A Predation Bioassay to Quantify the Ecological Significance of Sublethal Responses of Juvenile Chinook Salmon (Oncorhynchus tshawytscha) to the Antisapstain Fungicide TCMTB

Abstract

The lumber protection antisapstain fungicide 2-(thiocyanomethylthio)benzothiazole (TCMTB) is leached by rainfall from treated wood awaiting shipment from outdoor lumber storage facilities. Stormwater discharges of this pesticide into estuarine reaches of the Fraser River, British Columbia, have raised concern about toxic effects on juvenile salmon rearing in this habitat. Simulated stream flow conditions were used to expose underyearling chinook salmon (Oncorhynchus tshawytscha) to a sublethal (10 μg∙L−1) concentration of TCMTB. Equal numbers of fin-clipped control and exposed fish were subsequently transferred to a vertically stratified (fresh over seawater) 15 500-L outdoor tank where they were challenged with the combined stressors of salinity and the presence of a marine predator (yellowtail rockfish (Sebastes flavidus)). After 5 d under these simulated estuarine conditions, survivors were enumerated. Chinook salmon that had been exposed to the toxicant were consumed in preference to the control group by a factor of 5.5:1. Inasmuch as predator avoidance represents the successful integration of appropriate biochemical, physiological, and behavioural responses, the predator bioassay provides an ecologically relevant technique to determine the significance to survival of the complex multifactorial interactions of individually "sublethal" stressors.