Temporal variations of secondary production in the marine bivalve Spisula subtruncata off the Po River delta (Italy)

Abstract

As a part of a benthic monitoring programme, size-frequency analysis was used to study the life history and production of a population of Spisula subtruncata (da Costa) inhabiting the sandy deposits of the Northern Adriatic Sea in front of the Po river delta. From grab samples collected quarterly at six stations during the years from 1979 to 1984, all the individuals were measured. Growth and mortality rates were determined following cohorts through time; production was calculated as the sum of growth increments. The life-cycle is mostly annual, recruitment occurring during the summer months, and only a few individuals surviving to the following summer. The values of secondary production over the entire life-cycle for each of the five cohorts studied ranged from 5·13 g m −2 to 58·06 g m −2 at 5 m deep stations, and from 1·43 g m −2 to 36·92 g m −2 at 8 m deep stations. The P B ratio calculated for the first year of life of the five cohorts ranged from 2·70 to 3·76 at 5 m and from 2·64 to 7·43 at 8 m. The results are discussed in relation to the environmental conditions during the study period.