Summary 



1. The operation of MNPS could affect fish 

 assemblages in the Millstone area in several 

 ways. Juveniles and adults could be lost due 

 to impingement on the intake screens. The 

 mortality rates of early history stages could 

 be increased by entraining eggs and larvae 

 through the condenser cooling water system. 

 The local distributions could be altered by 

 the thermal plume. This report emphasizes 

 the comparison of data collected during two- 

 unit operations to those collected since the 

 start-up of Unit 3. 



2. Impingement monitoring at Unit 2 was dis- 

 continued on December 11, 1987 because 

 losses have been well-documented and all 

 feasible mitigative measures have been inves- 

 tigated. There was a significant decline in 

 total impingement in recent years, which was 

 attributed to physical changes near the Unit 

 2 intake and possible changes in water circu- 

 lation patterns because of the operation of 

 Unit 3. Losses due to impingement by 

 MNPS were reduced with the installation of 

 fish return sluiceways at Units 1 and 3. 



3. Over 100 fish taxa have been collected in the 

 monitoring programs since 1976. These pro- 

 grams were demersal trawl, shore-zone seine, 

 impingement, and ichthyoplankton. Eight 

 taxa were selected for detailed examination 

 due to their prevalence in entrainment or 

 impingement collections or their abundance 

 in the shore-zone area of Jordan Cove, an 

 area which may be impacted by the thermal 

 plume. 



4. The American sand lance was primarily col- 

 lected as larvae and was a dominant entrained 

 taxon. A decline in larval abundance since 

 the early 1980s was attributed to a regional 

 decrease in adult stock size. 



5. Several life history stages of anchovies were 

 very abundant in some sampling programs. 

 Adults were present in impingement collec- 



tions, juveniles were caught by trawl, and 

 eggs and larvae were abundant in entraiimient 

 samples. The numbers impinged have de- 

 clined in recent years. Comparison of annual 

 egg and larval abundance indices suggested 

 compensatory mortality during the early life 

 history stages, which could help mitigate 

 losses due to entrainment. 



6. Sticklebacks and Atlantic tomcod were pri- 

 marily found in impingement collections. 

 The impact of MNPS impingement on 

 sticklebacks has been reduced due to high 

 ( > 70%) survival of individuals returned by 

 sluiceways at Units 1 and 3. There was a 

 marked decrease in Atlantic tomcod numbers 

 impinged at Unit 2 since the start-up of Unit 

 3. 



7. Silversides dominated the shore-zone area of 

 Jordan Cove and adults were abundant in 

 winter trawl and impingement collections. 

 There was a recent decline in the number 

 impinged at Unit 2, but this was not evident 

 in the number caught by trawl. There were 

 no apparent changes in length-frequency dis- 

 tribution or seasonal abundance in Jordan 

 Cove related to the three-unit thermal plume. 



8. Grubby larvae were present in ichthyoplank- 

 ton collections and juveniles and adults were 

 present in trawl and impingement collections. 

 Larval abundance has declined in recent years 

 to levels similar to the late 1970s. Numbers 

 impinged at Unit 2 have decreased and those 

 returned by the Units 1 and 3 sluiceways had 

 high survival (>74%). Except for station 

 NR, there has been no long-term changes in 

 the mean length or abundance indices of 

 adults collected by trawl. 



9. The tautog is an important recreational fish 

 in the Millstone area and the greatest potential 

 impact of MNPS on it is through the en- 

 trainment of eggs. Egg abundance, the best 

 index of adult stock size, has increased in 

 recent years, but larval densities declined to 

 levels similar to the late 1970s. There was 



Fish Ecology Studies 289 



