Migrating eastern North Pacific gray whale call and blow rates estimated from acoustic recordings, infrared camera video, and visual sightings

Regina A Guazzo,John W Durban,John A Hildebrand,Gerald L D’Spain,David W Weller,Hollis M Europe

doi:10.1038/s41598-019-49115-y

Abstract

During the eastern North Pacific gray whale 2014–2015 southbound migration, acoustic call recordings, infrared blow detections, and visual sightings were combined to estimate cue rates, needed to convert detections into abundance. The gray whale acoustic call rate ranged from 2.3–24 calls/whale/day during the peak of the southbound migration with an average of 7.5 calls/whale/day over both the southbound and northbound migrations. The average daily calling rate increased between 30 December–13 February. With a call rate model, we estimated that 4,340 gray whales migrated south before visual observations began on 30 December, which is 2,829 more gray whales than used in the visual estimate, and would add approximately 10% to the abundance estimate. We suggest that visual observers increase their survey effort to all of December to document gray whale presence. The infrared camera blow rate averaged 49 blows/whale/hour over 5–8 January. Probability of detection of a whale blow by the infrared camera was the same at night as during the day. However, probability of detection decreased beyond 2.1 km offshore, whereas visual sightings revealed consistent whale densities up to 3 km offshore. We suggest that future infrared camera surveys use multiple cameras optimised for different ranges offshore.

Highlights

Estimating population sizes of marine mammals is a challenging problem
Visual observers confirmed that no substantial numbers of other species were in the area that could be confused with gray whales on the infrared system
Guazzo et al.[8] estimated that only 10% of gray whales traveled past Granite Canyon outside of the array, but using visual sightings we found that the proportion of whales swimming outside the array was greater than assumed, www.nature.com/scientificreports www.nature.com/scientificreports decreasing the proportion that migrated through the hydrophone array

Summary

Introduction

Estimating population sizes of marine mammals is a challenging problem. Traditionally population size has been calculated using visual surveys including aerial and ship-based line transect surveys and shore-based surveys. The abundance of the eastern North Pacific population of gray whales has been estimated using shore-based visual surveys during the southbound migration since the 1967–1968 migration[1]. After accounting for probability of detection and amount of time with effort, NOAA estimated the eastern North Pacific gray whale population size to be 28,790 individuals during the 2014–2015 season[2]. Bryde’s whales call at different rates depending on their geographic region with whales in the Eastern Tropical Pacific producing calls at a slower rate (intercall interval of 0.3–5.5 min) than those in Japan (intercall interval of 0.05– 0.29 min)[5] These studies exemplify the importance of estimating calling rate for the time and location of interest. Gray whales were thought to produce fewer calls while migrating than while in their breeding areas[6,7], but Guazzo et al.[8] estimated that 4,854 calls were produced within a search area offshore of the NOAA visual observation site, resulting in an average calling rate of 5.7 calls/whale/day

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