Report of a cetacean survey in the Western Approaches of the ...

these were not collected in the same manner. Furthermore, the presence of trawlers may well stronglyinfluence the behaviour and occurrence of cetaceans (see de Boer et al., 2005).Buckland & Turnock (1992) developed a method for simultaneous correction of both animals missedon the track line and responsive movement of the animals prior to detection. The method requires asecondary platform to detect animals before they move in response to the survey vessel and does notrequire the probability of detection on the trackline, g(0), to be unity. Further, if sighting distances andangles of the secondary platform are measured without error, the method is unbiased when sightingdistances and angles recorded by the primary platform are subject to bias or error. Data from theprimary platform are used to estimate the encounter rate (number of detections per km 2 ), while datafrom the secondary platform allow the effective width of search from the primary platform to beestimated.Using the program ‘Distance’ (Laake et al., 1993) the conventional estimate of density (groups/km 2 )was obtained by equating the number of detections from the primary platform (n p ) with the numberexpected. When assuming g(0) equals 1, the equation is:D p = n p f(0) / 2L (1)Where;n pf(x)L= number of primary detections= probability density of perpendicular distances x recorded from the primary platform= length of transect (km)The density estimate is biased if the animals move in response to the survey vessel before detectionfrom the primary platform or if the probability of detection on the trackline is less than unity. Anestimate that is asymptotically unbiased in the presence of both effects is:D c = n p f p (0) / 2L g p (0) (2)Where;f p (x)g p (y)= probability density of perpendicular distances prior to responsive movement, of animalssubsequently detected by the primary platform=probability that an animal detected from the secondary platform at perpendicular distance yfrom the trackline of the primary platform is subsequently detected from the primary platform(= detection function for the primary platform)g p (y) = n ps f ps (y) / n s f s (y) (3)If the secondary platform is not in continuous operation, the above procedure is carried out on datacollected while both platforms were in operation and a correction factor is calculated as:c = D c / D p (4)The density for the entire survey area is then estimated by cD, where D is estimated from the sightingsdata from the primary platform for the full survey, calculated assuming g(0) = 1. The abundanceestimate is calculated by N=D A (where A is the survey area in km 2 ) and the corrected abundanceestimate by N c = c D A.The coefficient of variation (CV) of the corrected abundance estimate can be calculated with thefollowing equation (Turnock et al., 1995):CV (N c ) = √{CV 2 (c) + CV 2 (N)} (5), whereCV(c) = s.e (c) / c (6)Upper and lower 95% confidence intervals for N c can be calculated by using the Satterthwaite degreesof freedom procedure outlined in Buckland et al. (2001, p.120-121).10