Annual population estimates of Southern Elephant Seals at Macquarie Island from censuses made annually on October 15th.

Brief description

INDICATOR DEFINITION
Count of all adult females, fully weaned pups and dead pups hauled out on, or close to, the day of maximum cow numbers, set for October 15.

TYPE OF INDICATOR
There are three types of indicators used in this report:
1.Describes the CONDITION of important elements of a system;
2.Show the extent of the major PRESSURES exerted on a system;
3.Determine RESPONSES to either condition or changes in the condition of a system.

This indicator is one of: CONDITION

RATIONALE FOR INDICATOR SELECTION
Elephant seals from Macquarie Island are long distance foragers who can utilise the Southern Ocean both west as far as Heard Island and east as far as the Ross Sea. Thus their populations reflect foraging conditions across a vast area.

The slow decline in their numbers (-2.3% annually from 1988-1993) suggests that their ocean foraging has been more difficult in recent decades. Furthermore, interactions with humans are negligible due to the absence of significant overlap in their diet with commercial fisheries. This suggests that changes in 'natural' ocean conditions may have altered aspects of prey availability. It is clear that seal numbers are changing in response to ocean conditions but at the moment these conditions cannot be specified.

DESIGN AND STRATEGY FOR INDICATOR MONITORING PROGRAM
Spatial Scale: Five beaches on Macquarie Island (lat54 degrees 37' 59.9' S, long 158 degrees 52' 59.9' E): North Head to Aurora Point; Aurora Point to Caroline Cove; Garden Cove to Sandy Bay; Sandy Bay to Waterfall Bay; Waterfall Bay to Hurd Point.

Frequency: Annual census on 15th October

Measurement Technique: Monitoring the Southern Elephant Seal population on Macquarie island requires a one day whole island adult female census on October 15 and a daily count of cow numbers, fully weaned pups and dead pups on the west and east isthmus beaches throughout October.

Daily cow counts during October, along the isthmus beaches close to the Station, provide data to identify exactly the day of maximum numbers. The isthmus counts are recorded under the long-established (since 1950) harem names. Daily counts allow adjustment to the census totals if the day of maximum numbers of cows ashore happens to fall on either side of October 15. Personnel need to be dispersed around the island by October 15 so that all beaches are counted for seals on that day. This has been achieved successfully for the last 15 years.

On the day of maximum haul out (around 15th October) the only Elephant seals present are cows, their young pups and adult males. The three classes can be readily distinguished and counted accurately. Lactating pups are not counted, their numbers are provided by the cow count on a 1:1 proportion. The combined count of cows, fully weaned pups and dead pups provides an index of pup production.

The count of any group is made until there is agreement between counts to better than +/- 5%. Thus there is always a double count as a minimum; the number of counts can reach double figures when a large group is enumerated. The largest single group on Macquarie Island is that at West Razorback with greater than 1,000 cows; Multiple counts are always required there.

RESEARCH ISSUES
Much research has been done already to acquire demographic data so that population models can be produced. Thus there will be predicted population sizes for elephant seals on Macquarie Island in 2002 onwards and the annual censuses will allow these predictions to be tested against the actual numbers. The censuses are also a check on the population status of this endangered species.

LINKS TO OTHER INDICATORS

Lineage

Progress Code: onGoing

Statement: Two different methodologies have been employed to collect these data. From 1988 to 2004 a full census of all colonies on the island were conducted each year between the 13th and 17th of October - details of which are presented below. From 2005 onwards, owing to diminishing resources, a decision was made to only census a representative sample of the southern elephant seal colonies on the island. 5 colonies were selected as being representative of the whole island. Further information about this 'partial' census can be found in a word document available for download from the URL given below.

In the data graphs displayed below, the 'Full Census' graph present data collected between 1988 and 2004 using the full census method. The 'Partial Census' graph present data from 1988 onwards using the partial census method - for the period 1988-2004, the figures for the 5 colonies used in the partial census have been obtained from the full census data.

Full Census Method:

Female Elephant seals haul-out to pup on island beaches according to a very regular pattern (Gaussian or Normal distribution) and timetable (peak haul-out between 13 and 17 October). They group together in 'harems' with a few associated dominant males and are readily counted due to their obvious locations on open beaches and their lack of movement once ashore and having pupped. Thus counts of females can be made accurately to within +/- 5%.

The counts of the adult females (cows), fully weaned pups and dead pups are summed daily, and separately, through October for the west and east isthmus beaches. These totals represent the time distribution of numbers of cows ashore for each of these beaches. Corrections to the census totals are made in two stages:

Firstly, Gaussian curves are fitted to these distributions by least-squares calculations. From the equation to the curve for the west isthmus, the day of maximum numbers is determined as the calculated mean day. If the day of maximum numbers is not October 15, then the total for October 15 is increased by the proportional difference between it and the peak total of the fitted curve.

Secondly, the totals of the numbers for each of the five island zones counted on October 15 are increased to numbers predicted for the actual day of maximum cows ashore. This is done by transforming the fitted curve for the west isthmus into daily values of the proportion of cows present, from 1 (which is the number of cows at the peak of the curve) to fractions for each day on either side of the peak. Then the zone totals are divided by the fraction appropriate for their number of days delayed. This is done because the island beaches are grouped into five zones (see figure in instructions) which have characteristic day delays in their elephant seal cow numbers reaching a maximum with respect to the west isthmus area.

These delays are given in days:
ZONE- DAYS (delay from calc. peak day, west Isthmus)
North Head to Aurora Point - 0
Aurora Point to Caroline Cove - 3
Garden Cove to Sandy Bay - 2
Sandy Bay to Waterfall Bay - 3
Waterfall Bay to Hurd Point - 5

The total for the census is the sum of all these five adjusted zone totals.