Distribution and abundance estimation of sperm whales (Physeter macrocephalus) along the Hellenic Trench in eastern Mediterranean

Abstract

Sperm whales (Physeter macrocephalus) of the Hellenic Trench, Mediterranean Sea, illustrate a constant summer distribution and abundance. The sperm whale population of the Mediterranean Sea has been characterized as “Endangered” by the IUCN (2012) although areas of high occurrence should be under a wider conservation planning. Here, I modelled sperm whale distribution in the Hellenic Trench in order to quantify the distribution of the sperm whale along the Hellenic Trench. To do this a combined method of GAMs-GEEs were used to account for the autocorrelation existed in the data. Social groups and solitary or loosely aggregated males varied significantly in the habitat use within the study area, with males using habitat closer to the shore and social groups to present an affinity for higher Sea Surface Temperature (SST) and Sea Level Anomaly (SLA) values. The covariates remained in the model for the combined dataset (social groups-males) are depth, seabed steepness and distance from the shore, distance from 1km depth contour, SST and SLA. Point transects sampling was used for the abundance estimation of the summer sperm whale population from a combined acoustic and visual survey and an estimate of 27 [19.7, 32.08] individuals was derived with 95% CI. An acoustic detection function was modelled with a Generalized Linear Model (GLMs) with data derived from an experimental dataset. The detectability of sperm whales was influenced by group size, so stratification sampling was applied to take into account the bias introduced by the number of individuals in each group. An acoustic effective range of 13 – 21 km was derived, with bigger sized groups being detected at greater distances than the smaller ones. The Hellenic Trench presents apparently an important area for the sperm whale sub-population of the Mediterranean Sea. The Hellenic Trench has been recommended to be an MPA for the protection of the sperm whale by ACCOBAMs (Agreement on the Conservation of Cetaceans of the Black Sea, Mediterranean Sea and contiguous Atlantic Area).