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 that was not 

 evident in the number caught by trawl. No apparent changes occurred in length-frequency distribution 

 or seasonal abundance in Jordan Cove related to the 3-unit thermal plume. 



Grubby larvae were present in ichthyoplankton collections and juveniles and adults were taken in trawl 

 and impingement collections. Larval abundance declined in recent years to levels similar to those of the 

 late 1970s. Numbers impinged at Unit 2 have decreased in recent years, most likely due to physical 

 changes near the intake and because of the start-up of Unit 3. Individuals returned by the Units 1 and 3 

 sluiceways had higli survival ( > 74''/o). Except for recent years at the Niantic River station, no long-term 

 changes were found in the mean length or in abundance indices of adults collected by trawl. 



The tautog is an important recreational fish in the Millstone area and the greatest potential impact of 

 MNPS on it is through the entrainment of eggs. Egg abundance, the best index of adult stock size, 

 increased in recent years, but larval densities declined to levels similar to the late 1970s. An apparent low 

 egg to larval survival was found in all years examined. Because the tautog takes 2 to 4 years to reach 

 maturity, the possible impact of entrainment by 3-unit operations on adult stock size will not be evident 

 for several years. 



Abundances of all life history stages of cunner collected near MNPS have recently declined. In 1986, the 

 numbers of eggs and larvae were among the lowest found since 1976. Decreases in the trawl catch were 

 evident at stations closest to MNPS. Part of the declines in both impingement and in juvenile and adult 

 abundance at a station near the intakes could have been caused by physical changes to the habitat, but 

 reasons for the decline at other stations are not known. The apparent low egg to larval survival in all 

 years was significantly correlated with that for the tautog, suggesting similar factors affecting their repro- 

 ductive success. Monitoring will continue to determine if the recent decrease in the cunner population 

 was due to a natural fluctuation in abundance or from the operation of MNPS. 



Hydrothermal Studies 



During 1987 the extent and configuration of the 3-unit thermal plume were determined during various 

 tidal regimes. A dye survey was conducted on 26 August 1987 and supplemental temperatures were 

 recorded by automatic data loggers, which were deployed during October. The configuration and extent 

 of the thermal plume produced during 3-unit operation, as measured by dye concentration, generally 

 matched predictions during all four tidal regimes. Water temperatures recorded continuously at selected 

 locations were also generally what was expected, based on predictions. The plume is highly dynamic and 

 those regions influenced by increased water temperatures generally experienced a respite from warm water 

 for at least several hours during a tidal cycle. Based on both the survey and supplemental temperature 

 data, at no time during the study period did the 4°F (2.2°C) isotherm appear to extend past the 8,000-ft 

 (2,439 m) limit imposed by the NPDES permit. Based on dye concentrations at Unit 1 intakes, very little 

 recirculation of discharge water occurs. 



Executive Summary 



