Distribution of north Atlantic Right Whales, Eubalaena glacialis, in Eastern Canada from line-transect surveys from 2017 to 2022

Abstract

In response to the unusually high number of North Atlantic Right Whales (NARW) carcasses (N=12) reported in the Gulf of St. Lawrence (GSL) in 2017, an unprecedented aerial survey effort was deployed for the monitoring of NARW presence in Canadian waters starting in late August 2017. Between 2017 and 2022, a total of 561,187 km of systematic transect lines have been surveyed over periods of up to 7.5 months in some years, involving up to three aircraft simultaneously. Survey effort was deployed in potential NARW foraging areas across eastern Canadian waters, by covering the entire GSL each summer, and the Scotian Shelf and the continental shelf around Newfoundland and southern Labrador every second summer. A total of 185 NARW groups (246 whales) were observed by primary observers during the systematic surveys, with 6 groups in 2017, 25 groups in 2018, 23 groups in 2019, 43 groups in 2020, 31 groups in 2021, and 57 groups in 2022. The vast majority of NARW sightings (93%) occurred in the two survey stratum in the southern GSL (i.e., separated into southeastern and southwestern stratum), however this area accounted for ~58% of the total survey effort. Abundance estimates in this study were calculated based on a distance sampling approach, and reported for each survey pass of each stratum in order to compare abundances among strata and over time (i.e., within a survey season in the case of repeated surveys, and also across years). Two correction factors specific to NARW and to the region surveyed were computed to correct inherent biases of aerial surveys, i.e., availability bias, for animals underwater when the aircraft passed overhead, and perception bias, for animals at the surface of the water that are missed by observers. These two corrections increased abundance estimates by a factor of ~3. While NARW were consistently detected at the beginning (May to mid-June) and at the end (September to November) of the survey season in the southeastern GSL stratum, they also occur in this area in July and August. Indeed, the highest abundance estimate in this stratum across survey years was recorded for the survey conducted in mid August 2022 (97 animals, CI: 31-308). In the southwestern GSL stratum, peak abundances were observed consistently observed each year between early June and early August. The highest fully corrected abundance for the study period came from a pass of the southwestern GSL stratum in mid-June 2018, with 281 animals (CI: 100-790). All surveys of the southwestern GSL stratum conducted between June and end of August reported observations of NARW. Systematic aerial surveys are one of the set of tools available to monitor NARW which, combined with other approaches such as acoustic monitoring, provide useful information required for the conservation of the species.