Before and After Logging in Carnation Creek, British Columbia, and Some Implications for Overwinter Survival

Abstract

Numbers of juvenile coho salmon (Oncorhynchus kisutch) in streams are reduced substantially in winter compared to those that occur in summer. Most of this reduction occurs early in autumn with the onset of the first seasonal freshets. Stream sections containing adequate winter habitat in the form of deep pools, log jams, and undercut banks with tree roots and debris lost fewer fish during freshets and maintained higher numbers of coho in winter than sections without these habitat characteristics. These features provide shelter and reduce stream velocities. Microhabitats occupied by coho juveniles in winter after logging were unchanged from those described before logging — all microhabitats were characterized by low water velocities (≤ 0.3 m/s). Up to 48% of the coho population inhabiting stream sections with adequate shelter remained there by midwinter (Jan. 3). This percentage was typical of stream sections where at least some trees remained after logging. Streamside trees stabilized the banks and prevented their collapse. In contrast, two of three study sections that had been clear-cut logged had unstable banks which collapsed during winter freshets. Almost no coho remained in these sections in winter. Many coho emigrate from the main stream to seek the shelter of low-velocity tributaries and valley sloughs concurrent with the decline of coho populations in Carnation Creek during autumn and early winter. This seasonal shift in distribution reverses in the spring when large numbers of coho reenter the main stream. Fish overwintering in these sites have a high apparent survival rate. Before logging a 4-yr mean of 169 ± 44 coho entered one tributary (a slough called 750-m site) in autumn. Of these numbers entering, 72.2% came out in spring. During and after logging, an annual mean of 288 coho entered the same site. The apparent survival rate during and after logging was 67.4%, essentially unchanged from the prelogging value. Logging has neither reduced the numbers of coho juveniles that enter such sites in autumn to overwinter, nor reduced the numbers leaving these sites to reenter Carnation Creek in spring.

Winter Disturbances and Riverine Fish in Temperate and Cold Regions

“…McMahon and Hartman (1989) reported a series of experiments in which highly branched root material provided a preferred refuge from floods for juvenile coho salmon in artificial outdoor flumes by deflecting flows and creating current-protected, turbulence-free pockets. These experiments corroborate findings from the field, in which structurally rich accumulations of woody debris provided crucial refugia from floods for juvenile coho salmon (Tschaplinski and Hartman 1983).…”

“…Overbank flows result in a reconnection of nutrient-rich floodplain habitats, such as flooded fields or drainage ditches (Masters et al 2002, Durand et al 2011, and intermittent tributaries (Brown TG and Hartman 1988). Second, the high discharges cause higher velocities and shear stresses that can cause streambed scour (Seegrist and Gard 1972); bank erosion (Brown RS et al 2001); and in-stream structures, such as beaver dams, to wash out (Tschaplinski and Hartman 1983). The abiotic effects of flooding, and especially the corresponding erosive forces, are exacerbated in the presence of different types of river ice that confine and divert the flow and constitute direct scouring agents themselves.…”

“…The washout of overwintering habitat, such as woody debris or beaver ponds, can evoke a considerable to complete reduction in the densities of juvenile and adult fish in small streams (Seegrist andGard 1972, Tschaplinski andHartman 1983), either because of a decrease in carrying capacity or because of displacement or direct kills. Substrate scour leads directly to a mechanical grinding and crushing of substrate-bound species and life stages.…”

“…The longitudinal arrangement of refugia is particularly relevant in the presence of disturbances with a directional pattern, such as floods (Lake 2000). For instance, juvenile coho salmon densities increased by a factor of four following multiple winter freshets in a river section offering stable refugia (Tschaplinski and Hartman 1983). The fish probably washed in from lessstructured river sections, located in a large clearcut area farther upstream.…”

Nature and Functions of Cover for Riverine Fish.

“…Daytime predation risk from diurnally endothermic predators (birds and mammals) has been suggested as the major factor governing diurnal concealment (VALDIMARSSON and METCALFE, 1998;METCALFE et al, 1999). It also reduces downstream displacement and physical injury during freshets or ice formation (BUSTARD and NARVER, 1975;TSCHAPLINSKI and HARTMAN, 1983). At a higher scale, juvenile salmonids underwent shifts towards quiet lateral habitats, small tributaries, beaver ponds or springs with groundwater acting as a thermal refugium, all of which are overwintering sites with higher survival rates than main streams (BUSTARD and NARVER, 1975;CUNJAK and POWER, 1986;JAKOBER et al, 1998).…”