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The abundance of common bottlenose dolphins (Tursiops truncatus) in the former special marine reserve of the Cres-Lošinj Archipelago, Croatia

There is a resident common bottlenose dolphin population inhabiting the Kvarnerić area of the northern Adriatic Sea, Croatia. The observed decline in population size between 1995 and 2003 led to the declaration of a part of the Kvarnerić as the Cres-Lošinj Special Marine Reserve (CLSMR) in 2006. Protection lasted 3 years, before lapsing. A boat based photo-identification study was conducted between 2004 and 2011 with the aim to estimate the number of bottlenose dolphins inhabiting the CLSMR area, investigate the factors influencing their habitat use and explore the possibility that this area is representative of the population for the whole Kvarnerić. Standard mark–recapture methods and the Mth estimator of Chao for closed populations were applied. The lowest value was estimated in 2008 (N = 112 ; 95% CI = 94–150) and the highest in 2006 (N = 310 ; 95% CI = 265–392). The statistically significant inter- annual variability is likely to reflect natural and anthropogenic driven shifts in habitat use rather than real changes in population size. The average monthly sighting rate was lower in the tourist season (Jun–Aug) than in the pre- and post tourist-season, suggesting that developed nautical tourism may influence shifts in habitat use. Other potential causes are distribution of prey and fishing effort. Inclusion of data from the rest of the Kvarnerić for the years of 2005 and 2011 did not reveal a significant increase in estimated abundance (2005: z = −0.413, P = 0.3398 ; 2011: z =  −1.749, P = 0.0401). This indicates that the CLSMR area is representative of the whole Kvarnerić. The number of bottlenose dolphins using the CLSMR area seems to have increased compared with the previous study (1995–2003). This could be due to a shift in habitat use or an increase in population size. However, yearly variations indicate processes causing displacements within the dolphin population home range. Conservation measures aiming at reducing the displacements are proposed.

common bottlenose dolphin; Tursiops truncatus; abundance; population size; Adriatic; conservation; ecology; protected area

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