Interactions between Mediterranean bottlenose dolphins (Tursiops truncatus) and gillnets off Sardinia, Italy

Abstract

A first attempt at analysing interactions between bottlenose dolphins and gillnets along the northeastern coast of Sardinia (Italy) was conducted between October 1999 and December 2004. A new approach was used: combining interviews with fishers with boat-based direct observations and behavioural and group size analysis. Fishers on monitored boats noted 2556 days on which gillnet damage was caused by bottlenose dolphins, 68.7% of the total fishing days, with no difference between seasons. An annual estimate of the number of bottlenose dolphins caught in the gillnets was 1.47 (0.98 immatures and 0.49 adults). In all, 317 days were spent making observations at sea, of which 330.6 h were spent directly observing the dolphins. There were no changes in the distribution of daily encounter rate among seasons, group size, or number of immatures, in the presence or the absence of gillnets. However, bottlenose dolphins spent more time both in the study area and feeding when gillnets were present. The extent of the estimated bycatch is worrisome in terms of the ability of bottlenose dolphins off Sardinia to sustain such an annual loss.

Whistle characteristics in free-ranging bottlenose dolphins (Tursiops truncatus) in the Mediterranean Sea: Influence of behaviour

“…One observer monitored the focal group and two observers reported position and behaviour of other members of the group in order to control changes in group composition. The encounter continued until the focal group changed composition or was lost; a group was considered lost after 15 min without a sighting (Díaz López, 2006a). The group size was assessed visually in situ, and the data were later verified with photographs and videos taken during each sighting.…”

“…and aerial activity. The definition and duration of each behavioural category was attempted a posteriori following data analysis strictly based on objective, non-discrete parameters, including specifically observed behavioural events, area, dive duration, swimming direction and speed, contact among individual dolphins, presence of fishing gears, and other variables (Díaz López, 2006a).…”

Interaction between bottlenose dolphin (Tursiops truncatus) and trammel nets in the Archipelago de La Maddalena, Italy

“…Recently, in the context of declining fish stocks, the fishing industry has increasingly viewed these interactions as a problem, particularly so for small-scale fisheries (Lauriano et al, 2004;Díaz-López, 2006, 2009Rocklin et al, 2009).…”

Changes in the catch composition of artisanal fisheries attributable to dolphin depredation in a Mediterranean marine reserve

“…Dolphins have now learned to consider catches of fishing operations as a new food resource (Reeves et al, 2001); they remove fish directly from nets, resulting in a loss of fish for fishers and damage to nets. The consequences of dolphin interactions with fishing activities, however, have often been neglected (Noke and Odell, 2002), although negative impacts on fisheries have already been reported from Morocco (Zahri et al, 2004), Sardinia (Lauriano et al, 2004;Díaz-Ló pez, 2006), and the Balearic Islands (Brotons et al, 2008).…”

“…In the Mediterranean Sea, the status of marine resources is considered to range from fully exploited to overexploited (FAO, 2005), and artisanal fishing is declining (Gó mez et al, 2006), so depredation by dolphins may become even more critical to the activity. Until now, however, a lack of data has limited our ability to evaluate the expanse and nature of dolphin -fisheries interactions (Díaz-Ló pez, 2006). Indeed, few studies have attempted to fill this gap, and most information is anecdotal or unpublished (Reeves et al, 2001;Bearzi, 2002).…”