Abstract
The depleted population of belugas (Delphinapterus leucas) inhabiting the St Lawrence estuary, Canada, was monitored by periodic photographic aerial surveys. In order to correct counts made on aerial survey film and to obtain an estimate of the true size of the population, the diving behaviour and the visibility from the air of these animals was studied. A Secchi-disk turbidity survey in the belugas’ summer range showed that water clarity varied between 1.5 m and 11.6 m. By studying aerial photographs of sheet-plastic models of belugas that had been sunk to different depths below the surface, we found that models of white adults could be seen down to about the same depth as a Secchi disk, but no deeper. Smaller models of dark-grey juveniles could only be seen down to about 50% of Secchi-disk depth. By observing groups of belugas from a hovering helicopter and recording their disappearances and re-appearances, it was found that they were visible for 44.3% of the time, and that an appropriate correction for single photographs would be to multiply the photographic count by about 222% (SE 20%). For surveys in which there was overlap between adjacent frames, the estimated correction would be 209% (SE 16%). This correction factor was slightly conservative and gave an estimate of the true size of the population, based on a single survey, of 1,202 belugas (SE 189) in 1997. An estimate for 1997 based on smoothing 5 surveys 1988–1997 was 1,238 (SE 119).
Highlights
The population of belugas (Delphinapterus leucas) inhabiting the St Lawrence estuary was seriously depleted by intensive hunting—for commercial products, to protect fisheries, and for recreation—which was not controlled in any way until it was closed in 1979
Belugas dive out of sight in the turbid water of the St Lawrence estuary, and cannot be seen from above, so photographic counts only provide an index of numbers
The Secchi-disk depth used as a standard measure of water turbidity gave an adequate indicator of the depth at which a white adult beluga could be seen on aerial survey film, at least for vertical viewing, either visually or photographically, and in turbidities ranging from moderately turbid to moderately clear
Summary
The population of belugas (Delphinapterus leucas) inhabiting the St Lawrence estuary was seriously depleted by intensive hunting—for commercial products, to protect fisheries, and for recreation—which was not controlled in any way until it was closed in 1979. Belugas dive out of sight in the turbid water of the St Lawrence estuary, and cannot be seen from above, so photographic counts only provide an index of numbers. This could generate information on trend, distribution, and, by inference from the size of the animals seen on the photographs, on the age structure of the population (Sergeant and Hoek 1988, Kingsley and Hammill 1991, Kingsley 1993, 1996). While trend information is important in evaluating the status of declining populations, for stationary or increasing populations the absolute size is more important (IUCN 1994). The absolute historical size of the population in the late 1800s was estimated by calculating back using data on catches made over the years (Laurin 1982), so an absolute estimate of its size would provide a measure of population recovery
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