Abundance and Breeding Cycle of the Australian Sea Lion in Western Australia

Brief description

This study assessed sea lion pup production at 9 breeding sites in Western Australia (ranging from Houtman Abrolhos Islands to Six Mile Island at the start of Great Australian Bight) from 1988 to 2001 to estimate population size overall. Trends in pup production, pup mortality and the timing of the breeding season were monitored to gain an understanding of population dynamics across the range of the species in Western Australia.

The influence of environmental variables, in particular activity of the Leeuwin Current, in demographic processes was also investigated.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: Nine breeding sites were selected as target colonies for the purpose of ongoing population monitoring. The cost of repeated surveys of every breeding colony in Western Australia was prohibitive, so a sub-sample of colonies representing the geographic and temporal breeding variation was chosen. The locations were Houtman Abrolhos Islands, Beagle Island, North Fisherman Island, Buller Island, Hauloff Rock, Red Islet, Kimberley Island, Glennie Island and Six Mile Island. Sampling was not consistent among all target colonies from 1989 to 2001 due to a number of logistical constraints. The three west coast colonies in the Jurien Bay area (Beagle Is, North Fisherman Is and Buller Is) were visited most frequently and represent the most comprehensively surveyed colonies. Previously published data on pup production and mortality (Gales et al. 1992 & 1994) for the target colonies for the period 1998-1992 were incorporated into this study to look at patterns over an extended timeframe.

Breeding colonies were visited towards the end of the predicted breeding season to ensure that the majority of that season's newborn pups would be present. All animals were counted and defined according to one of the following seven classes as originally described by Gales et al. (1992).

Bulls - Mature males which have developed a white cap. These animals are of breeding age.

Cows - Females of breeding age, grey brown coat with creamy ventral surface. Often recognisable by presence of a pup.

Sub-adult males - Males which are distinguishable from mature females by their larger size, broader skull and face and darker ventral coat (spotted or dark grey-brown)

Juveniles - Male or female animals which have same colouration as females but are smaller.

Newborn pups - Newborn pups which still retain the chocolate-brown natal coat.

Moulted pup - Recently moulted pups which are developing the colouration of juveniles, but on the basis of size are less than one and a half years old. After this age it is difficult to distinguish them from the previous cohort which is classed as juveniles.

Dead pups - Pups that were considered to have been born that breeding season and found dead.

Colonies were transversed by two or three individuals and all sea lions (live or dead) counted. Extensive searching was conducted as these animals are capable of using all terrain on the islands and pups are sometimes hidden away in well protected rock crevices and under vegetation. A single count was maintained by the recorder and other researchers would report sightings of animals to the recorder, whilst trying to prevent unnecessary disturbance to the animals. This ensured the maximum accuracy of the count and also prevented disturbing mother-pup pairs. Dead pups were also included in the counts, and used as an estimate of pup mortality during the breeding season.

The numbers of pups counted (p) represented a proportion of the overall pup production (P) of the colony for that season. To arrive at P it was necessary to know at what time of the season the count was conducted. It was assumed that all breeding seasons lasted approximately 4-5 months, and that the distribution of births throughout the time period of the breeding season was linear. Knowing the ratio of moulted to newborn pups, and that pups moult at around 3-4 months, it is possible to gauge how long the breeding season had been going, and then to extrapolate from pup counts to calculate P. The extent of male defense of oestrus females was also used to indicate the progression of the breeding season. It is likely that this technique results in an underestimate of pup production, as some pups may be overlooked during the count, as some may have moved from the colony to a nearby haulout, or have died and their carcasses not been present during the survey. This technique was originally used for N. cinerea by Gales et al. (1992 & 1994), and it is assumed that the application of similar methods will enable an assessment of population trends to be made over a longer time scale than previously reported.