Predation and Harvest 



Predation is another important interaction among individuals of the same or 

 different species. Predation, including harvest by people, influences the 

 number of individuals in a population. Fishes are preyed upon by marine 

 mammals, seabirds, wading birds, terrestrial birds, terrestrial mammals, 

 waterfowl, and other fishes. Harvest by humans, specifically over-harvest, 

 has had historic impacts on fish populations (see "Importance to Humanity," 

 this chapter). People affect predation by stocking prey and predator species. 

 Predation is essential for population regulation and must be wisely considered 

 in management decisions. Human predation (harvest) limits must be maintained 

 so as to allow for natural regeneration, during which only excess individuals 

 should be harvested. 



Diseases and Parasites 



Fishes are subject to a wide variety of diseases and parasites, including 

 viral, fungal, and bacterial infections, and parasitic protozoans, worms, 

 crustaceans, and sea lampreys. Deficiency and degenerative diseases, such as 

 cancer, rickets, blindness, and liver dysfunction, are common. Fish 

 populations in the wild usually are not impaired seriously by disease and 

 parasites and epidemics are rare. 



Hatchery fishes, however, are very susceptible to large scale infestations and 

 may serve as carriers to the wild. Furunculosis ( Bacillus salmonicida ) is a 

 disease that has spread from hatchery reared salmon to natural populations 

 (Clayton et al. 1976). Disease can be a significant limiting factor in 

 recovering populations. The market value of some species (cod, for example) 

 is diminished by the presence of parasites. The problems and possible 

 mechanisms of "codworm" infestation are discussed in chapter 13, "Marine 

 Mammals." Diseased or parasite-carrying fishes may be more susceptible to 

 other causes of mortality. People increase fish exposure to disease-causing 

 agents and parasites, primarily through disposal of wastes in waters (table 

 11-6). People also introduce potentially detrimental species to an area. The 

 sea lamprey (a parasitic fish) was inadvertently introduced and became 

 landlocked in the Great Lakes, where it has all but eliminated some of the 

 commercial and recreational fisheries. Its habitats in Maine presently 

 include the open ocean, coastal rivers, and their tributaries. There is as 

 yet no evidence of harm to Maine's freshwater fish populations from sea 

 lampreys (Everhart 1958). 



Dams and Obstructions 



Physical obstructions, such as water falls and artificial dams, dikes and 

 weirs, are barriers to migrating fishes. The majority of the existing dams in 

 the coastal zone are impassable for many anadromous fishes (American shad, 

 Atlantic salmon, alewife, sturgeon, and blueback herring), and many resident 

 migratory freshwater fishes (e.g., trout). Young of the catadromous eel 

 (elvers) can surmount most of these barriers (personal communication from C. 

 Walton, Maine Department of Marine Resources, Hallowell, ME; May, 1978). Dams 

 with heights as low as 2 feet (0.6 m) can be effective barriers at low water 

 levels . 



11-31 



10-80 



