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Experimental field manipulation of stream temperatures and suspended sediment concentrations : behavioural and physiological effects to juvenile chinook salmon Quigley, Jason Trevor

Abstract

The early life history behaviour of juvenile chinook (Oncorhynchus tshawytscha) rearing in the Torpy River, British Columbia was investigated to assist in interpreting the significance of the effects of forest harvesting on juvenile chinook. Investigations of side-channels, tributaries and the mainstem Torpy River in 1997 and 1998 suggested that groundwater controlled, nonnatal tributaries were the highest value habitats in all seasons (spring, summer, fall/winter). Small, non-natal tributaries contained both the greatest abundance and largest individuals in terms of length and mass for all age classes. Tributaries also provided the longest growing season and juveniles in these habitats displayed very high residency. Side-channels consistently provided the lowest value habitats. Juvenile chinook over-wintered in groundwater controlled non-natal tributaries, or emigrated from the mainstem in August. The exodus of juvenile chinook from the mainstem was likely triggered by sedimentation of substrate compartments coincident with summer low flows, precluding successful over-wintering. I simulated increases in suspended sediment concentrations and stream temperatures in side-channels and tributaries, to mimic those induced by forest harvesting and quantified their biological effects on juvenile chinook salmon. Experimentally elevated suspended sediment concentrations in un-logged tributaries (602 mg/L) and side-channels (2797 mg/L) resulted in physiological and behavioural sub-lethal stress responses in juvenile chinook. Even though tributary sedimentation trials were stressful to juvenile chinook, these individuals endured these conditions rather than exhibit an avoidance response. Side-channel sediment trials invoked upstream avoidance in juvenile chinook. Small increases in stream temperatures (0.97 - 3.87 °C), representative of post-forest harvesting conditions, also caused sub-lethal stress responses in juvenile chinook salmon. Temperature increases did not exceed the preferred range for the species indicating increased variation in temperature typical of post forest harvesting conditions could have profound effects on juvenile salmonids. Sub-lethally stressed fish in side-channel heating trials displayed upstream avoidance and aggregative behaviour. Avoidance was not part of the behavioural repetoire of tributary rearing individuals. Ecological motivation, due to the high value of tributaries, likely caused juvenile chinook to temporarily override adaptive avoidance responses and endure sub-lethally stressful conditions. The propensity for tributary rearing juvenile chinook to endure obviously unfavourable conditions makes them vulnerable to forest harvesting induced changes to water quality. The ecological relevance of sub-lethal stress and consequences of relatively small changes in water quality from land use activities, in the context of life history behaviour, are discussed.

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