Ratios of expected values (REX), a method for impact evaluation of thermal discharges on the fish community in the Pickering littoral waters of Lake Ontario

Abstract

Discharge effects on the fish community are assessed for the Pickering power station and a creek in the neighbourhood, located in littoral waters of the northern shore of Lake Ontario. Since the ambience is cold in spring and fall, when the main spawning migration takes place, any positive thermal variation is selected with preference by the onshore migrants. Our goal is to identify these effects when significant or, alternatively, the community richness reduction, and even the biological damage to fish. Ratio to Expected Values (REX) involves a comparison of the diversity indexes from locations under impact versus ambient samples, in order to identify the effect magnitude. To clear the background records of local effects, they were checked for outliers, which have been replaced based on missing-value techniques. Applying the method, the slight thermal increase of the creek discharge was found to produce a higher richness compared to station discharges, hence offering more attraction for onshore migrants. Despite the high attraction of station thermal plumes, the general result is negative, due to a process described here as ambiguous attraction. It is preliminarily identified as a fish attraction, due to favourable thermal conditions at the plume edges, followed by repulsion and fish exhaustion, due to low oxygen level and absence of spawning conditions for the attracted fish, further in the plume. The paper also introduces a new statistical approach of effect evaluation for significance, based on pair comparison of records in the suspected area versus background records.