Research Document - 2015/044

Instantaneous availability bias correction for calculating aerial survey abundance estimates for narwhal (Monodon monoceros) in the Canadian High Arctic

By Cortney A. Watt, Marianne Marcoux, Natalie C. Asselin, Jack R. Orr, and Steven H. Ferguson

Abstract

Twenty-four narwhals were fitted with satellite tags near the communities of Arctic Bay and Pond Inlet, Nunavut every August from 2009-2012 and provided information on the time narwhals spend at different depths. Aerial surveys to estimate narwhal abundance were also conducted in August 2013. An accurate estimate of narwhal abundance is needed for advising on a total allowable catch; however, not all narwhals are available for viewing during the survey as most may be submerged and therefore invisible to observers. Narwhals diving to greater than 2 m depths are not distinguishable from an aircraft in clear waters, while narwhals in highly turbid waters, such as that found in fiords where glacial runoff enters, may not be distinguishable at >1 m depths. A correction factor for this availability bias is needed to adjust survey estimates to account for narwhals that may have been present but were either not visible to observers, or not distinguishable from beluga whales. An instantaneous availability correction factor used to correct aerial surveys can be estimated from the proportion of time diving animals spend near the surface where they can be detected and identified. The proportion of time narwhals spent at 0-1 m, and 0-2 m depths was analyzed in a mixed effect model with whale as a random variable and period of August (mid versus late), time of day (day or night), sex, and area of tagging as fixed factors. The chosen model included no variables for the 0-1 m bin and period of August for the 0-2 m bin. Tagged narwhals spent 20.4 ± 0.78 % of their time in the 0-1 m bin in August and we recommend an instantaneous availability correction of 4.90 (± 0.187) for the 2013 survey in regions with highly turbid waters where visibility may be limited. Narwhals spent 31.4 ± 1.06 % of their time in the 0-2 m bin in mid August and we recommend an instantaneous correction of 3.18 (± 0.107) for 2013 survey strata occurring in clear waters.

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