Community interactions are shown by means of a partition plot (figure 11-5). 

 From this diagram it is clear that the Northwest Atlantic gadids show a 

 reasonable degree of food partitioning, since major prey, except for broadest 

 categories (e.g., other fishes and other Decapoda) is rarely shared by more 

 than two or three predators. A similar situation has been described for a 

 number of freshwater and other marine fish communities. Langton and Bowman 

 (1978) support the contention that the cod fish evolved in a system where the 

 availability of food was the controlling factor. In other words, competition 

 for food, as the limiting resource, resulted in the development of different 

 food habits by each species of fish. 



FACTORS AFFECTING DISTRIBUTION AND ABUNDANCE 



Environmental factors, both natural and human-originated, influence the 

 abundance, distribution, and behavior of fish populations. These factors 

 include water temperature, salinity, food availability, competition, 

 predation, rate of harvest, disease and parasites, water quality, and dams and 

 other obstructions. Their effects on fish may be direct (e.g., causing 

 deaths) or indirect (e.g., decreasing food supplies). Early life stages, egg 

 and larvae, are most vulnerable to stress from the environment, since they are 

 less mobile, and usually occur close to shore where human activity is more 

 concentrated (Clayton et al. 1976). 



Water Temperature 



Temperature is a major factor affecting the distribution of most fish 

 populations. Seasonal and daily movements, gonad development, spawning 

 activities, growth rates, osmoregulation, respiration, and the duration and 

 success of egg and larval development vary with temperature. In general, 

 marine fishes have a narrower range of temperature tolerance than estuarine 

 fishes. This reflects the relative stability of the marine environment as 

 compared to the fluctuating conditions of estuaries. Most estuarine and 

 anadromous fishes are adapted to the warmer water temperatures typical of 

 shallow estuarine or riverine environments in summer (16 to 26°C; 61 to 79°F). 

 Pelagic fishes are generally more sensitive to temperature changes than 

 demersal fishes. 



Targett and McCleave (1974) looked at the distribution and abundance of fishes 

 in Bailey Cove (Sheepscot estuary, region 2) during the summer in relation to 

 water temperature. Mummichogs, smooth flounders, Atlantic silversides, and 

 Atlantic herring were the dominant fishes captured (98% of the catch). The 

 mummichogs and Atlantic silversides were caught primarily in the inner cove 

 (warmer, shallower water). Atlantic herring, smooth flounder, winter 

 flounder, alewivcs, and Atlantic tomcod were captured near the outer margin 

 (deeper, cooler, water) of the cove; American eel and blueback herring were 

 found to use the cove primarily at night, when waters were cooler (McCleave 

 and Fried 1975). The latter two groups of fishes tend to avoid the tidal cove 

 when the waters become too warm. 



Other examples of temperature preference were shown in a study of the seasonal 

 abundance of pelagic fishes in the deeper, main channels of the Sheepscot 

 River estuary. Rainbow smelt were found to be the only year-round resident in 

 the upper estuary (Recksiek and McCleave 1973). The relatively warm Back 

 River estuary supports abundant populations of alewives, blueback herring, and 



11-28 



