Ecology of planktonic stages of the anchovy, Engraulis encrasicolus (LINNAEUS, 1758), in the central Adriatic

Abstract

       The results of analyses of spatial-temporal distribution and survival of anchovy planktonic stages collected from the open sea and channel area for fifteen years and from the coastal area for nine years are presented. This study was undertaken to contribute to the knowledge of their ecology as well as of the fluctuations in their numbers to show what caused them in the first place.

        Temperature effects on egg developmental time and the rate and properties of growth of anchovy larvae and post­larvae under experimental conditions were examined for the purpose of these analyses. Mathematical approximations of these relationships were also made. They provided the basis for the calculations of the age of anchovy developmental stages in the plankton. Global, monthly and annual mean mortality coefficients of anchovy eggs and larval stages were calculated from the known age from which the periods of specific mortality were also established.

       Egg production and the numbers of anchovy larvae and postlarvae per unit time and area were calculated for each individual sample from known egg developmental time as affected by temperature and from mortality coefficients. Thus the data on plankton catches were made intercompa­rable irrespective of the time and place they were realized at.

       The analysis of long-term monthly means showed that spatial-temporal distribution of anchovy planktonic stages during spawning season was affected more by the dynamics of biotic environmental factors (primary production and zoo­plankton quantity) than by that of the abiotic ones (tem­perature, salinity).

       The study of the relationship between anchovy planktonic stages and adult anchovy catches from the study area showed that during intensive reproduction anchovy were far less accesible.

       Long-term egg production fluctuations, the numbers and survival of anchovy postlarvae were observed in relation to the above mentioned abiotic and biotic environmental fac­tors and anchovy catch fluctuations applying the spectral analysis method.

        The similarity between oscillation periods of all analysed parameters led to the conclusion that the entire anchovy population responded to environmental changes with a phase lag of about a year. It seems to be largerly affected by the trophic basis changes.

         Calculated basic oscillation period provide a good basis for eventual long-term forecast of anchovy population fluc­tuations in the Adriatic.