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    Antarctic fur seal (Arctocephalus gazella) pup growth rates obtained at Cape Shirreff, Livingston Island, South Shetlands: 1994/95 to 1997/98 (CEMP Index C2, Procedure B)

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    Document Number:
    WS-AREA 48-98/18 Rev. 1
    R. Hucke-Gaete, V. Vallejos and D. Torres (Chile)
    Agenda Item(s)

    Antarctic fur seal, Arctocephalus gazella, pup growth rates were measured using CEMP Standard Methods (C2 - Procedure B) over four consecutive Austral summer season (1994/95-1997/98) at CEMP Site and SSSI N°32 Cape Shirreff, Livingston Island, South Shetlands. Every fortnight (15 days) 100 pups (50 of each sex) were weighed by cross-sectional sampling. Pup growth rates were determined from least square linear regression of weight against age. Strong correlation was found between variables for both sexes, and assumptions of linearity and homogeneity of variance proved to be adequate (? =0.05).
    For comparison purposes we contrasted our data with values reported for Bird Island, South Georgia (1973-1987). From this comparison, Cape Shirreff's male pups resulted to have a faster growth with less deviation from the mean, while females on both sites had very similar growth rates. If these growth rate patterns remain similar for equal time series, it could make sense to suggest that Cape Shirreff's male pups grow faster in order to undergo with better body conditions higher latitude associated problems, although female data would not support this hipothesis.
    Highest growth rate for males was attained in 1995, and females in 1994. Conversely, lowest growth rate for males was attained in 1994, and females in 1995 and 1996. This could be explained by differential maternal expenditure between pup sexes, although this hipothesis has been suggested as invalid by Lunn and Arnould (1997). Further conclusions will have to be tested with larger data sets in order to establish possible differences on reproductive strategies between populations of A. gazella.
    An important decrease in A. gazella's population was observed during 1997/98. No evidence for an ENSO phenomenon influence is explained by pup growth rates until present.
    Pup growth rates might be influenced indirectly by sea-ice extent, and directly by krill or salp dominance, since the highest value of male growth rate (1995/96) was attained when large sea-ice extent was reported, while the two lowest growth rates for males (1994/95 and 1996/97) were attained when less sea-ice was evidenced. This pattern is surprisingly not followed by females, which appear to have no relation with sea-ice variation.
    Four years of data is apparently not enough to sustain all hipotheses described due to some incoherence between sexes, and conclusions drawn are necessarily tentative, but could serve to expose limitations of particular schemes and to give a first insight into variation of Antarctic fur seal reproductive performance in the CEMP site with relation to the local marine environment.