Evaluation of biological resources in the Adriatic Sea based on plankton investigations

Abstract

        The results of complex investigations of plankton in the Central Adriatic are given. An estimate of the carbon production in the Adriatic and the classification of this sea in this sense was made possible by the determination of the primary pro­duction, which took several years. Four productive zones were quantitatively defi­ned. It was possible to confirm them also by phyto — and zooplankton biomass. The total yearly primary production of the Adriatic was estimated at about 9 milion tons of carbon, about 100,000 tons of zooplankton biomass, and the yearly zooplankton production at about two and a half million tons. All this shows that the Adriatic is not a higly productive sea except in a few smaller zones. In the spawning areas of sardine in the Central Adriatic the average amount of eggs in the channel region is 44/m² and in the open central Adriatic 48/ m².

       The seasonal variations of the phyto-zooplankton are not parallel. Besides the phytoplankton abundance sometimes large and sometimes small quantities of zooplankton occur. During one year phytoplankton has two blooms, and zooplan­kton a productive and a non-productive period. However, the zooplankton quantity follows closely the yearly fluctuation of the primary production, which shows that the amount of the food produced still determines greatly the size of the zooplan­kton populations. The quantity of sardine eggs in the Central Adriatic shows the revers yearly trend of the phytoplankton and zooplankton biomass. This might be due to a greater concentration of the spawning fish consumming thus more food.

        Consideralble year - to - year fluctuations of plankton biomass and production have been found. These fluctuations are connected with some climatic parameters such as the baric situation over a large area of the northern Atlantic, Europe and the Mediteranean. These parameters condition an unequal advection of the eastern water in the Adriatic, which, with the nutrient salts it brings with, has a favourable effect on the production.

       The fish production was estimated from the primary production and the zooplankton production. The two values agree fairly well and are about 300,000 and about 380,000 tons per year respectively. The possible deviation is estimated at + 60 an - 25% respectively with a possibility of forecasting based on climatic factors and the primary production. On the basis of the egg quantity the sardine and anchovy stocks in the Adriatic were estimated (at about 380,000 tons) so that the values of the estimated production and stock could be compared.