This paper reports the results of a ground survey of fur seal colonies from in the South Shetland Islands in 2007/08. Multiple counts of pups at each colony were conducted to establish confidence limits on pup production. Total pup production was 7,602 (±103) pups down 24.4% from the last census in 2001/02 (10,057 ±142 pups born). Dead pups accounted for 1.64% of the total. A comparison with previous censuses over a 20yr period (1987, 1992, 1994, and 1996) indicates the rate of increase in fur seal populations has diminished substantially. The averaged annual rate of increase from 1987-1994 was between 13.5-13.9%. From 1994-1996 it was 8.5% and from 1996-2002 the average annual rate was +0.9%. Since 2001/02, pup production for the entire archipelago declined at an average annual rate change of -4.3%. Sources of error in pup production estimates include unobserved neonate mortality prior to census, natality rates of the adult female population and leopard seal predation on pups. Neonate mortality up to 30 days after the median date of pupping was estimated for the years 1997-2007 at 4.5% (±0.60). Adult female natality for the same period was 0.79 (±0.02) and in 2007/08 was below average at 0.73. Leopard seal predation was estimated from the rate of mother-pup failed pairs after pups begin entering the water (~30d after the median date of pupping [MDP]). Leopard seal predation for the first 75d post MDP for 2003-2007 was estimated at 0.376 (±0.043) and for the 2007/08 was above average at 0.522. Thus, higher neonate mortality, reduced natality, and an increase in leopard seal predation all contributed to lower pup production estimates in 2007/08.
Document Number:
WG-EMM-PSW-08/14
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Abstract