Estimation of biological parameters for the common dolphin (Delphinus delphis) and its importance on dynamic models. An approximation of the mortality-at-age and the influence of the bycatch

The short-beaked common dolphin (Delphinus delphis) is the most abundant small cetacean in the north eastern Atlantic and in some regions by-catch is considered to be unsustainable. To help inform conservation measures, population models are needed to explore how the population will react under different scenarios. In order to build a population model, mortality-at-age curves derived from stranding data were constructed, using a Siler model fitted to the total observed dataset. Due to the paucity of data for the youngest age classes, a separate model was fit to a subset of the data that did not include the youngest age classes. Finally, a Heligman-Pollard model was fitted to the second subset of data as well, taking into account the proportion of by-caught dolphins in the sample. The survivorship curves for the Siler models reported that a median of 30% of the females achieve maturity when we used the total observed data set but only the 18% if we remove the youngest ages. The Heligman-Pollard model showed a more realistic shape since by-catch mortality was taken into account. The results of this model show an effective growth of 0.912 which indicate a population decline, likely due to high rates of by-catch.