Ecology and comparison of coastal and offshore bottlenose dolphins (Tursiops truncatus) in California

Abstract

Populations of coastal and offshore bottlenose dolphins (Tursiops truncatus) are considered to differ ecologically, with implications for their protection and management. We assessed occurrence, distribution and behaviour of coastal and offshore populations of dolphins during a photo-identification study in Santa Monica Bay and nearby areas (1997–2007). Bottlenose dolphins occurred year-round and were encountered on 44.2% of all surveys (n = 425). We photo-identified 647 individuals; of these, 375 (58.0%) were coastal (<1 km from shore), 241 (37.2%) offshore (1–65 km from shore) and 31 were both (4.8%). Dolphins mostly travel (69.0%) and travel-dive (61.5%), with offshore dolphins socialising more (22.6%) than coastal. There were low re-sighting rates for both coastal and offshore dolphins. Low re-sighting rates of coastal individuals provide little evidence of year-round site fidelity, suggesting their range is greater than the study area. Several individuals were re-sighted between and over 1-year periods, often foraging. This suggests that coastal dolphins: (1) are highly mobile within inshore waters, but spend time foraging in the study area, and (2) range >1 km from shore, contrary to what has been previously reported.

Marine open cage aquaculture in the eastern Mediterranean Sea: a new trophic resource for bottlenose dolphins

“…Such an increase in dolphin concentration around fish farms has been suggested to be related to the high density of wild fish species in the vicinity of the fish cages (e.g. Díaz López & Bernal Shirai 2007, Bearzi et al 2009). Sea-cage fish farms act as 'super-FADs' (FAD: fish aggregation device; Dempster et al 2002), attracting a large variety of wild fish that feed on large food pellets lost through the cages, and/or on particulate organic matter (POM) from broken pellets and feces (Dempster et al 2002, Machias et al 2004.…”

“…Also, scientists have qualitatively observed bottlenose dolphins Tursiops truncatus in the proximity of fish farm cages in several coastal areas of the northern and central Mediterranean (e.g. Díaz López & Bernal Shirai 2007, Bearzi et al 2009). In our study area, surveys at sea conducted between 1997 and 2008 have evidenced a relative increase in the occurrence of bottlenose dolphins searching for food and feeding around coastal fish farms (Bearzi et al , 2009.…”

“…Díaz López & Bernal Shirai 2007, Bearzi et al 2009). In our study area, surveys at sea conducted between 1997 and 2008 have evidenced a relative increase in the occurrence of bottlenose dolphins searching for food and feeding around coastal fish farms (Bearzi et al , 2009. Such an increase in dolphin concentration around fish farms has been suggested to be related to the high density of wild fish species in the vicinity of the fish cages (e.g.…”

“…Few studies have looked at the effects of open cage aquaculture on higher levels of the food web, including top predators such as dolphins (although see Díaz López et al 2005, WatsonCapps & Mann 2005. The importance of these effects in terms of habitat loss, potential for incidental mortality, and benefits deriving from increased prey availability or ease to catch such prey is currently being debated (Würsig & Gailey 2002, Bearzi et al 2009). In several areas of the Mediterranean Sea, scientists have observed an increased occurrence of common bottlenose dolphins Tursiops truncatus (hereafter 'bottlenose dolphin') near fish farms (Díaz López & Bernal Shirai 2007, Bearzi et al 2009), where these dolphins engage in opportunistic foragingpossibly following a decline in their prey throughout the basin (Stergiou & Koulouris 2000, Coll et al 2008.…”

Group size and composition of Tursiops truncatus (Cetacea: Delphinidae), in a coastal insular habitat off southeastern Brazil

“…The variation in group size observed in this study especially after 2008 may be a consequence of one or more of the following factors: (a) the flexible and highly adaptable behavior of the species (SHANE et al, 1986;BEARZI et al, 2008); (b) the fluid nature of a fission-fusion society that undergoes frequent changes in group size, composition and social relationships (see BEARZI et al, 1997;CONNOR et al, 2000 and references therein); (c) the irregular distribution of prey items in coastal habitats (WELLS et al, 1987;WELLER, 1999); and (d) the decrease in biological diversity within the archipelago due to human-induced stresses (RANGEL et al, 2007;ECOMAR/SEAP, 2009). During the censuses, the observed groups often formed small ephemeral subgroups whose composition changed frequently (i.e., the groups showed the fission-fusion behavior) (LODI, 2009).…”