24 



• Tides 



Fish populations seek certain environmental conditions for certain 

 activities: temperature that provides food, depths that provide food, 

 currents that provide food or aid in spawning activity, certain kinds of 

 bottom to feed and spawn, etc. 



The fluid nature of the ocean may cause large environmental changes 

 that can be daily, seasonal, or \/ery long-term in trend. These changes 

 in turn affect the conditions within which that ecosystem operates. 



There do exist many constant aspects of the marine environment: the 

 Gulf Stream flows north along the United States east coast, the surface 

 waters get warmer in the summer, tides are regular and can be measured, 

 marine currents have been observed and charted, certain kinds of weather 

 produce certain environmental conditions; yet, as the kind of information 

 sought grows ever more precise, the precision of the data--dependent as 

 it is upon so many variables--most often does not follow. 



Further, fish populations occupy certain favorable niches in that 

 marine environment, niches determined by depth, salinity, temperature, 

 and type of bottom. As those niches are not solely bottom-dependent, 

 they move and so must the fish population. 



Many species migrate, many more of the same species may not; spawning 

 grounds, however, have been determined to be relatively area-precise. 

 In general, populations of fish and even some shellfish move great dis- 

 tances according to the variables outlined above; those movements that de- 

 pend upon seasonal fluctuations can be rather closely observed. 



Fish, then operate in an environment occupied by other species with 



