O 1000 2000 

 FREQUENCY 

 SE 1 NE 



100 200 300 

 FREQUENCY 



1 O 20 

 FREQUENCY 



I MP I NGEMENT 



Fig. 8. Seasonal length-frequency distribution by 20-mm size-classes of silversides in seine, trawl, and MNPS Unit 2 

 impingement collections during 1986 and 1987. 



catches were within the range of historic data. 

 The length-frequency distribution of the 1986 

 catch was compared to the long-term frequency, 

 and the size composition found in 1986 was sim- 

 ilar to previous data (NUSCO 1987b; Fig. 8). 

 Seasonal patterns of occurrence in 1986-87 were 

 also similar to historic patterns. 



The estimated 136 silversides impinged in 1987 

 was a historic low. The pattern of decreased 



impingement was previously discussed and was 

 not specific to silversides, but was probably due 

 to physical changes near the MNPS intakes. The 

 5-mean catch in trawls, an index of winter adult 

 abundance, for the 1986-87 report period was 

 similar to previous catches (Table 15), indicating 

 that the decrease in impingement was not due to 

 a change in adult abundance. 



TABLE 15. Seasonal 5-mean catch (CPUE) and 95% confidence interval of silversides by trawls at selected stations 

 by report period (June-May). 



Season used for calculating S-mean was November-February at IN, NB and NR and October-January at JC. 



276 



