Annual Cycle of Fur Seals, Arctocephalus Forsteri (Lesson), on the Open Bay Islands, New Zealand I

Pacific Science (1975), Vol. 29, No.2, p. 139-152 Printed in Great Britain Annual Cycle of Fur Seals, Arctocephalus forsteri (Lesson), on the Open Bay Islands, New Zealand I EDWARD H. MILLER2 ABSTRACT: Fur seals, Arctocephalusforsteri (Lesson), were studied on the Open Bay Islands, South Island, New Zealand in 1970-1971. Few adult males were present at the colony site during the winter, but many arrived ashore in November to vie for territories. Individual territorial males remained ashore and fasted for up to 63 days before losing or abandoning their territories. A few adult males re appeared briefly at the colony site a few weeks after abandoning their territories, anclleft again. Subadult males were common at the colony and other parts of the Open Bay Islands at the start of the breeding season, but their numbers declined steadily throughout it. Adult females frequented the colony site throughout the year. Some pregnant females appeared some weeks before parturition in areas where they subsequently gave birth, then left to feed. The tendency of pregnant females to feed heavily in the weeks prior to giving birth resulted in few females being ashore in mid-November. Pregnant females landed ashore about 2.1 days before parturition. After having given birth, they remained ashore with their pup for about 8.8 more days before leaving to feed. Parturient females entered estrus and copulated about 7.9 days postpartum; sexual receptivity was observed to last up to 14 hrs. Parturient females were absent for about 4.4 days on their first feeding trip after having given birth, and were ashore with their pups for about 2.8 days immediately thereafter. Subsequent feeding periods at sea were longer. Mothers nursed their pups fm about one-third of the time that the former were ashore. The fraction of time spent with mothers by pups on land changed little between Dec ember and May, and the female-pup nutritional bond extended in some cases for up to a year. Nonbreeding adult (?) females increased in numbers near the colony as the summer progressed, then declined near the end. Very young males and some older subadult males were common at the colony site in May, but relatively few very young females were then present. An estimated effective sex ratio of 6.1: 1.0 (females: males) prevailed in the colony during breeding. Sex ratios based on census data consistently underestimated this figure. The annual cycle is characterized by marked synchrony of births: about three-fourths of them fall in a 22-day period. A temporal equivalent of McLaren's "marginal male effect" may selectively favor a short period of pupping and copulation by females, helping to maintain a brief breeding period in the face of ecological determinants of breeding synchrony that are weaker for A. forsteri in New Zealand than for populations of some other pinnipeds. OUTSTANDING STUDIES of the annual cycle of by Bartholomew and Peterson (Bartholomew the fur seal Callorhinus ursinus have been made and Hoel 1953; Peterson, unpublished, 1965, 1968), but tEen: is only limited information on I Manuscript received 20 September 1974. feeding and nursing rhythms, seasonal move 2 University of Canterbury, Department of Zoology, ments, and seasonal changes in population Christchurch, New Zealand. Present address: Dalhousie University, Department of Biology, Halifax, Nova composition of Arctocephalus species. Some Scotia, Canada. information is available on population com- 139 140 PACIFIC SCIENCE, Volume 29, April 1975 position and the annual cycle of an Australian missed censuses, and influences of circadian, population of A. forsteri (Stirling 1971a, b), tidal, and meteorological factors on patterns but only scattered and generally vague data of haul-out necessitated, for parts of the study, have been published on New Zealand popu the summing of several days' data throughout lations (for references, see "Discussion"). In the summer period. Some kinds of data, this paper, I discuss the annual cycle of A. therefore, were summed over 5-day periods forsteri as based on research performed on the (see below). Where censuses were missed, Open Bay Islands, South Island, New Zealand, estimates for day-blocks were made based on which support a colony of 2,000 to 3,000 the mean figures for sampled days within the animals (Crawley and Brown 1971). day-blocks. Correspondences between dates Species nomenclature follows that of Repen and day-blocks are as follows: 1, 1-5 Nov.; ning, Peterson, and Hubbs (1971). 2, 6-10 Nov.; 3, 11-15 Nov.; 4, 16-20 Nov.; 5, 21-25 Nov.; 6, 26-30 Nov.; 7, 1-5 Dec.; 8, 6-10 Dec.; 9, 11-15 Dec.; 10, 16-20 Dec.; MATERIALS AND METHODS 11,21-25 Dec.; 12, 26-30 Dec.; 13,31 Dec.-4 Censuses and observations were carried out Jan.; 14,5-9 Jan.; 15, 10-14 Jan.; 16, 15-19 on Taumaka, the larger of the two Open Bay Jan.; 17, 20~24 Jan.; 18, 25-29 Jan.; 19, 30 Islands, from 27 October 1970 to 13 February Jan.-3 Feb.; 20, 4-8 Feb.; 21, 9-13 Feb. 1971 and from 26 May to 2 June 1971. General observations were made there from 17 to 24 August 1970. (For descriptions of habitat see RESULTS Burrows 1972 and Miller 1971.) Census figures for females, yearlings, and During the 1970-1971 austral summer, adult territorial males present on the colony early morning censuses (0400-0700 hrs) regu study areas are summarized in Figure 1. Some larly provided for each day the highest census territorial males were present on the islands figures. Therefore, these figures are used in the in late October, and their numbers increased following data descriptions. Censuses were slowly until mid-December. Yearlings, some made almost daily for a grassy clearing near still nursing, were seen most days until 9 the north end of Taumaka, for the rocky reefs December. Some gaunt starveling yearlings off the north end of Taumaka, and for the trail were seen but most yearlings appeared to be connecting the hut to the observation blind well fed. The female population reached a local overlooking the main study area on the colony maximum in mid-November, reached a mini site. Daily censuses were made on the main mum about 1 week later, then increased rapidly study area. until the end of the 1st week in December. During May and June 1971, it proved neces It then fluctuated erratically and declined sary to capture juvenile seals in order to slowly until late January. A second increase confirm their sex. This had been unnecessary and subsequent decline in female numbers during the preceding summer because most occurred in early February. immature seals then present were large enough Male seals encountered on the trail and to display diagnostic sexually dimorphic fea grassy clearing ("plateau") could be unequi tures (e.g., shape and massiveness of the face, vocally classed as adults or subadults but I shoulders, and foreflippers). found it difficult to distinguish members of All territorial males and certain subadult these classes when censusing the outlying reefs males (SAMs) could be individually distin through binoculars. The general similarity guished on the basis of features such as color between the shapes of the curves for total of the pelage, broken canines, scars, shape and males and for SAMs (Figure 2a) suggests, other~charactetlstics -brine -face,-ana vocaliza~ howevef, that errors inClis1:ingliishihg members tions. Individual adult females were more ofthe classes were minor. The SAM population difficult to identify regularly, but a small fluctuated erratically until about day-block 7, number was identifiable by similar features. then declined steadily. Low numbers of some classes of seals, A SAM "arrival" at the main study area Annual Cycle of Fur Seals on the Open Bay Islands-MILLER 141 90 70 30 10 earlings 5 5 15 25 5 15 25 5 15 Dec Jan Feb FIGURE 1. Seasonal trends shown in census figures taken during early morning for adult females, territorial males, and yearlings on the main study area. was considered to be the arrival or hitherto (Figure 2b). The class was probably comprised unrecorded presence of a SAM there. Seven largely of females, for the following reasons. individually recognizable SAMs were recorded When the outlying reefs were visited and as returning to the main study area after as long censused at close quarters, most classifiable as 56 days, and 12 SAMs were recorded there seals were females. Thus, on 31 October two on from 2 to 8 consecutive days. The number females were seen there, none was present on of SAM arrivals per hr on the main study area 2 or 3 November, 18 were counted on 20 declined steadily over the summer (Figure 3). December, 17 on 13 January, 20 on 4 February, Only one SAM was recorded landing on the and 27 on 5 February. Second, since the SAM main study area after day-block 8, although population declined over the summer (Figure SAMs were then still common on noncolony 2a) it can be inferred that the population of un census areas (Figure 2a). However, census recognized male"neuters" also declined. Final figures for SAMs on the main study area ly, female BT, who gave birth on the main were highest before the population decline on study area and whose pup died, was twice noncolony census areas became apparent observed on the outlying reefs in the weeks (Figure 2a), suggesting that the dispersion of following her pup's death, suggesting some SAMs over the islands as a whole obscured ,a use of that location by females who have lost decline in population that was present from their pups.

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