stations .K' and WP were sampled biweekly on both tides. Both sites are within the area projected to be 

 influenced by the thermal discharge from three-unit operation. But because only 2 yr of 3-unit pre-operational 

 data could be collected on the ebb tide, a new sampling scheme was adopted based on a control-treatment 

 pairing (CrrP) design (Skalski and McKenzie 1982; Bernstein and Zalinski 1983). The critical requirement 

 of the CTP design is the selection of the control-treatment (nonimpacted-impacted) pairs of stations where 

 the abundance of an organism responds similarly to changes in environmental parameters. The ratios of 

 control-to-treatment data from each pair of stations, are compared between pre-operational and operational 

 phases to detect impacts. An analysis of 15 yr of data showed significant correlations between GN (control) 

 and WP (treatment) data using silverside and total fish abundance as response variables. Based on the 

 results of this analysis, a new sampling scheme was adopted. Our control-treatment pair (GN, WP) is 

 now sampled biweekly. Because work to date will serve as a baseline for a future assessment of three-unit 

 operation, no data from this special study are presented here. 



Data handling 



To assess impacts it was necessary to identify potentially affected species, document their spatial 

 distribution, and describe the natural temporal fluctuations of their life history stages collected near MNPS. 

 Although sampling at NUEL has occurred since 1969, the changes in each program have limited the 

 comparability and usefulness of data collected before 1976. Therefore, when data were available, analyses 

 were restricted to 1976-1985, the period of two-unit operation at MNPS. The only exception to this was 

 that the 1969 through 1985 seine data were used in the time-series analysis of silversides. 



The selection of potentially affected species was based on their prevalence in entrainment or 

 impingement samples. Spatial distribution patterns were based on the catch of a species at each station. 

 Temporal fluctuations were described by annual (median and/or mean) catches and by the forecasts pro- 

 vided by time-scries models, which used log-transformed catch data. The distribution of life history stages 

 in impingement, trawl, and seine collections was inferred from the morphological characteristics and sizes 

 of the fish. Identification of each fish species in all programs was made to the lowest possible taxon. 

 Some specimens were identified to genus or family if they were juveniles, adults that could not be easily 

 identified in the field, or if they were species of uncertain taxonomic status due to inadequate descriptions 

 in the literature. The taxa which included more than one species are listed in Table 6. This table also 

 includes the programs in which each taxon was used, the reason for combining species, and the probable 

 species that were included in each group identification. 



15 



