Toward a comparative evaluation of human impacts on fishery ecosystems of enclosed and semi‐enclosed seas

Abstract Fisheries of enclosed and semi‐enclosed seas provide the first basis for evaluating human impacts on marine ecosystems. These have become of serious concern before similar changes are detectable in oceanic systems, thus emphasizing their value as laboratories for comparative study of man‐induced changes. The paper discusses the relevance of the cline, oligo‐meso‐eu‐dys‐trophic to stressed marine systems, and focuses on impacts on fisheries of enhanced nutrient runoff, noting common features with marine systems subject to natural enrichment, but also with well‐studied freshwater systems. It is suggested that under nutrient enrichment and heavy fishing, both “top down”; and “bottom up”; trophic mechanisms act in synchrony to change the trophic chain, leading initially to increased fishery productivity of formerly oligotrophic systems, followed by more drastic and negative changes as nutrient input passes beyond a state that may be called mesotrophic. Nutrient enrichment and overfishing have similar and synergistic effects: a decline in diversity, an initial increase in productivity of benthic/demersal and pelagic food webs, then the progressive dominance of the production system by short‐lived, especially pelagic, species. The separation of these two anthropogenic effects on semi‐enclosed seas is difficult and has been confounded by the synchronous growth of industrial fishing, and of downstream impacts of growing populations, industrialization and agro‐industry, and growing water usage, especially since World War II. A description of human impacts on several semi‐enclosed seas emphasizes the Marine Catchment Basin (MCB) concept, noting that ranking by the relative extents of catchment area and marine basin explains much of the observed impact. A systems approach to management of inland and coastal seas is suggested, with priority on the control of nutrient flows between terrestrial and aquatic systems, and the establishment of accurate statistical systems integrating fisheries and environmental data over whole basins and coastal seas.