Patterns in the mortality of common dolphins (Delphinus delphis) on the Portuguese coast, using stranding records, 1975-1998

Abstract: Spatial and temporal patterns of distribution of common dolphins (Delphinus delphis) off the Portuguese continental coast were examined using a stranding database, for the period 1975 to 1998. Information on sex and size composition of the strandings and by-catch events was analysed to elucidate the social organization of this species. Studies were based on 294 stranded specimens and 124 confirmed by-catches. The annual number of dolphins stranded increased during the study period, probably as a result of a higher observer effort. Stranding records suggested that common dolphins are present off the Portuguese coast in all seasons and months of the year and occur in all the regions. However, geographic distribution of mortality was not homogeneous—larger numbers of strandings were recorded in the northern and central areas of the country. Three factors may be responsible for this pattern: differences in the distribution and/or abundance of the species, oceanographic conditions, and/or topographic features of each area. Significant differences were found in the number of strandings per season, with 37% occurring in the spring and 33% in the winter months, which may be a consequence of the severe weather conditions at this time of the year. Sex ratio of the stranding and by-catch records was significantly biased towards males in all geographic areas and seasons. Similarly, a significantly higher proportion of immature individuals were found both in the stranding and by-catch datasets. These results could suggest either the existence of differential mortality by sex and maturity, or the occurrence of age and sex segregation in the population. Stranding data also suggest that fishery interactions could be responsible for up to 44% of mortalities for this population.


Document Type: Research article

Pages:  88-98

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