The aim of this work is to investigate the consistency of data collected using ferries as platform of opportunity, for estimating cetacean distribution and abundance in the Pelagos sanctuary. Distance sampling analysis is aimed to transform line transect data into densities calculated on a strip defined by Effective Strip Width (ESW). During summer 2011, 32 different Marine Mammal Observers (MMO) collected data onboard of 10 different ferries along 4 different routes crossing the Ligurian sea. Each ferry trip was considered as a strip-transect, surveyed once a week, all long the season. The study analyzes the influence of MMO and ferry characteristics (height of bridge and speed) as bias on data. To do so, ESW is estimated along each route as a measure of the effectively surveyed area. GLMs have been applied to verify the effects of the three studied variables (MMO, height of bridge and speed) on the perpendicular distances. Strong differences have been found among ESW along the four different routes with consequences on the estimated sampled strip (which ranged between 0.6 and 2.7 km2 width). Furthermore, we established that the ESW differences are mainly due to differences in species composition per route (e.g. average ESW for fin whales, bp, is 1640 m while average ESW for striped dolphins, sc, is 441 m). Thus, GLMs model have been fitted separately for striped dolphins and fin whales. For both species, the only studied parameter affecting sighting perpendicular distance is the bridge's height (P_value_sc=1.01E-07; P_value_bp= 8.05E-02), while ferry speed and MMO don't show any significative statistical influence. Regional differences and platform characteristics should then be taken into account when analyzing this kind of dataset for density estimation purposes. Nevertheless, data collected from ferries are a valuable source for monitoring programs on cetacean presence and distribution within wide areas.
File in questo prodotto:
Non ci sono file associati a questo prodotto.