Survival and Growth Responses of Juvenile Salmonines Stocked in Eastern Lake Ontario Tributaries

To evaluate the species‐specific and stream‐specific suitability of juvenile salmonine habitat in the southern Lake Ontario watershed, we studied the effects of multiple environmental gradients on the first‐summer apparent survival and growth of various combinations of Atlantic salmon Salmo salar , rainbow trout Oncorhynchus mykiss , and coho salmon O. kisutch stocked in tributaries. Costocking of either Oncorhynchus species had no detectable effect on the apparent survival or change in cohort biomass of Atlantic salmon, but their growth rates were reduced slightly when they were stocked with rainbow trout. Generally, Atlantic salmon outperformed their putative competitors. Summer temperatures were near the physiological optimum for Atlantic salmon but may have limited the success of rainbow trout and especially coho salmon. Total salmonine biomass was maximized at sites in which only Atlantic salmon were planted. Apparent survival and biomass elaboration of Atlantic salmon varied inversely with stream size, temperature, and the abundance of wild salmonines and piscivores, whereas growth rate responded positively to moderate increases in summer temperature, agricultural development, and nutrient enrichment. These regional and species‐specific differences in stocked salmonine success may, in part, be explained by variations in temperature, geomorphology, and anthropogenic influences. We recommend that the feasibility of restoring Atlantic salmon continues to be evaluated, especially in those tributaries considered to be of marginal quality for other salmonines.