Pattern and mechanism of resource partitioning between stream populations of juvenile coho salmon (Oncorhynchus kisutch) and coastal cutthroat trout (Salmo clarki clarki)

Anadromous populations of juvenile coho and cutthroat frequently occur sympatrically in coastal streams of western North America. Their apparently similar external morphology and macrodistribution in streams suggest they might broadly overlap in resource use. This study examines resource partitioning between these two salmonids in streams and in laboratory experiments. In small coastal streams, during late summer low flow, sympatric populations of juvenile coho and cutthroat were partially segregated in microhabitat use and diet. Abundance and biomass of coho were highest in low velocity microhabitats (pools, glides) whereas that of cutthroat were highest in riffles and lowest in pools. In all microhabitats examined, adult insects were more common in the diet of coho than cutthroat, whereas chironomid larvae and pupae showed the reverse pattern. In laboratory stream experiments, partitioning of space between underyearling coho and cutthroat from sympatric populations was similar to that in streams. When tested together in summer at 12-14 °C, coho numerically dominated pools and trout dominated riffles. When tested separately, their microhabitat use was similar (60-75% of either species occupied pools). In winter, at 3 °C, both species showed strong preference for pools and overhead cover, whether tested separately or together. In laboratory stream experiments, coho and cutthroat fry displayed a similar array of aggressive activity although non-contact aggression was more frequent in coho and nipping more frequent in cutthroat. Both salmonids were most aggressive when food was presented, regardless of season. When tested together in summer at 12-14 °C, intensity of aggression was high and cutthroat more actively defended territories in riffles and coho in pools; in winter at 3 °C, aggression was low and both species weakly defended pools. Microhabitat use and aggressive behavior of allopatric and sympatric cutthroat tested separately in the laboratory stream were similar, although sympatric trout defended riffle territories more vigorously, responded to the feeding cycle with greater synchrony, and used components of aggressive display apparently more suited to high water velocity habitats. When fed ad libitum in the laboratory, underyearling coho grew faster than cutthroat in winter, irrespective of temperature range (5-15 °C) and photoperiod (8-16 h), whether tested separately or together; in summer, growth was similar for both species at the same test conditions as above. When cohabiting during summer in two coastal streams, underyearling cutthroat grew faster than coho, possibly because of greater behavioral diversity in feeding and microhabitat utilization, and lesser social dissipation of energy. The data provide evidence of interactive segregation (Nilsson 1956) and illustrate the flexible behavior of these two salmonids, cutthroat slightly more so than coho, possibly due to subdominance. Behavioral flexibility may counteract heterogeneity and instability of stream environments, and may permit opportunistic exploitation of broadly overlapping niches when resources are plentiful.Science, Faculty ofZoology, Department ofGraduat