ECOLOGICAL STRUCTURES - EXPANSION AND REPLACEMENT

From an ecological point of view, the middle-sized pelagic fish, speci ally Trachurus spp and Scomber spp, are located between the little-pel agic fish (pilchards and anchovies) and the big migrators ones (tuna a nd related species). Its ecological rol has been considered as a parti cular level between the zooplanktivorous fish and the strictly piscivo rous ones. These species have a global biomass between the other two g roups of fish, that is to say, less that the little one and more than the big one. It is necessary to consider three distinctive aspects of the middle-sized pelagic fish: 1st. Its longevity with several age-gro ups in the different stocks of these species. 2nd. Its trphic plastici ty, and 3rd. the high swimming capacity and resistant. As results of t hese three characteristics the populations of these species show a hig h stability and a major capacity of expansion, occuping, by replacemen t, the niches leaved by the over-exploited little-sized pelagic fish s pecies. The total catches of pelagic fish species, in a high scale, do not show this capacity of replacement in a clear way. The analysis of specific fisheries shows that when the biomass of the little-sized pe lagic fish decreased, it is followed by a significative increase of th e middle sized pelagic fish biomass. These are several fishing grounds in the world that have experimented a collapse of stocks of little-si zed fish and quick rise, in biomass, of the middle-sized ones. The fis heries off the namibian coast are an excellent example. In this area t he fisherie on Sardinops ocellata collapsed and was follow by a strong increase in catches of Trachurus trecae and T. capensis. The japanese fisherie of Sardinops melanosticta has experienced a significative de cline compensate by a high expansion and increase in catches of Scombe r japonicua. If a longer longevity with a more age groups in the popul ation mean a higher stability, the wide trophic spectrum (zooplankton and small fish, and even phytoplankton) capacitate to these species to be much more competitives than other fish species during low favourab le trophic conditions. The high swimming capacity, in the horizontal a nd vertical planes, facilitate to search for food in a more extensive areas, than that covered for the little-sized pelagic fish strongly de pendents of the productive areas on the shelf (upwelling). As a result of all these characteristics,the middle-sized pelagic fish species sh ow an important and quick expansion of their populations, occuping the depletioned zones and support a high fishing presure.