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

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Estimation of the critical distancePalka and Hammond (2001) developed a method that uses animal orientation data collected onshipboard line transect surveys to determine whether, and at what critical distance, cetaceans areresponding to the survey ship. We assume that the observed orientation of an animal at the surface isrepresentative of the direction of travel and that the animal’s swim speed is typically much less thanship speed.Swim directions were defined as relative to the transect line and measured clockwise from 0° to 360°,where 0° describes an animal swimming parallel to and in the same direction of the ship, and 90°describes an animal swimming perpendicular to the transect line and to the right. Observers werescanning from the trackline to 90° on either side of the trackline. Swim directions were pooled intoquadrants where quadrant 1 represents swim directions 0-90°; quadrant 2 swim directions of 90°-180°;quadrant 3 swim directions of 180°-270°; and quadrant 4 swim directions of 270°-360°. The product ofvalues in all quadrants is assumed to equal 1, and factors are assumed to be independent. In linetransect sampling, factors affecting detection of animals are assumed to be the same on either side ofthe platform; thus, we ‘folded’ over starboard sightings onto the port side and used achieved data toestimate the ‘critical distance’ at which the effect of responsive movement begin.The expectation is that animals do not react to ships when sighted at sufficiently far distances (greaterthan a critical distance r c ), and so the ratio n 3 /n 1 (when r> r c , where n i are the number of sightingsdetected with a swim direction in quadrant i) would not be significantly different from 1. At closerdistances, if responsive movement occurs, then the ratio n 3 /n 1 (when r< r c ) would be eithersignificantly greater than 1 (for avoidance) or significantly less than 1 (for attraction). If no responsivemovement occurs, then the ration n 3 /n 1 would be 1 at all radial distances.The findings are illustrated in Figure 19, where the ratio of n 3 /n 1 are shown for different radial distancecategories for the primary platform (n=118). It is evident that the ratio n 3 /n 1 becomes close to 1 atradial distances > 1100m. Sample size for the secondary platform did allow conducting similar analysis.The method requires swimming directions of animals to be measured without error at all distances.However, observers found it difficult to judge the direction of travel, especially at greater distancesfrom the secondary platform. A video camera mounted on top of the binoculars may increase theaccuracy of the measurement or at least evaluate observer judgement. More observer training andpractice should also improve the measurement. Alternatively, it may be easier for observers to estimatethe direction of travel in relation to eye of sight in stead of relatively to the ship’s track.The ratio n3/n1 for primary dataRatio n3/n110.90.80.70.60.50.40.30.20.10
DISCUSSIONThe estimated corrected density for the 2005 survey (Stratum E) was 1.312 individuals/km 2 (CV=0.41;95% CI=0.59-2.89) and the corrected estimated abundance 5,417 (95%CI=2,457-11,949). Whenpooling data across years the corrected density estimate by stratification for Stratum E was 0.74individuals/km 2 (CV=0.39; 95%CI=0.34-1.59) with a corrected abundance estimate of 3,055 (95%CI=1,425-6,544).In the absence of movement we would expect the correction factor, c, to equal g(0) -1 . A c much smallerthan g(0) -1 indicates movement towards the vessel, while a c much larger than g(0) -1 indicatesmovement away towards the vessel. The double platform survey indicated that primary observers onlymissed 7% of the dolphins on the trackline (g(0)=0.93), but that a strong responsive movement towardsthe boat resulted in apparent densities 1.5 as high as they normally would be. The ESW of thesecondary platform is rather narrow (316m) compared to the radial distances at which animals weredetected. This is due to the narrow secondary platform search sector (30° on either side of the trackline,due to platform restrictions). Animals could therefore approach the vessel from outside this searchsector and still be detected by the Primary observers. Indeed, 42% of all primary platform sightingsduring double platform sightings were made outside 30° (see fig. 20). This means that the strip widthfor duplicates may well be underestimated and is possibly the reason why the obtained ratio ofESW duplicates /ESW primary (1.5) is rather small compared to other studies (see Table 14).Source Species g(0) Correction factor(c)Hammond et al. Harbour porpoise0.31 2.7 0.37(1995)(Phocoena phocoena)Hammond et al. White-beaked dolphin 0.57 0.56 1.79(1995)(Lagenorhynchusalbirostris)Canadas et al. Common dolphin0.8 0.17 5.9 *(2004)(Delphinus delphis)Dawson et al. Hector’s dolphin0.89 0.5 2(2004)(Cephalorhynchus hectori)Buckland and Pacific Dall’s porpoise 0.6 0.13 7.7Turnock (1992) (Phocoenoides dalli)This surveyCommon dolphin0.93 0.68 1.5(Delphinus delphis)Table 14. Overview of estimates for g(0), the correction factor c and its reverse using the BT methodin different surveys and this survey. * The c -1 for common dolphins reported by Canadas et al (2004)was calculated as ESW duplicates /ESW primaryThe overall relative abundance of cetaceans was high (18.3 sightings per 100km effort), which ishigher compared to 2004 (16.9). These repeatedly high relative abundances reaffirm that the surveyarea, which is co-incident with the pair-trawl fishery, is important for common dolphins during winter.Common dolphin group sizes in our surveys (2004 χ=7.1 & 2005 χ=5.2) were markedly lower thanthose of other studies. For example, during the SCANS survey, of their Block A, the mean group sizeof of common dolphins was 10.8 (see Hammond et al., 1995). Macleod and Walker (2004) alsoreported a higher mean group size of 37.7 (SD 70.8). However, they noted that group size variedseasonally and reported a relatively low mean group size of 16.9 (SD 34.7) during winter.c -133
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 and 10: also to report the presence of dolp
Page 11 and 12: RESULTSPart 1. Cetacean distributio
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: Fig. 18. Histograms of perpendicula
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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