Several criteria and problems are examined in relation to the separation between middle-sized and small-sized pelagic fish and big migrators. Differences in longevity and rhythm of growth are considered good parameters for separation. Behavioural pattern of the most representative species is also analyzed. Special attention is given to fishing activity during the last decades, and the social stimuli that conditioned this activity. The importance of middle-sized pelagic fish is notable if the increment in catches is borne in mind. Finally, the main factors that motivate the collapses of fish populations, namely the influences of environmental changes and of fishing, are examined. The houndry between both factors is always difficult to settle. It is important to indicate the value of these analysis for the correct exploitation of the resources.