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

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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
RESULTSPart 1. Cetacean distribution, behaviour and environmental parametersSurvey effort and summary of sightingsWe conducted systematic survey work (during high effort and transect survey modes) on 21 days out ofa total survey period of 38 days. In addition, fisheries observations were carried out on 19 days.Systematic surveys following transects lines were conducted for a total of 22:16 hours over 167.3nautical miles. In addition, high effort surveys were conducted for a total of 43:38 hours, with 32:59hours over 251.8 nmiles within the main survey area. Table 2 shows an overview of visual effort duringdifferent survey effort modes for the main survey area and elsewhere, including details on survey effortduring the 2004 survey.Survey modeEffort(nauticalmiles)Effort(km)Survey effort(hr:min)ProportionofEffort(%)2004 2005 2004 2005 2004 2005 2004 2005FastmodeSlowmoden/aT (in survey area) 226.1 167.3 418.7 309.8 26:02 22:16 3.39 20.20S (in survey area) 232.6 251.8 430.2 466.3 27:18 08:59 3.56 8.15S (French channel) 137.6 109.6 254.8 203 15:26 10:39 2.01 9.66S (Portland) 15.9 0 29.4 0 1:53 0 0.25 0FOF 29.1 1.4 53.7 2.6 05:16 00:14 0.69 0.21TS 25.2 15.3 47.1 28.4 4:40 02:20 0.61 2.12SLOW 164.2 36 303.7 66.7 25:52 05:52 3.37 5.32SLOW (French Channel) 30.8 0 57 0 6:13 0 0.81 0SLOW (Portland) 26.7 0 49.6 0 5:21 0 0.70 0FOS 68.5 21.8 127 40.3 16:38 05:07 2.17 4.64Low Effort (Total) 608.9 613.9 1128 1137 112:10 03:48 14.62 3.45Off Effort (Total) 2261 1783 4186 3301 369:29 20:03 48.16 18.19Monitoring dead dolphins 25.6 44.5 47.4 82.5 08:16 09:58 1.08 9.04Other 1 730.5 196.9 1353 364.6 153:00 20:59 19.94 19.03Total Track 4582 3241 8486 6002 767:14 110:15 100.00 100Table 2. Extent of visual effort and tracks lined for various survey modes within the Survey Areaunless stated otherwise (e.g. the total expedition area (Total), the French part of the English Channeland an area off Portland). 1 Include fisheries observations which differ from FOS and FOF because ofthe presence of ≥1 rigged-hulled inflatable in vicinity of survey vessel and pair trawlers.A total of 286 sightings of approximately 1,290 animals were made during the entire expedition. Thisis less then during the 48 days survey in 2004 during which a total of 469 sightings of 3,707 animalswere made.Five different cetacean species were identified (see Table 3). In order of decreasing numbers ofdetections these included the short-beaked common dolphin, harbour porpoise (Phocoena phocoena),bottlenose dolphin (Tursiops truncatus), Risso’s dolphin (Grampus griseus) and fin whale(Balaenoptera physalus). Basking sharks (Cetorhinus maximus) were also seen on seven occasions. Ofall cetacean sightings, 6.3% remained unidentified. These included 18 sightings of unidentifieddolphins (42 animals). Seventeen sightings were made outside the survey area (e.g. south of 49°20’N),including 13 sightings of common dolphins, one sighting of Risso’s dolphins, fin whale, bottlenosedolphin and an unidentified dolphin.Cetaceans were seen with a sighting rate of 4.5 sightings/hour during transect and high effort modeswithin the survey area, which was higher compared to the findings from 2004 (2.74 sightings/hourduring both transect and high effort modes). The lowest concentration of cetaceans sighted during thevoyage occurred in the French part of the Channel (1.6 sightings/hour). The highest number ofcetaceans seen on a single day during both transect and high effort modes occurred on the 18 th ofFebruary (n=28). The average sighting rate for that day was 6.7 sightings/hour.11
Page 1 and 2: Report of a cetacean survey in theW
Page 3 and 4: EXECUTIVE SUMMARYThe Western Approa
Page 5 and 6: INTRODUCTIONLittle is known about t
Page 7 and 8: Two important assumptions of line t
Page 9: also to report the presence of dolp
Page 13 and 14: SpeciesFAST MODENumber of sightings
Page 15 and 16: Initial sighting cuesDuring fast su
Page 17 and 18: Harbour porpoiseThe harbour porpois
Page 19 and 20: Part 2. Fisheries observationsFishi
Page 21 and 22: On the 23 rd of March, at 08:23, a
Page 23 and 24: Fig 7. Frequency distribution of si
Page 25 and 26: The estimates of abundance generate
Page 27 and 28: Proportion of sightings by perpendi
Page 29 and 30: Proportion of sightings by componen
Page 31 and 32: Fig. 18. Histograms of perpendicula
Page 33 and 34: DISCUSSIONThe estimated corrected d
Page 35 and 36: extrapolated total estimated mortal
Page 37 and 38: Tregenza, N.J.C. and Collet, A. 199
Page 39 and 40: Dolphin 2SPECIES: Common dolphin (D
Page 41 and 42: CETACEAN POSTMORTEM REPORTDolphin 7

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