Effects of side channel structure on productivity of floodplain habitats for juvenile coho salmon

Numerous artificial side channels have been constructed in British Columbia and the Pacific Northwest to compensate for habitat loss from floodplain development. We reviewed data from published studies on natural and restored side channel habitats to determine how design features influence productive capacity for juvenile coho salmon Oncorhynchus kisutch. Average density and biomass of coho salmon parr were significantly higher in stream-type side channels (3.4 parr/m(2) and 8.01 g/m(2), respectively) than in pond-type side channels (0.8 parr/m(2) and 2.37 g/m(2)). Although total part biomass was three times higher in stream-type side channels, average part weight was 47% lower, suggesting greater density-dependent limitation of growth from higher recruitment of juveniles to stream-type habitats. Parr abundance declined from late summer to early spring in both side channel types but appeared to decrease more quickly in stream-type side channels, suggesting greater self-thinning in stream-type habitat from mortality or immigration to slower pond or main-stem habitat as fish sought lower velocities for overwintering. Fish density in a single off-channel complex that contained both stream and pond habitats (fish were able to move between habitats) was also higher in stream habitats, although fish were significantly larger in pond habitats than in stream habitats. Parr density in stream-type side channels was constant with increasing channel size, whereas density in pond-type side channels was a decreasing function of side channel area. Smolt production data were more limited and variable, and production was not significantly different between stream- and pond-type side channels. Smolt density (smolts produced/m(2) of channel habitat) was also a decreasing function of total side channel area, indicating that the optimal side channel habitat size (or pond size within a side channel complex) was below 5,000-10,000 m(2). Side channels that incorporate a diversity of flowing- and standing-water areas are most likely to provide the variety of habitats (i.e., spawning, summer rearing, and overwintering) required by salmonids to complete their life cycle.