that feed on suspended material (filter feeders, e.g., mussels) usually live 

 where the currents are strong enough to provide a sufficient amount of food. 

 They are found most commonly in sandy or rocky environments, where deposit 

 feeders are relatively scarce. Other requirements include shelter, hard 

 substrates for nonmobile organisms (e.g., anenomes), appropriate sediments to 

 make cases, and algal substrates for some polychaetes and bryozoans. 



Fish. The estuarine system supports a slightly lower diversity of fish 

 species than the marine system. Fishes of coastal Maine are discussed in 

 detail in chapter 11, "Fishes." Temperature and salinity are major factors 

 determining the seasonal and local distribution of fish in estuaries. Fish 

 populations in Maine's estuarine systems are dominated by resident demersal 

 (bottom-dwelling) species, such as the Atlantic tomcod, flounders, sculpins, 

 skates, cunner, rock gunnel, and sea raven. The estuaries also host a variety 

 of fishes on a seasonal basis, including the anadromous fishes (alewife, 

 American shad, blueback herring, sturgeons, Atlantic salmon, sea lamprey, and 

 rainbow smelt), Atlantic herring, Atlantic menhaden, hakes, and the 

 catadromous American eel. The rainbow smelt is perhaps the only year-round 

 resident in the upper estuaries. Most fish leave the estuaries for deeper, 

 warmer waters in winter. 



Fish are important components in the energy flow of the estuarine system 

 because of their abundance (biomass) and representation on many levels of 

 aquatic food chains (primary, secondary, and tertiary consumers). Many fish 

 utilize the estuaries as primary spawning or nursery areas (wrymouth, rock 

 gunnel, sculpins, sea snails, snakeblenny, sticklebacks, mummichog, tomcod, 

 silversides, winter flounder, and Atlantic herring). 



Estuarine fish are classified on the basis of their principal feeding habits 

 as planktonic, nektonic, or demersal/semidemersal (figure 5-25). Estuarine 

 systems are dominated by demersal/semidemersal-feeding finfish populations, 

 followed by nektonic feeders, and then planktonic feeders. The cunner and 

 Atlantic cod are both nektonic and demersal feeders. Planktivores , such as 

 the herrings (Atlantic herring, blueback herring, alewife, and American shad), 

 Atlantic menhaden, and American sand lance are water column feeders that feed 

 on pelagic crustaceans, mostly (copepods, mysids, euphausiids, and amphipods). 

 Nektonic feeders consume pelagic crustaceans and fish. These active feeders 

 include the majority of the area's summer migrants (the hakes, spiny dogfish, 

 bluefish, Atlantic mackerel, and striped bass) as well as rainbow smelt, 

 Atlantic salmon and white perch. The dominant demersal-feeding fishes are 

 flounders, codfish, and skates, the sculpins, the ubiquitous American eel, 

 Atlantic tomcod, the anadromous sturgeons, and the sticklebacks. Major food 

 items are crustaceans, echinoderms, bottom fish, fish eggs, benthic diatoms, 

 molluscs, and polychaete worms. 



Birds . Waterbirds, including seabirds, shorebirds, wading birds, and 

 waterfowl, use the estuarine system for breeding (mostly the islands), feeding 

 (all habitat classes), moulting, migrating, and wintering. Twenty percent of 

 waterbirds are permanent residents. In addition, several raptors, including 

 the resident endangered bald eagle, utilize the estuarine system. Waterbirds 

 are described in detail in chapters 14 and 15, "Waterfowl" and "Waterbirds" 

 respectively. The bald eagle and other raptors are described in chapter 16, 

 "Terrestrial Birds." Most estuarine bird species are high level consumers. 

 Seabirds feed on zooplankton, benthic invertebrates, cephalopods, finfish, and 



5-44 



