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Marine Pollution Bulletin 52 (2006) 1077–1080

Note

Entanglement of Antarctic fur seals at Bouvetøya, Southern Ocean

G.J. Greg Hofmeyr a,*, Marthan N. Bester a, Steve P. Kirkman a,Christian Lydersen b, Kit M. Kovacs b

a Mammal Research Institute, Department of Zoology and Entomology, University of Pretoria, Pretoria 0002, South Africab Norwegian Polar Institute, Polar Environmental Centre, 9296 Tromsø, Norway

Abstract

Entanglements of Antarctic fur seals Arctocephalus gazella were recorded during four summers from 1996 to 2002 at the subantarcticisland, Bouvetøya. Rates of entanglement varied between 0.024% and 0.059%. These rates are low for a pinniped population and mightbe because of the geographic isolation of the haulout site. An apparent decrease in the levels of entanglement over the course of the studywas likely due, at least in part, to the removal of entanglements by observers. At least two-thirds of entangling materials were generatedby fishery sources. Since there is no known local source of anthropogenic marine pollution, seals become entangled either in waters dis-tant from the island, or when materials drift into local waters. Significantly more subadults were found entangled than expected from thepostulated population age class distribution.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Antarctic fur seal; Anthropogenic debris; Arctocephalus gazella; Bouvetøya; Entanglement; Southern Ocean

1. Introduction

The entanglement of pinnipeds in marine pollution is ofimportance when considering both their welfare and theirconservation. Entanglements have the potential to causethe suffering and death of individual animals by eitherrestricting or decreasing efficiency of movement (Feld-kamp, 1985; Feldkamp et al., 1989) or by causing injuries(Feldkamp, 1985; Fowler, 1988; Pemberton et al., 1992).In addition, if this source of mortality is sufficiently preva-lent it can have significant demographic effects (Fowler,1988; Harcourt et al., 1994). Increasing rates of entangle-ment are being registered for many populations of pinni-peds (Waldichuk, 1978; Fowler, 1987; Stewart andYochem, 1987; Walker et al., 1997; Zavala and Mellink,1997; Hofmeyr et al., 2002), possibly because of changesin the use of synthetic materials, and also changes in thegeographical extent and growth of fishing industries (Laist,1987; Pemberton et al., 1992; Hofmeyr et al., 2002; Page

0025-326X/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.marpolbul.2006.05.003

* Corresponding author. Tel.: +27 12 420 4573; fax: +27 12 420 2534.E-mail address: ghofmeyr@zoology.up.ac.za (G.J. Greg Hofmeyr).

et al., 2004). While the implementation of regulations deal-ing with the handling of potential sources of marine pollu-tion has been followed by decreases in the entanglementrates of some populations (Arnould and Croxall, 1995;Hanni and Pyle, 2000), at other sites such regulations seemto have had no effect or have been followed by increases inrates of entanglement (Jones, 1995; Henderson, 2001; Auri-oles et al., 2003; Page et al., 2004). The recovery of dis-carded or lost anthropogenic material on islands of theSouthern Ocean (Bonner and McCann, 1982; Gregory,1991; Slip and Burton, 1991) show that their isolation doesnot ensure protection from the effects of marine pollution.The presence of debris in this area is especially notablesince the Antarctic Circumpolar Current is believed to pro-vide a partial barrier around the Southern Ocean (Walkeret al., 1997). Since 1984 the Commission for the Conserva-tion of Antarctic Marine Living Resources (CCAMLR)has requested the assessment of the impact of anthropo-genic debris on marine wildlife in this area (Morris,1985). Following this initiative, the entanglement of pinni-peds has been reported for many sites within the Antarcticregion (Arnould and Croxall, 1995; Croxall et al., 1990;

(0.059)

(0.024)(0.032)

(0.044)

0

0.1

0.2

0.3

0.4

0.5

0.6

'96/97 '98/99 '00/01 '01/02Season

Num

ber o

f sea

ls p

er d

ay

Fig. 1. Mean number of entangled Antarctic fur seals recorded per day atNyrøysa, Bouvetøya. Numbers in parentheses are the entanglement rates(percentage of entangled seals per estimated total population).

Table 1Percentages of different types of entanglement materials recorded onAntarctic fur seals at Nyrøysa, Bouvetøya

Entanglement material Number recorded Percentage of total

Fishing net 51 48.1Polypropylene packaging strap 19 17.9Rope or string 15 14.2Penguin skin 3 2.8Ski handle 1 0.9Plastic bag 1 0.9Rubber O-ring 1 0.9Unknown 15 14.2Total 106 100

0

10

20

30

40

50

60

70

80

Adult males Adult females Subadults Yearlings Pups

Age and sex category

Num

ber o

f ent

angl

ed s

eals

ExpectedObserved

Fig. 2. Observed and expected numbers of entangled Antarctic fur seals ofvarious age and sex categories recorded at Nyrøysa, Bouvetøya. Expectedvalues represent the sex–age structure of the entire population, followingPayne (1979a).

1078 G.J. Greg Hofmeyr et al. / Marine Pollution Bulletin 52 (2006) 1077–1080

Slip and Burton, 1991; Hofmeyr et al., 2002). Bouvetøya(54�25 0S, 3�20 0E) lies within this area. It supports the sec-ond largest breeding population of Antarctic fur sealsArctocephalus gazella worldwide (Hofmeyr et al., 2005).This study provides the first report of the rates and effectsof entanglement of this species at this site. Bouvetøya alsosupports a small population of southern elephant sealsMirounga leonina (Kirkman et al., 2001). None were seenentangled during this study.

2. Methods

Data on entanglements were collected incidentallyduring other studies at the Antarctic fur seal rookery ofNyrøysa over a total of 301 days from mid-December tolate February or early March during each of four australsummers (1996/1997 – 75 days, 1998/1999 – 81 days,2000/2001 – 85 days and 2001/2002 – 60 days). Sightingeffort remained approximately constant, with daily visitsby observers to areas inhabited by most of the seals, andweekly visits to all areas. The sex and age class of entangledindividuals (defined following Bester, 1987), severity ofentanglement (defined following Croxall et al., 1990), andthe type of the entangling material were recorded.Attempts were made to exclude duplicate sightings of indi-vidual entangled seals. Animals seen in any one seasonwere assumed not to have been sighted in a previous sea-son. Entanglements were removed when possible. Annualrates of entanglement were calculated by comparing thenumbers of entangled seals to the mean annual populationsize during the period of the study (66 000 – Hofmeyr et al.,2005). The age and sex class distribution of entangled ani-mals was compared to expected values derived from thepostulated structure of the Antarctic fur seal populationat Bird Island (Payne, 1979a), because this information isnot documented for Bouvetøya, using a G-test to test forgoodness-of-fit (Sokal and Rohlf, 1969).

3. Results

A total of 119 sightings of a minimum of 106 entangledseals were noted during the study with the minimum num-ber of entangled animals recorded per season ranging from16 to 39 and the entanglement rate varying between 0.024%and 0.059% of the total population. The mean minimumnumber of entangled animals sighted per day decreased sig-nificantly (R2 = 0.91, F = 19.87, d.f. = 1.2, p < 0.05) overthe period of the study (Fig. 1). Fifty six percent of identi-fied entangling material was fishing net, with other commonentangling materials including polypropylene packagingstraps and rope and string (Table 1). In addition to anthro-pogenic material, three seals were found with collars of pen-guin skin. Most entangled animals were subadults (67%),with significantly more animals of this age class, and moreadult females than expected, and concomitantly signifi-cantly fewer of other age/sex classes (GG = 107.17,d.f. = 4, p < 0.01 – Fig. 2). One of the 106 entangled animals

recorded was found dead, likely as a result of its entangle-ment. Where the severity of observed entanglements wasknown (87.6%), the entangling material had only cut intothe skin (26.1%), or had penetrated the skin and underlyingfat layer (25.0%). The other half of the entangled animalsshowed no apparent injury. Two-thirds of entanglementssighted on live animals were removed.

4. Discussion

The rates of entanglement of Antarctic fur seals at Bou-vetøya (0.024–0.059%) are a degree of magnitude lower

G.J. Greg Hofmeyr et al. / Marine Pollution Bulletin 52 (2006) 1077–1080 1079

than those determined for other fur seal populations of theSouthern Ocean (Croxall et al., 1990; Hofmeyr and Bester,2002; Hofmeyr et al., 2002) and elsewhere (Shaughnessy,1980; Fowler, 1987; Stewart and Yochem, 1987; Hender-son, 2001; Page et al., 2004; Boren et al., 2006). Sinceentanglement-induced mortality of fur seals at these otherSouthern Ocean sites is considered to be negligible in thecontext of the sizes of their populations, the same isundoubtedly the case for Bouvetøya. The recent rapidincrease in the Bouvetøya population (Hofmeyr et al.,2005) supports this contention. While a decrease in thenumber of entanglements sighted per day over the courseof the study was recorded, the time series of recordings istoo short to relate entanglement rates with trends in out-side influences. Elsewhere changes in rates of entanglementhave been recorded over longer time periods and, in somecases, decreases have been related to the implementation ofregulations dealing with the handling of anthropogenicwaste (Arnould and Croxall, 1995; Hanni and Pyle,2000). The decrease in the number of entanglementssighted per day over the course of this study is likely relatedto the removal of entanglements. This would, however,require that at least some entanglements are carried forlong periods, and not merely a year as is thought to bethe case.

Assuming that the entangling polypropylene packagingstraps are from bait boxes (Jones, 1995), the fishing industrywould be responsible for at least two thirds of entangle-ments seen at Bouvetøya. This is similar to the situationdescribed in many other studies of entanglement of bothAntarctic fur seals (Croxall et al., 1990; Arnould and Crox-all, 1995; Hofmeyr and Bester, 2002; Hofmeyr et al., 2002),and other species of seals (Shaughnessy, 1980; Fowler, 1987;Lucas, 1992; Pemberton et al., 1992; Harcourt et al., 1994;Hanni and Pyle, 2000; Page et al., 2004). As no fishingindustries are thought to operate in the waters around Bou-vetøya, and since the island lies relatively far from shippinglanes, entangling marine pollution found here is unlikely tobe of local origin. Most of this debris must therefore haveeither drifted into waters around the island, or been encoun-tered by seals when foraging considerable distances awayfrom the island. Antarctic fur seals are known to capableof moving long distances (Shaughnessy and Burton, 1986;Payne, 1979b). Any material encountered in waters closeto Bouvetøya would likely have been carried in by theAntarctic Circumpolar Current from sources within theSouthern Ocean, such as southern South America orthe Scotia Sea, where fishing industries operate. Markingson some of the debris encountered washed up on the bea-ches of the study site indicates South American production(G. Hofmeyr, S. Kirkman pers. obs.).

Almost three times more subadults were recorded entan-gled than expected. Other studies of Antarctic fur seals(Croxall et al., 1990; Arnould and Croxall, 1995) and otherspecies of otariids (Fowler, 1987; Stewart and Yochem,1987; Pemberton et al., 1992; Hanni and Pyle, 2000; Hof-meyr and Bester, 2002; Page et al., 2004) have also found

that the majority of entangled seals are subadults or pups.The high incidence of entanglement of these young ageclasses has been ascribed to their inquisitive disposition(Fowler, 1987; Pemberton et al., 1992; Page et al., 2004),and to their frequenting higher risk foraging areas, com-pared to older animals (Henderson, 2001; Page et al.,2004). Disproportionate abundances of any one age orsex class recorded amongst entangled animals should, how-ever, be interpreted with caution, as the age and sex classdistribution of animals ashore may differ from that of theentire population. Aurioles et al. (2003) showed a signifi-cant correlation between the ages and sexes of entangledanimals and the population of California sea lions ashoreat La Paz Bay, Gulf of California.

Slightly more than half of the observed entanglementscaused obvious injury. A similar proportion (59.2%) isfound for the combined Antarctic and subantarctic fur sealpopulation at Marion Island (Mammal Research Institute,unpublished data). These proportions are considerablymore than the 30% described by Croxall et al. (1990) forthe South Georgia population of Antarctic fur seals. It ispossible that more seals at the latter site are in earlier stagesof entanglement because of their proximity to sources ofpollution, especially the local fishing industry (Walkeret al., 1997). Studies of other populations of otariids, how-ever, do show that populations that inhabit waters close tofairly large fishing industries can have large proportions ofanimals suffering from obvious wounds (Pemberton et al.,1992; Page et al., 2004). While it has been suggested thatall entangled animals will eventually die without humanintervention (Croxall et al., 1990; Page et al., 2004), obser-vations of animals bearing scars indicates that some entan-glements might deteriorate and come off naturally (Fowler,1987; Stewart and Yochem, 1987; Pemberton et al., 1992).This seems unlikely at Bouvetøya, considering the strengthand relatively undeteriorated state of the majority of thematerial recovered.

Acknowledgements

This study was financed by the Norwegian Polar Insti-tute and NORAD. Further support was provided by theMammal Research Institute of the University of Pretoria.We thank Ludvig Krag, Alf Næstvold, Brian Flascas,Wendy Wilson, Onno Huyser, Bruce Dyer, Ross Wanless,Dave Keith and Bianca Harck for their valuable assistancein the field. In addition, we are grateful to Kjell Isaksen andBjørn Krafft for the parts they played as leaders of theteams in the field, as well as for the roles they played indata collection, and to Ian Gjertz for his administrativecontributions to the fur seal work at Bouvetøya.

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