Co-Occurrence and Habitat Use of Fin Whales, Striped Dolphins and Atlantic Bluefin Tuna in the Northwestern Mediterranean Sea

Abstract

Different dolphin and tuna species have frequently been reported to aggregate in areas of high frontal activity, sometimes developing close multi-species associations to increase feeding success. Aerial surveys are a common tool to monitor the density and abundance of marine mammals, and have recently become a focus in the search for methods to provide fisheries-independent abundance indicators for tuna stock assessment. In this study, we present first density estimates corrected for availability bias of fin whales (Balaenoptera physalus) and striped dolphins (Stenella coeruleoalba) from the Golf of Lions (GoL), compared with uncorrected estimates of Atlantic bluefin tuna (ABFT; Thunnus thynnus) densities from 8 years of line transect aerial surveys. The raw sighting data were further used to analyze patterns of spatial co-occurrence and density of these three top marine predators in this important feeding ground in the Northwestern Mediterranean Sea. These patterns were investigated regarding known species-specific feeding preferences and environmental characteristics (i. e. mesoscale activity) of the survey zone. ABFT was by far the most abundant species during the surveys in terms of schools and individuals, followed by striped dolphins and fin whales. However, when accounted for availability bias, schools of dolphins and fin whales were of equal density. Direct interactions of the species appeared to be the exception, but results indicate that densities, presence and core sighting locations of striped dolphins and ABFT were correlated. Core sighting areas of these species were located close to an area of high mesoscale activity (oceanic fronts and eddies). Fin whales did not show such a correlation. The results further highlight the feasibility to coordinate research efforts to explore the behaviour and abundance of the investigated species, as demanded by the Marine Strategy Framework Directive (MSFD).

Abundance, activity and critical habitat of the striped dolphinStenella coeruleoalbain the Gulf of Taranto (northern Ionian Sea, central Mediterranean Sea)

“…Dispersal of the striped dolphins was observed exclusively around midday, characterized by small groups engaged in travelling towards other not investigated offshore areas or within the survey area. Although the reasons are unclear, the dispersal pattern seems very similar to that observed for the striped dolphin in the north‐western Mediterranean Sea (Bauer et al, ; Gannier, ).…”

Risk of lethal vessel strikes to humpback and fin whales off the west coast of Vancouver Island, Canada

“…(Marsh & Sinclair 1989, Buckland et al 2001, Dawson et al 2008. For aerial surveys conducted in other regions, availability bias has been calculated for humpbacks at 0.67 (breeding ground off Brazil; Andriolo et al 2006) and 0.26 (breeding ground off Hawaii; Mobley et al 2001), and for fin whales at 0.25 (feeding ground in the Mediterranean Sea; Bauer et al 2015). While we cannot apply measures of availability bias from other study areas to our calculations, these values give us a rough indication of the level of underestimation that may be present in our whale counts.…”

Estimating cetacean density and abundance in the Central and Western Mediterranean Sea through aerial surveys: Implications for management

“…Nonetheless, these parameters have been previously obtained for the striped dolphin and the fin whale over large portions of the Central and Western Mediterranean Basin, highlighting seasonal, annual and geographical patterns. Panigada et al (2011) and Bauer et al (2015) provide a synthesis of the available information on the species abundance, density and encounter rates in the Western portion of the Basin and present the most recent seasonal abundance and density estimates for the Pelagos Sanctuary, for both striped dolphins and fin whales uncorrected for perception and/or availability biases. Bauer et al (2015) also provide estimates of density -corrected for the availability bias -for the same species in the Gulf of Lions.…”

“…Panigada et al (2011) and Bauer et al (2015) provide a synthesis of the available information on the species abundance, density and encounter rates in the Western portion of the Basin and present the most recent seasonal abundance and density estimates for the Pelagos Sanctuary, for both striped dolphins and fin whales uncorrected for perception and/or availability biases. Bauer et al (2015) also provide estimates of density -corrected for the availability bias -for the same species in the Gulf of Lions. In this paper, we present a correction factor for the perception bias, obtained through a MRDS approach (Burt et al, 2014;Laake and Borchers, 2004) for the striped dolphin.…”