Ecological Archives E096-248-A1

Alastair M. M. Baylis, Rachael A. Orben, John P. Y. Arnould, Fredrik Christiansen, Graeme C. Hays, and Iain J. Staniland. 2015. Disentangling the cause of a catastrophic population decline in a large marine mammal. Ecology 96:2834–2847. http://dx.doi.org/10.1890/14-1948.1

Appendix A. Detailed description of archipelago-wide census methods and count error and southern sea lion (Otaria flavescens) breeding colonies at the Falkland Islands.

A1: Archipelago-wide census methods and count error.
Southern sea lion breeding colonies were identified by surveying the coast line from a boat. Where possible, observers walked the coast line and interior of each island to ensure all breeding groups were found. Most breeding colonies (80 %) were direct counted from land. Direct counts from the boat (or zodiac) were the preferred method for seven breeding colonies (Table A1). We were unable to land at seven breeding colonies because access was not granted (Table A1). One breeding colony (Beauchêne Island) (n = 3 pups) was counted in 2011 by Falklands Conservation (unpublished data), but not in 2014 due to logistical constraints. Five small breeding colonies reported in 2003 (≤ 30 pups) did not exist in 2014. However, we found six new breeding colonies (Table A1).

Several potential sources of count error exist. These are (1) pups were born after visiting breeding colonies, (2) pups died prior to counting and the carcass was not detected and (3) live pups at some breeding colonies may have been hidden by tussac grass (i.e., had moved away from the main breeding beach) (Hamilton 1939, Thompson et al. 2005).

We cannot account for the first two sources of error, although previous studies suggest they are less than 5 % (Hamilton 1939, Thompson et al. 2005). To explore potential count error associated with pups hidden by tussac grass, we used the average number of pups counted in tussac grass as a proxy for potential error. In 1995, the number of pups counted in tussac was 4 % (based on breeding colonies counted after 31 January 1995) (Thompson et al. 2005). In 2014, all pups remained on breeding beaches until the 24 January. After the 24 January, pups at 14 breeding colonies were found in tussac grass on the periphery of breeding beaches (Table A1). Based on these 14 breeding colonies, the number of pups in tussac grass was, on average, 14 ± SD 12 % of the total count. We estimated the potential count error for all breeding colonies counted after 24 January. Count error was derived by drawing a fractional error from a uniform distribution, assuming the uncertainty of the original count was between 4 % and 14 %. A value (count) was then randomly drawn from a sampling distribution based on the error. The procedure was repeated 10,000 times and the resulting 10,000 possible estimates generated for each breeding colony were summed. The 2.5 and 97.5 percentiles calculated as the lower and upper band of the 95 % confidence interval.

One known breeding colony (Port Louis) was not counted in 2014 because of issues associated with site access. In 2003, 64 pups were counted at Port Louis. To estimate the number of pups at Port Louis in 2014, we used the same bootstrap approach detailed above, but with an error range that was based on the population change between 2003 and 2014 for colonies of a similar size (>60 pups). The lower and upper error used was 17 % and 94 %, respectively.

For 36 breeding colonies counted on or after 24 January 2014, the total number of pups estimated using a count error of between 4 % and 14 % to account for pups that could have moved from the beach into surrounding tussac grass, ranged between 2,923 and 2,951 (lower and upper 95 % CI). Therefore, the total number of pups born in 2014 could be as high as 4,591 or a 66 % increase in the number of pups since 2003, using upper 95 % confidence intervals for breeding colonies counted before (1,513) and after (2,951) 24 January and Port Louis.

Table A1. Breeding colonies of southern sea lions at the Falkland Islands counted in 2014, 2003, 1995, 1965 and the 1930s. Dead pups are included in the total column, and reported as the value within parenthesis. D = direct count. DB = preferred method, direct count from boat.

Breeding colony reported in 1934–1937, but not in 1995, 2003, or 2014

1 Includes 750 pups ‘estimated’ rather than direct counted
2 Location unknown. Description rather than name of island.
3 Access not granted.
*Counts were ‘estimates’ rather than direct counted

Literature Cited

Hamilton, J. E. 1939. A second report on the southern sea lion (Otaria byronia) (de Blainville). Discovery Reports 19. Pages 121–164.

Thompson, D., I. Strange, M. Riddy, and C. Duck. 2005. The size and status of the population of southern sea lions Otaria flavescens in the Falkland Islands. Biological Conservation 121:357–367.

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