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Fish Prey of Fur Seals Arctocephalus Spp. at Subantarctic Marion Island

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Fish Prey of Fur Seals Arctocephalus Spp. at Subantarctic Marion Island Marine Biology (1998) 131: 559±566 Ó Springer-Verlag 1998 N. T. W. Klages á M. N. Bester Fish prey of fur seals Arctocephalus spp. at subantarctic Marion Island Received: 16 July 1997 / Accepted: 2 March 1998 Abstract The analysis of scats collected between 1989 Marion Island, while breeders of the smaller A. gazella and 1995 from the two fur seal species resident on population are found on the southern coast. Their adult subantarctic Marion Island, Arctocephalus gazella and population sizes were approximately 39 000 and 1000 A. tropicalis, showed that they fed predominantly on ®sh seals, respectively, in 1994/95 (Hofmeyr et al. 1997). of the family Myctophidae (lantern®shes). Scat compo- In 1989 a sampling programme of fur seal scats sition (prey species, abundance) was very similar for the (faeces) was initiated in order to address our lack of two species. The seven species of myctophids that knowledge of the diets of both species of fur seals resi- formed numerically 90 and 86% of the scat composition dent on Marion Island. Speci®c aims of this investiga- for A. gazella and A. tropicalis, respectively, all showed tion were to determine to what extent their ®sh diets seasonal ¯uctuations in their contribution to seal diets. dier and whether temporal changes in the prey species Electrona carlsbergi, E. subaspera, Metelectrona ven- composition are evident. It was also of interest to what tralis and Gymnoscopelus fraseri increased in winter in degree fur seal ®sh diets overlap with those of king both species of fur seals, whereas Gymnoscopelus piabilis, penguins (Aptenodytes patagonicus) on Marion Island, Protomyctophum choriodon and P. tenisoni showed the as they were perceived as potential competitors for food. opposite trend. Seal diets overlapped substantially with those of the king penguins (Aptenodytes patagonicus) resident on Marion Island, but no evidence for com- Materials and methods petitive exclusion could be found between these two major warmblooded consumers of marine resources at Scats of Arctocephalus tropicalis were collected at the Cape Davis the Prince Edward Islands. breeding colony site from August 1989 to November 1994. Scat sampling from A. gazella was done at the Watertunnel Stream breeding colony site from November 1989 to April 1995. An attempt was made to clear the study sites from old scats prior to collection, Introduction thereafter only the freshest material was taken. Neither age group nor sex of the seals frequenting the beaches was determined at the Two species of otariids, the subantarctic fur seal Arc- time of sampling as the haulout pattern of both species of fur seals at Marion Island is well known (Condy 1978; Kerley 1983). A break- tocephalus tropicalis and the Antarctic fur seal A. gazella down of the numbers collected by month and by year for each breed on Marion Island (46°54¢S; 37°45¢E), southern species is given in Table 1. At collection in the ®eld, scats were kept Indian Ocean. Each species favours its own breeding sites separate, i.e. the material was not pooled. On return to base they on the island, although some hybridization occurs in were provisionally washed with water in a sieve with 0.5 mm mesh size to remove most of the non-diagnostic soft material and then mixed colonies (Kerley 1984). Breeding sites of A. tropi- stored dry until analysis. In the laboratory, the hard prey remains calis are mainly con®ned to the exposed west side of contained in the faeces (mostly otoliths, several squid beaks) were cleaned further using an ultrasound shaker, then dried and sorted. The otoliths were identi®ed by comparison with reference specimens held in the collections of the Port Elizabeth Museum, and with il- Communicated by O. Kinne, Oldendorf/Luhe lustrations in the pertinent literature (Nolf 1985; Williams and McEldowney 1990; Smale et al. 1995). Otolith diameters of pristine N.T.W. Klages (&) Port Elizabeth Museum, P.O. Box 13147, specimens were measured under a dissecting microscope ®tted with a graticule, while eroded otoliths were only identi®ed and counted. Humewood 6013, Republic of South Africa Prey lengths and masses were estimated from regressions published M.N. Bester in Adams and Klages (1987), Hecht (1987), Williams and McEl- Department of Zoology and Entomology, downey (1990), Cherel et al. (1997) and Olsson and North (1997). University of Pretoria, Pretoria 0002, Insucient data precluded estimating length and mass of some ®sh Republic of South Africa species of minor numerical importance. 560 Table 1 Arctocephalus gazella (A.g.), A. tropicalis (A.t.). Numbers of scats sampled from both fur seal species resident on Marion Island each month and year from 1989 to 1995 Month 1989 1990 1991 1992 1993 1994 1995 Totals A.g.. A.t A.g. A.t. A.g. A.t. A.g. A.t. A.g. A.t. A.g. A.t. A.g. A.t. A.g. A.t. Jan 6 17 9 5 5 5 9 4 29 31 Feb 5 8 10 3 6 11 5 5 26 27 Mar 30 11 6 3 5 5 15 56 19 Apr 7 10 5 5 6 5 5 5 28 20 May 2 3 1 5 4 15 5 12 23 Jun 3 3 4 5 2 10 3 12 18 Jul 3 1 6 6 8 3 10 17 Aug 7 2 3 1 4 5 3 6 19 Sep 8 3 1 5 6 11 Oct 5 3 3 1 2 3 11 Nov 1 4 3 5 2 9 2 5 21 Dec 16 20 8 3 3 5 4 31 28 Sums 17 44 63 70 33 24 27 45 37 53 23 9 24 0 224 245 The scat material was augmented with a small collection of prey 5 g as the frequency distributions of the prey masses remains ¯ushed from the stomachs of ®ve subadult males and one show (Fig. 2). The heavier prey items with masses of 20 adult female Arctocephalus tropicalis at Marion Island. The seals had been restrained during methodological trials of stomach lavage to 30 g were mostly G. piabilis. Overall, the median ®sh techniques in April 1990 (Ferreira and Bester 1998). lengths estimated from the scats were 75 and 83 mm SL for A. tropicalis and A. gazella, respectively. The overall median ®sh masses were 5.8 and 7.3 g, respec- Results tively, for the two predators. Thus, the ratio between predator and prey was approximately 0.05 in length and Otoliths of the mesopelagic ®sh family Myctophidae 0.0001 in weight, as the cows of both species of Arc- (lantern®sh) were by far the most numerous hard-prey tocephalus can weigh up to 50 kg and attain a total body components identi®ed in the scats of both species of fur length of 1.4 m (Jeerson et al. 1993). For males the seals, with up to six dierent species making up an in- ratios would be even smaller. dividual scat. Fish from other families were rare, con- Scat collection was too irregular over the years (Ta- tributing <1% by numbers to the diet of Arctocephalus ble 1) to permit a full-scale investigation of inter-annual gazella or A. tropicalis (Table 2). Squid beaks appeared and seasonal changes in their composition, but dier- in small numbers only. They were invariably of such a ences became evident when the data were divided into small size or so damaged that positive species identi®- summer and winter collection periods. These trends were cation was impossible. Remains of other prey classes, essentially identical for both species of fur seals. Winter e.g. crustaceans, were not found. Prey species lists and was de®ned as those 6 months of the year when the mean numerical compositions of the scats from both fur seals sea surface temperature at Marion Island drops below (Table 2) showed much similarity, although statistically 5 °C (1 May to 31 October; Schulze 1971). Summer there was a highly signi®cant dierence (v2 1524, comprised the remaining half year. All seven species that df 32, p < 0:0001). A. gazella had a slightly more di- formed 90 and 86% of the scat composition by numbers verse diet (28 versus 25 taxa), but the two predators still for Arctocephalus gazella and A. tropicalis, respectively, shared 21 out of 32 taxa (65.6%) identi®ed in the scats. showed seasonal ¯uctuations in their contribution to Using equations given in Pianka (1975), the percent- seal diets (Table 4). Electrona carlsbergi, E. subaspera, numerical diet data of both fur seals indicate a dietary Metelectrona ventralis and Gymnoscopelus fraseri in- overlap of 0.86 (where 1.0 indicates complete overlap creased in winter in both species of fur seals, whereas and 0.0 indicates none). G. piabilis, Protomyctophum choriodon and P. tenisoni The length and weight characteristics of ®sh con- showed the opposite trend. A sensitivity analysis showed sumed by Arctocephalus gazella and A. tropicalis were that these trends were also discernible, although not as also very similar (Table 3) although not identical pronounced, when winter was assumed to start a month (v2 1703, df 1352, p < 0:0001). The frequency dis- later (1 June instead of 1 May) or when the start of tribution of the sizes (Fig. 1) peaked at about 50 mm winter was moved a month forward (1 April instead of standard length (SL), and this peak was entirely due to 1 May). Protomyctophum tenisoni. A second elevation of both The stomachs of all six animals of Arctocephalus curves at 80 to 90 mm SL originated from a species mix tropicalis subjected to lavage were nearly empty. Re- chie¯y belonging to the genera Electrona and Gym- sults of the identi®cation of the few hard parts that noscopelus.
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