Comportement d'échouage du phoque commun (Phoca vitulina vitulina) et facteurs de correction pour les survols aériens effectués en Atlantique canadien entre 2019 et 2021

Abstract

Aerial surveys were conducted between 2019-2021 to estimate the abundance and distribution of harbour seals (Phoca vitulina vitulina) throughout Atlantic Canada: coastlines of the St. Lawrence Estuary and Gulf of St. Lawrence, the Bay of Fundy, southwest Nova Scotia, Eastern Shore and Cape Breton, as well as the Newfoundland and Labrador Shelves. These surveys were conducted in favourable conditions, during the pupping period of harbour seals, except in some areas of Newfoundland and Labrador Shelves where the survey was conducted during the moulting period. To account for animals not available to be counted during aerial surveys (i.e., at sea), haulout correction factors were calculated from: 1) data collected via satellite telemetry deployments on harbour seals in the St. Lawrence Estuary; and 2) published literature values. Twelve harbour seals (combination of adults or juveniles, and pups) were instrumented with satellite transmitters in the St. Lawrence Estuary, providing data on their haulout behaviour during the survey period (May 15-June 30, 2022). Haulout periods were identified from the transmitted hourly percent dry timelines. The proportion of the population hauled out at any given time during the survey window was estimated using a bootstrap approach, which corrected for the unbalanced sex-age sample of tagged animals. This method estimated that a proportion of 0.33 (95% CI: 0.09-0.60) of the harbour seal population was hauled out on average at any given time during survey-like conditions, corresponding to a mean correction factor of 3.0 (CV: 41.7%). When combined with published pupping correction factors in the Northwest Atlantic (range: 2.30-2.58), the weighted mean of the proportion hauled out was 0.39 (95% CI: 0.27-0.52) for a correction factor of 2.55 (CV: 16.02%). For parts of Newfoundland and Labrador Shelves surveyed during the moulting period, we calculated a weighted proportion hauled out of 0.61 (95% CI: 0.50-0.71) from published literature estimates, corresponding to a correction factor of 1.64 (CV: 8.67%). The bootstrap framework developed for this analysis of haulout behaviour from satellite telemetry data provides an important way forward in capturing behavioural variability and generating correction factors for aerial surveys. Additional tagging effort in different regions and across multiple years would be required to provide correction factors for harbour seals that reflect local conditions and thereby improve estimates of abundance and the associated uncertainty.