Atlantic menhaden in the summer months; whereas Atlantic mackerel, Atlantic 

 herring and spiny dogfish are most abundant in the more marine (and therefore 

 cooler) Sheepscot River estuary. Prolonged, near-freezing temperatures, 

 rather than the annual temperature range, limit the habitability of temperate 

 estuaries by pelagic fishes (Recksiek and McCleave 1973). The authors 

 hypothesize that those pelagic species would be most affected by an altered 

 temperature regime. 



Data on temperature effects on fish, other than mortality, are scarce. 

 Potential thermal impacts on fish populations, therefore, must be considered 

 before activities that could alter the temperature regime of a body of water 

 are undertaken. Human activities that have the potential to alter water 

 temperature and, therefore, the habitat of fishes, are summarized in table 11- 

 6. These are primarily problems in freshwater and estuarine systems. Some 

 activities raise water temperature by increasing surface water exposure to the 

 sun. Examples are: the removal of stream cover vegetation (common in 

 agriculture, forestry, and construction practices), and water flow 

 impedimentation upstream from dams and impoundments. Another heat source is 

 the direct addition of heated effluent from municipal and industrial waste 

 disposal and power-producing operations. Eight power plants discharge cooling 

 water within the characterization area (see chapter 3, "Human Impacts on the 

 Ecosystem") . 



Salinity 



Marine waters generally are defined as those having a salinity concentration 

 of >30 ppt. Estuarine salinities typically range from 0.5 to 30 ppt and fresh 

 water is <0.5 ppt. Salinity is fairly constant (about 32 ppt) in the open 

 ocean and is not considered a major factor in determining the distribution of 

 marine fishes. In estuarine environments, however, salinity determines 

 distribution of most organisms (Recksiek and McCleave 1973). Each species, 

 and often each life stage, has a preference and a tolerance range. Anadromous 

 fishes such as the Atlantic salmon, alewife, rainbow smelt, American shad, and 

 blueback herring, spend their adult life in saline waters but return to 

 freshwater rivers and streams to spawn. The eggs and larvae of these fishes 

 develop properly only in fresh or slighly brackish water. Juvenile marine 

 fishes are generally more tolerant of low and fluctuating salinites than adult 

 fishes, therefore, estuarine and nearshore environments are usually dominated 

 by juveniles (TRIGOM 1974). Salinity regimes vary constantly in coastal Maine 

 (see chapter 5, "The Estuarine System"). Tidal flushing and freshwater inflow 

 are the dominant regulators. People alter estuarine salinity through removal, 

 impoundment, or addition of fresh water, and by altering water basin or 

 channel configuration, which may change currents or alter tidal flow (table 

 11-6). 



Competition 



Fishes compete for food, space, shelter, and spawning sites with members of 

 their own species (intraspecif ic competition) or other species (interspecific 

 competition). Competition is density-dependent; that is, it is governed by 

 numbers of individuals present in a certain area and the availability of 

 habitat. People sometimes increase competition in natural communities by 

 limiting available habitat and food supply and by introducing competing 

 species . 



11-29 



10-80 



