Research Document - 2007/025

Comparison of photographic and visual abundance indices of belugas in the St. Lawrence Estuary in 2003 and 2005

By Gosselin, J-F, M.O. Hammill and V. Lesage

Abstract

Beluga abundance in the St. Lawrence estuary and Saguenay River was estimated using photographic and visual aerial surveys from the middle of August to early September in 2003 and 2005. Transects covered an area of 5377 km² in the estuary which corresponds to the main summer concentration of animals. A total of 311 belugas were counted on 1108 photographs taken on 2 September 2003. This count was increased to 312 animals after taking into account that 0.2% of the area photographed was masked by glare from the sun. This count was also multiplied by an expansion factor of 2.021, to account for the 49.5% photo coverage of the estuary. Two animals observed in the Saguenay River were added to the final estimate resulting in a surface abundance index of 632 (SE = 116) beluga for the photographic survey in 2003. Systematic visual line transect surveys were completed along every second line of the photographic survey design in order for the whole area to be covered in a single day. Five visual surveys were flown at an altitude of 305 m in 2003, and another 14 surveys completed in 2005, alternated between altitudes of 305 m and 457 m. Distance analyses were done on the truncated distribution of perpendicular distances from the transect line to account for the areas of lower detectability of animals under (re. left truncation) and away (re. right truncation) from the plane. The perpendicular distance distribution was left-truncated at 99 m and right–truncated at 1569 m in 2003. Left and right truncations were 155 m and 2172 m respectively for the 305 m altitude, and 213 m and 2355 m respectively at the 457 m altitude in 2005. The combined abundance index of 934 (SE = 105) belugas from the visual line transect surveys in 2003 was 48% higher than the index from the photographic survey. In 2005, the combined abundance index of 675 (SE = 101) for the lower altitude (305 m) was not significantly different (F = 1.79, p = 0.21) than the combined index of 531 (SE = 62) at the higher altitude (457 m). The altitude did not have a significant effect on effective strip half width, estimated cluster size nor encounter rate. Belugas were more frequent and generally more abundant in the Saguenay River in 2005 than in 2003. Animals were detected in the fjord on 13 of the 14 surveys completed with an average of 39 individuals in 2005, compared to 3 out of 6 surveys with an average of 6 individuals in 2003. Although abundance indices from the visual and photographic methods were not significantly different, additional comparisons should be completed to ensure calibration of these two techniques.

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