Οικολογία και δυναμική των νεαρών σταδίων των ψαριών σε ένα παράκτιο οικοσύστημα της Δυτικής Ελλάδας

Abstract

The ecology, the dynamics and the distribution of the early benthic life stages were studied for four sparids (Diplodus vulgaris, D. sargus. D. annularis and Oblada melanura), in a coastal littoral, 2km east of Acheloos estuary. The benthic larval phase of each species was divided in six ontogenetic stages, according to specific discrete melanophore and chromatophore patterns that were found to follow a sequence and were conspicuous enough to be used in visual census counts. Preliminary observations in the study area showed tendency of the benthic larvae to aggregate in specific substratum types and depth range, so stratified sampling was considered as the most appropriate method. Strata were distinguished having as criteria the substratum types accounted in each of the three depth zones of 0-1m, 1-2m and 2-5m. Data concerned abundances of each ontogenetic stage in each stratum during a 15 month interval and they were collected by visual census. Moreover, data on site-attachment ontogenetically depended behaviour of benthic larvae of Oblada melanura were also collected. The later behavior was studied via an index devised, for quantifying different expressions of residence behavior. Two of the four studied species (D. vulgaris and D. sargus), settled during the cold period of the year, while the other two settled during summer. Metamorphosis rates were faster at the first ontogenetic stages for all species, while they were faster in total for the summer species. Temporal and spatial distribution of the species was arranged in order to minimize competition. D. vulgaris and D. sargus settled in the same substratum types in different periods, and O. melanura and D. annularis settled during the same priod in different substrata. D. vulgaris remained in the nursery area the longest period, exhibiting the broadest ecological requirements, been distributed in different substrata and depths. On the other side, O. melanura remained half as long in the nursery area, exhibiting the narrowest ecological requirements, staying always over hard substratum in shallow waters. The other two species exhibited intermediate conditions, with D. sargus preferring hard substratum and D. annularis, seagrass beds. None of the studied species preferred soft substratum. The shallowest part of the littoral appeared to be iv very important for three species (D. vulgaris, D. sargus, O. melanura) and the transition zones (between two different substrata), for all four of them. Benthic larvae of Oblada melanura commenced settlement in small shoals that resided for varying number of days in specific sites over stones or rocky substratum. Gradually as metamorphosis proceeded they became more kinetic forming larger shoals that expanded their home range