Natural markings of Cuvier's beaked whale Ziphius cavirostris in the Mediterranean Sea

Abstract

Errors in analysis using natural marks have been recognised since the early development of mark-recapture techniques. In this nine-year study, the mark types present on Cuvier's beaked whales Ziphius cavirostris in the Mediterranean Sea were categorised in order to assess the reliability of natural marks for long-term photo-identification studies. Mark prevalence, abundance and gain/loss rates among age/sex classes were investigated. Results showed that Cuvier's beaked whales were extensively marked (96% of population; mean = 48 marks per individual, range 0–169), with up to 10 different mark types. However, only five mark types should be considered reliable: notch, large scar, back indentation, medium scrape and large stripe. Marks caused by intraspecific interaction were the most abundant and prevalent mark types. In high-quality photographs, 71% of the population was reliably marked, but reliable marks were unevenly distributed within the population and their prevalence appeared to be strongly correlated to age and sex of individuals. The gain rate of scrapes that could be used reliably for identification was six times higher in mature males than females of the same age. This was the case for mature as well as immature males, suggesting that aggressive interaction among males may start early in their life, before reaching maturity. This difference between sexes may cause an over-representation of males in a typical photo-identification database, especially among immature individuals.