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

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Both strata2004Areakm 2EffortL(km)Numberof schoolsnn/L f(0) ESW Expectedgroup size(s)Density (D)ofindividuals(ind/km 2 )AbundanceNEstimate 8,872 448.9 42 0.094 3.402 0.294 6.8767 1.094 9,708%CV 41.76 24.24 12.43 20.52 33.44 33.44Lower 95%Confidence LimitUpper 95%Confidence LimitStratum E2004(by stratification)Area(km 2 )EffortL(km)Numberof schoolsn0.037 2.099 0.228 4.562 0.541 4,7990.236 5.512 0.377 10.367 2.214 19,639n/L f(0) ESW Expectedgroup size(s)Density (D)ofindividuals(ind/km 2 )AbundanceNEstimate 4,129 305.9 18 0.059 3.402 0.294 6.8767 0.688 2,841%CV 35.23 24.24 12.43 20.52 47.96 47.96Lower 95%Confidence LimitUpper 95%Confidence LimitStratum E2005(by stratification)Area(km 2 )EffortL(km)Numberof schoolsn0.026 2.099 0.228 4.562 0.041 1690.136 5.512 0.377 10.367 1.335 5,512n/L f(0) ESW Mean groupsize(s)Density (D)ofindividuals(ind/km 2 )AbundanceNEstimate 4,129 268 47 0.175 4.251 0.235 5.212 1.943 8,022%CV 31.57 17.93 17.93 8.78 37.18 22.26Lower 95%Confidence LimitUpper 95%Confidence LimitStratum E2004+2005(pooled)Area(km 2 )EffortL(km)Numberof schoolsn0.091 2.98 0.336 4.375 0.528 2,1790.339 6.064 0.165 6.209 3.358 13,865n/L f(0) ESW Mean groupsize(s)Density (D)ofindividuals(ind/km 2 )AbundanceNEstimate 4,129 573.9 63 0.109 3.88 0.257 5.158 1.099 4,536%CV 28.76 18.51 18.51 8.93 36.4 36.4Lower 95%Confidence LimitUpper 95%Confidence LimitStratum E2004+2005(by stratification)Area(km 2 )EffortL(km)Numberof schoolsn0.061 2.689 0.372 4.317 0.534 2,2040.197 5.600 0.179 6.164 2.261 9,335n/L f(0) ESW Mean groupsize(s)Density (D)ofindividuals(ind/km 2 )AbundanceNEstimate 4,129 573.9 63 0.109 3.949 0.253 5.063 1.097 4,529%CV 31.41 13.93 13.93 10.53 35.94 35.94Lower 95%0.058 3.0 0.333 4.112 0.324 1,337Confidence LimitUpper 95%0.207 5.198 0.192 6.235 1.869 7,720Confidence LimitTable 12. Estimated model parameters, f(0) (the probability density function evaluated at zero distance), Effectivehalf Strip Width (ESW), density and abundance of short-beaked common dolphins in the Western Approaches ofthe English Channel during the winter months for the total area (both strata in 2004) and for the eastern stratum(Stratum E) in 2004 and 2005 and pooled effort in stratum E (2004+2005) and by stratification.24
The estimates of abundance generated here are based on a number of assumptions including that theprobability of detecting dolphins on the trackline, g(0), is assumed to be one, i.e. every animal on thetrackline is detected. However, this assumption could lead to a slight downward bias in the abundanceestimation because, in practice, some animals may have been undetected. Another assumption of theline-transect methodology is that the animals do not respond to the approaching survey vessels beforethey are detected. The effect of responsive movement is further explored below.Although biased, as outlined earlier, the above estimates do allow for some comparisons between years.Group sizes of common dolphins ranged between 1-22 individuals (2005) with a mean of 5.2. This islower than the 2004 survey and there is some evidence that at the 10% level this is significant (7.1;Students T test, p=0.067).When comparing the density estimates for the different years and survey strata (see Fig.8) it is evidentthat the density for 2004 Stratum E (‘2004 E STRAT’, see Fig.2) is much lower. This may well be aresult of the small sample size (n=18). Based on sample size (n=63) and achieved effort, the2004+2005 estimate by stratification for Stratum E (‘0405 E STRAT’, see Fig 8) is the best estimatefor common dolphins (when assuming g(0) =1 and no responsive movement) with a density D of 1.097individuals/km 2 (CV 35.9%).Provisional estimations for density of Delphinus delphisDensity (ind/km2)43.532.521.510.5004 E+W 0405 E+W 2004 ESTRAT2005 ESTRATSurvey/stratum0405 E 0405 ESTRATFig. 8. Provisional estimations for density (individuals/km2) of Delphinus delphis for different survey years andsurvey strata: 2004 both strata (04 E+W); 2004+2005 pooled data for both strata (0405 E+W); 2004 stratum E bystratification (2004 E STRAT); 2005 primary data (2005 E STRAT); 2004+2005 pooled data collected in stratumE (0405 E); and 2004+2005 stratum E by stratification (0405 E STRAT).25
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: Fig 7. Frequency distribution of si
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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