frequency ranges from continuous washing during storms to at least once every 8 h. Impingement samples 

 were taken by sorting fish from all the material washed from the screens during a 24-h period, usually 

 beginning and ending at about 0800. Fish were identified to the lowest possible taxon, counted, and up 

 to 50 specimens of each species were measured to the nearest mm in total length, (ilatch was recorded as 

 number impinged per 24-h period. 



Routine impingement sampling began at Unit 1 in 1972, although some qualitative observations were 

 made as early as 1971. Sampling at Unit 2 was initiated in September 1975. The primary objective of 

 impingement monitoring at Millstone has been to quantify total annual species-specific mortality. Ways 

 of minimizing this mortality were evaluated and plant design changes were recommended when appropiate. 

 Throughout the 13 yr of monitoring, various changes have been implemented. These occurred mainly in 

 four major areas, including the frequency of daily counts, the way in which fish lengths were recorded, 

 the method used to estimate the number of fish impinged per 24-h period, and the elimination of 

 impingement monitoring at Unit 1 after 16 December 1983 when a fish return sluiceway was installed. 



Changes in the frequency of impingement monitoring are outlined in Table 3. From 1972 to March 

 1977, impinged organisms accumulated over a 24-h period and were counted daily. In 1977, sampling 

 effort was reduced to 3 counts/week. Before this reduction, mean daily impingement estimates for each 

 month based on 7 counts/week were compared to mean daily estimates extrapolated from 3 counts/week. 

 The differences between actual monthly totals based on a complete census and the estimated totals ranged 

 from 20 to 50%, depending upon the species (NUSCo 1978). At the level of effort reduced to 3 

 counts/week, more than 85% of all species were represented. In 1982, the impingement program was 

 evaluated to determine if the precision of the impingement data could be improved by redistributing and 

 optimizing effort (NUSCo 1983). Historical data (3 counts/week) were stratified by month and effort and 

 reallocated according to Fl-Shamy (1979). The historical program (uniform effort - 3 counts/week) had 

 a precision value of 0.79. When the sampling effort was hypothetically redistributed so that more samples 

 were collected in those months when the variances of winter flounder [Pseudopleuronextes americanm) 

 counts were high and fewer samples in months when variances were low, the precision factor increased to 

 0.88. An optimal sampling scheme was implemented in December 1983 (Table 3). Since then, sampling 

 effort at Unit 2 was stratified by month so that 8 samples were collected in .lanuary, 15 in February, 14 

 in March, 5 in April, 4 per month from May through November, and 10 in December. The overall 

 sampling effort was reduced by approximately 40%. 



