Encounters with pelagic and continental slope cetacean species near the northern shore of the Gulf of Taranto, Italy

Abstract

Little is known about the distribution and species composition of cetaceans in the Gulf of Taranto, central Mediterranean Sea. Available information is limited to stranding reports and to scant preliminary observations at sea. Cetacean sightings were recorded between April 2007 and October 2009 during 23 daily boat surveys, totalling 1728 km of navigation. Three cetacean species were encountered: striped dolphins (n = 41), Risso's dolphins (n = 5) and fin whales (n = 3). An additional sighting of a sperm whale group is reported. Encounter rates of striped dolphins were the highest, suggesting a regular occurrence in the study area. Risso's dolphins seemed to be predictably present in continental slope waters off the coast of Basilicata. Fin whales and striped dolphins, normally inhabiting deep pelagic waters offshore, were found in continental slope waters relatively close to the coast, possibly due to the particular bathymetry of the Gulf of Taranto and to oceanographic conditions that enhance local productivity. High human impact and worsening environmental conditions in the northern Gulf of Taranto raise concerns over the future of cetaceans living in these waters. This study expands previous knowledge and aims to stimulate research on whales and dolphins in what seems to be an important portion of their Mediterranean range.

Keywords:

• Cetaceans
• Gulf of Taranto
• Mediterranean Sea

Introduction

The northern Ionian Sea portion delimited by the Italian regions of Calabria, Basilicata and Apulia, known as Gulf of Taranto, is among the least-known Mediterranean areas as far as cetacean populations are concerned. Whale and dolphin strandings in this area have been reported since the nineteenth century (Notarbartolo di Sciara & Bearzi 2005), but systematic research at sea was never conducted. Available information from recent times is limited to stranding reports (http://mammiferimarini.unipv.it/), together with a few preliminary observations at sea (Cerioni et al. 1995) and acoustic detections (Pavan et al. 1997). This study – one of few dedicated investigations in this part of the Mediterranean – contributes to knowledge on cetaceans living in the Gulf of Taranto and aims to stimulate interest in field research on free-ranging whales and dolphins in what may be an important portion of their range.

Materials and methods

The study area, approximately 1350 km² wide, is situated in the northern portion of the Gulf of Taranto, Ionian Sea. The bathymetric profile of the Gulf of Taranto is characterised by a central canyon 1000–2000 m deep, and a steep continental slope in nearshore waters west of Taranto (Figure 1). The Gulf of Taranto is characterised by high inter-annual and seasonal variability of hydrological and biological variables (Sardà et al. 2004). Primary production seems to be generally higher in the Gulf than in other parts of the Ionian Sea, as a result of significant upwelling (De Lazzari et al. 1999; Manca et al. 2006).

Figure 1. The study area, with survey effort and sighting positions.

Data were collected between April 2007 and October 2009 from speedboats equipped with outboard engines (average speed 15–25 km h^–1). Survey effort totalled 23 days at sea and 1728 km. Survey data were collected following pre-determined tracks (Figure 1) under daylight and long-distance visibility, sea state <1 Douglas, swell <1. Observations were interrupted if the sea state or weather deteriorated. A minimum of two standing observers were scanning the sea surface in search for cetaceans; observer's eye elevation was about 1.5 m above the sea level. Digital photos of the animals taken during the sightings helped to confirm species identification. Encounter rates of cetaceans groups were computed based on cells of 2′ latitude by 3′ longitude (3700 by 4260 m), containing a minimum navigation ‘on effort’ equal or greater than a cell's diagonal (5642 m). Encounter rates were calculated by the ratio n/L, where n is the total number of sightings in a cell and L is the total number of km spent in the same cell (Bearzi et al. 2009).

Results

Three cetaceans species were observed across 49 encounters (Figure 1): striped dolphins Stenella coeruleoalba (n = 41), Risso's dolphins Grampus griseus (n = 5) and fin whales Balaenoptera physalus (n = 3). Sighting positions are shown in Figure 1. An additional sighting of a group of four sperm whales Physeter macrocephalus in waters further offshore (40°12′23″ N 17°06′48″ E) was recorded on May 2007 and kindly reported by the Customs Police of Taranto. Group size (mean ± SD): striped dolphins 41 ± 6.0, Risso's dolphins 5 ± 7.0, fin whales 3 ± 1.0. Mean bottom depth (mean ± SD): striped dolphins 368 ± 183 m, Risso's dolphins 465 ± 165 m, fin whales 517 ± 158 m. Encounter rate (groups/100 km, mean ± SD): striped dolphins 8.6 ± 8.2, Risso's dolphins 6.7 ± 4.2, fin whales 5.7 ± 1.1. Encounter rates of striped dolphins in the three years of the study did not show significant differences (Kruskall–Wallis H = 2.89, p = 0.24, df = 2, n = 25).

Discussion

This study confirms the occurrence in the Gulf of Taranto of fin whales, sperm whales, Risso's dolphins and striped dolphins. Encounter rates of striped dolphins were particularly high. Risso's dolphins were found in upper continental slope waters close to the coast, consistent with present knowledge (Bearzi et al. 2010). Fin whales and striped dolphins typically inhabit deep pelagic waters, but they are also known to occur in productive continental slope waters (Notarbartolo di Sciara et al. 1993, 2003; Cañadas et al. 2002; Fortuna et al. 2007; Azzellino et al. 2008) and this may explain their occurrence in the nearshore waters of the upper Gulf of Taranto. Increased monitoring effort in this poorly studied area would allow assessment of cetacean population abundance and trends, as well as identification of the main anthropogenic threats and of possible mitigation measures. An appropriate research effort would support the formulation of management action intended to ensure a favourable status and long-term survival of marine mammals, as advocated by the EC Habitats Directive and the Marine Strategy Framework Directive and consistent with national commitments to preserve cetaceans and marine biodiversity.

Acknowledgements

This research is part of the following projects:

•	‘Jonian Cetacean Project’ supported by the University of Bari, Italy, through a financial grantship to AQ;
•	‘Cetacea’ supported by Fondazione Cassa di Risparmio di Puglia, Bari, Italy, through a financial grantship to AQ.