Use of Multiple Methods to Estimate Walrus (<i>Odobenus rosmarus rosmarus</i>) Abundance in the Penny Strait-Lancaster Sound and West Jones Sound Stocks, Canada

Abstract

Atlantic walrus (Odobenus rosmarus rosmarus) occur in Canada in a number of largely discrete stocks (Born et al. 1995, DFO 2002, COSEWIC 2006, Stewart 2008. In the Canadian Arctic Archipelago there are three recognized stocks (NAMMCO 2006, 2011, Stewart 2008: the Baffin Bay stock, the West Jones Sound stock and the Penny Strait-Lancaster Sound stock. Within the range of each stock, walrus are widely distributed in the open-water season when they are found at sea and on ice or land in numbers from 1 to over 1,000. This widely spread but clumped pattern is a challenge to quantitative survey design. However, censusing haulouts is a well-established technique (Buckland and York 2009) for estimating the population size of pinnipeds (e.g. harbour seals, Phoca vitulina: Olesiuk et al. 1990, Thompson et al. 1997, Jacobs and Terhune 2000, Boveng et al. 2003, Reder et al. 2003, Gilbert et al. 2005, Cronin et al. 2007 1977 (290). Regression analysis of MCP and density (number of walrus divided by number of haulouts surveyed) showed no significant trends over time. We also calculated bounded count estimates for comparison. Finally, we used broad-scale behavioural data to estimate the proportion of the total stock that could be considered countable, to produce two adjusted estimates. We selected recent surveys with good coverage and ignored adjusted estimates that were lower than MCP. For the PS-LS stock, the adjusted MCP (with 95% CL) was 672 (575-768) and 727 (623-831) walrus in 2007 and 2009, respectively. For WJS, the best estimates were the adjusted MCP of 503 (473-534) in 2008 and the adjusted bounded count of 470 (297-1,732) in 2009. While both stocks appear to have remained stable over three decades, differences in survey coverage and possible differences in walrus distribution make precise population estimation difficult.

A comparison of image and observer based aerial surveys of narwhal

“…Therefore, they may be harder to count during real-time observation. Imprecise group size estimates are not uncommon in the presence of numerous simultaneous sightings, and observers tend to underestimate the size of large groups (Norton-Griffiths 1974, Scott et al 1985, Mathews 1995, Lowry 1999, Stewart et al 2013. The tendency to underestimate group size has been observed to increase with group size (Gerrodette et al 2018), although this effect was observed in group sizes (>25 individuals/group) exceeding the ones observed during this survey.…”

“…, Mathews , Lowry , Stewart et al . ). The tendency to underestimate group size has been observed to increase with group size (Gerrodette et al .…”

Abundance of Atlantic walrus in Western Nares Strait, Baffin Bay Stock, during summer

“…Total counts from 20 August (hauled out and at sea) allowed a Minimum Counted Population (MCP, for a definition and description see Stewart et al 2014a). Based on information from the three tagged animals we established adjustment factors to account for the proportion of animals that were not hauled out.…”

“…Regression analysis (SigmaStat® v 3.11) was used to assess between-observer counts of photos taken from aircrafts and boats in the Canadian High Arctic during 1998-2008 and along Baffin Island during 2007-2008. These analyses indicated there was little inter-observer variation (Stewart et al 2014a) thus numbers of walruses in photographs from the survey in eastern Ellesmere Island in 2009 were counted by one observer (REAS). When there were data of more than one type available for a site, aerial photo-count was preferred over a visual estimate.…”

“…The nearest over-wintering population is at Coburg Island but the relationship of these summering walrus to those in west Jones Sound and on east Ellesmere Island remains unclear. In east Lancaster Sound, the haulout at Wollaston Island was abandoned several decades ago and recent surveys have found no concentrations at these sites in August (Stewart et al 2014a), although a sizeable group was seen on Cape Sherard in early August 2008 (M. Kuluguktuq, Grise Fiord HTA, pers. comm.…”