Combined line-transect and cue-count estimate of sperm whale abundance in the North Atlantic, from Icelandic NASS-2001 shipboard survey

Thorvaldur Gunnlaugsson,Daniel G Pike,Gisli A Víkingsson

doi:10.7557/3.2706

Thorvaldur Gunnlaugsson, Daniel G Pike + Show 1 more

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https://doi.org/10.7557/3.2706

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Abstract

Sperm whales (Physeter macrocephalus) pose a particular problem to shipboard surveys as they dive for extended periods and are therefore likely to be missed (not available) even if they are right under the track line. To address these problems the NAMMCO planning committee for the NASS 2001 survey drew up guidelines to be followed when sperm whales were sighted. This required every deep dive to be recorded and considered to be a cue, from which a cue-count estimate is calculated if the cue rate is known. For those whales that did not dive before coming abeam, a conventional line-transect estimate is calculated, which gives an instantaneous surface estimate from which a total estimate can be obtained if the proportion of the time spent at the surface is known. These estimates are compared and combined. Precise dive cycle information is missing for the mostly single all male sperm whales in this area but a preliminary estimate of 11,185 (cv 0.34) is obtained for the surveyed area with an assumed surface time of 20% and two deep dives per hour.

Highlights

The 2001 North Atlantic Cetacean Sighting Survey (NASS-2001) was a continuation of periodic synoptic sightings surveys conducted in the Eastern and Central North Atlantic in 1995, 1989 and 1987
Sigurjónsson and Víkingsson (1997) presented an estimate of sperm whale abundance based on NASS-87 Icelandic shipboard data, but as it was not clear where in the dive cycle the animal was when it was sighted, certain simplifying assumptions had to be made (Gunnlaugsson and Sigurjónsson 1990)
Sperm whales pose a particular problem to shipboard surveys as they dive for extended periods and are likely to be missed even if they are right on the track line

Summary

Introduction

The 2001 North Atlantic Cetacean Sighting Survey (NASS-2001) was a continuation of periodic synoptic sightings surveys conducted in the Eastern and Central North Atlantic in 1995, 1989 and 1987. Presented here is an analysis of the Icelandic vessel data with respect to sperm whales. The planning and conduct of the surveys is detailed in cruise-reports (Desportes et al MS 2002, Víkingsson et al MS 2002). Distances, whereas the proportion missed depends on the forward sighting distances and the dive cycle of the animal. Sigurjónsson and Víkingsson (1997) presented an estimate of sperm whale abundance based on NASS-87 Icelandic shipboard data, but as it was not clear where in the dive cycle the animal was when it was sighted, certain simplifying assumptions had to be made (Gunnlaugsson and Sigurjónsson 1990). Sperm whales pose a particular problem to shipboard surveys as they dive for extended periods and are likely to be missed even if they are right on the track line. The area covered, called the effective search width, depends on the distribution of perpendicular sighting

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