range of tides, producing the characteristic 

 zonation that parallels the shore line. The most 

 obvious environmental factors which may deter- 

 mine this zonation are the exposure to air which 

 the organism must tolerate, exposure to wave 

 action and winter ice, and salinity and thermal 

 stratification . 



Intertidally, the predominant species 

 living in sediments are the soft-shell clam, Mya 

 arenaria; a smaller clam, Macoma balthica; the 

 sand worm, Nereis virens; and an amphipod, 

 Corophium volutator. Another worm, Nephtys 

 caeca, is also abundant along the lowest edge of 

 the intertidal zone. In the corresponding zone 

 on hard substrate are the barnacle, Balanus 

 balanoides; the snails Littorina littorea and 

 obtusata; and the purple whelk, Thais lapillus . 

 Li the upper estuary. Nereis diversicolor re- 

 places N. virens and saxa tills replaces L. 

 obtusata. 



Fish populations in the estuary include 

 both migratory and indigenous species. Of the 

 former, there are such seasonal migrants as 

 the Atlantic herring and Atlantic mackerel; 

 anadromous species, such as the American 

 smelt, the alewife, and the Atlantic salmon; and 

 the catadromous American eel . Mackerel and 

 small herring appear in the waters of the lower 

 estuary during the summer months . There 

 appears to be some relationship between their 

 local abundance and the quantity of plankton in 

 the water, but this has not been conclusively 

 demonstrated. Inshore herring and mackerel 

 catches 2/^ere smaller in 1955 than in 1954, and 

 the summer plankton volumes were also smaller 

 during 1955 than the year before as is shown in 

 fig. 6. The appearance of a large school of 

 mackerel in a cove near the laboratory in August 

 1954 coincided with an extremely dense concen- 

 tration of zooplankton in that cove on which the 

 mackerel were feeding. 



No attempt has been made to analyze 

 further the tidal zonation, a subject which has 

 been treated thoroughly by the Stephensons (1949, 

 1954), and the principles of which are generally 

 applicable throughout this area, although there 

 are variations in particular species. Many 

 species that ecologically are not intertidal are 

 found in the intertidal zone since they occupy 

 only its lowest margin, scarcely above the 

 spring low tide level, and then are usually hid- 

 den in sheltered spots or in tide pools. These 

 species are usually members of subtidal or in- 

 fralittoral communities, and include lobsters, 

 rock crabs, starfish, sea urchins, anemones, 

 and a host of other interesting creatures. 



On the most seaward shores of the estu- 

 ary are a few sandy coves, fringed by ledges 

 and boulders, but protected from severe wave 

 action. In such places the number and variety 

 of Intertidal and infralittoral organisms are 

 relatively great. In the more sheltered places 

 the abundance of silt appears to discourage all 

 but the hardiest species or those which normally 

 live in silty sediments. 



Subtidal fauna has not been studied in 

 detail, but studies currently being made by the 

 biologists of the Clam Investigation of the U.S. 

 Fish and Wildlife Service will provide much in- 

 formation relative to the distribution of these 

 subtidal populations . 



The alewife (Pomolobus pseudoharengus) 

 ascends into fresh water in May, followed by the 

 glut herring ( aestivalis) in June . Sea lampreys 

 (Petromyzon marinus) ascend the river in great 

 numbers in May, but observations made at the 

 counting weir indicate they reach the head-of-tide 

 earlier than this, probably some time in April. 

 During May and June elvers of the American eel 

 ascend the river, while adults move downstream 

 in both spring and fall. Atlantic salmon are not 

 abundant but a few ascend each year, mostly in 

 the fall. Smelt (Osmerus mordax) enter the 

 estuary in the fall, remain in the tidal creeks 

 and the upper estuary during the winter, and 

 ascend the river and tributary brooks to spawn 

 in April. Striped bass (Roccus saxatilis) appear 

 in the upper estuary during the summer and are 

 said to have been abundant there all year round 

 when the dam was present at Sheepscot Falls . 



No definitive check list of fish indigenous 

 to the estuary has been compiled, but certainly 

 many of the species listed by Bigelow and 

 Schroeder (1953) are regular inhabitants, at 

 least in the more seaward parts of the estuary. 



2/ The information regarding the size of these 

 catches has been obtained from the herring catch 

 statistics on file at the Boothbay Harbor Labora- 

 tory and from interviews with local fishermen 

 and distributors. 



12 



