Analyzing mortality rates of small cetaceans using the harbour porpoise, phocoena phocoena, as a case study

Olivia OConnor

Life tables are an important tool for understanding the population demography of a species as well as its conservation status when coupled with models that predict susceptibility of age classes to anthropogenic effects. For cetaceans, however, undertaking assessments of live animals to populate cohort life tables is not possible for wide-ranging pelagic and oceanic species. Thus, one of the main sources of this kind of information arises from the observed age-structure of dead animals (i.e. stranded and bycaught cetaceans). To aid this approach, a freely accessible package strandCet was created for cetaceans in the statistical software R. The package can be used to estimate both natural and non-natural mortality at age from age-structured strandings, based on the Heligman-Pollard mathematical model of mortality, as well as perform life tables and assess population projections using the Leslie matrix. The latter can be used to determine population growth and generation time for a species.           

The current study will test this package under different scenarios using simulated data from an operating (underlying) model where all vital rates are known. Simulated observational data will reflect the features of the real data and the ability of the model to recover the truth will be appraised using a suite of performance indicators.  On satisfactory completion of simulation testing, strandCet will be applied to age data from over 600 harbour porpoises that stranded along the UK coastline between 1990 and 2013. Using the data from the 600 odd-aged porpoises from the England and Wales this will be the first study to perform life tables, calculate total mortality-at-age and produce Leslie matrices to derive population projections, including population growth, net production and generation time on that dataset.