Foraging ecology of grey-headed mollymawks at Marion Island, southern Indian Ocean, in relation to longline fishing activity
Introduction
The Prince Edward Islands group (southern Indian Ocean), comprised of Marion and the smaller Prince Edward Island, is one of only seven breeding localities for the grey-headed mollymawk (Thalassarche chrysostoma) and collectively support ca. 10% (ca. 9000 pairs) of the global annual breeding population (Gales, 1997; DCN unpubl. data for the Prince Edward Islands). Although the population on Marion Island appears to be stable and recently increasing, populations on South Georgia (representing 56% of the world population) and Campbell Island have shown long term decreases in numbers, leading to this species being listed as ‘vulnerable’ (Croxall and Gales, 1997). Incidental mortality due to birds being drowned when they attempt to snatch baited hooks deployed by longline fishing vessels, has been implicated as a main cause for the global decline (Gales, 1997).
Initial concern was focused on the pelagic tuna (Thunnus spp.) longline fishery (e.g. 5, 27). More recently, however, the development of demersal longline fisheries for Patagonian toothfish (Dissostichus eleginoides) on the shelf areas surrounding the main breeding localities of the sub-Antarctic avifauna has raised cause for concern (e.g. 3, 8, 7, 11). Sanctioned longline fishing for Patagonian toothfish within the 200 nautical mile Exclusive Economic Zone (EEZ) surrounding the Prince Edward Islands commenced in October 1996. This was after considerable ‘poaching’ by unsanctioned fishing vessels during the early part of 1996 (Purves, 1997). During the first year of sanctioned fishing large numbers of seabirds were killed (Ryan et al., 1997). This included at least 143 grey-headed mollymawks killed by licensed vessels (16% of total seabird bycatch), but as large scale unsanctioned fishing continued throughout this period, it was estimated that over 900 grey-headed mollymawks could have been killed during this year (Ryan et al. ). Examination of carcasses collected by scientific observers aboard sanctioned vessels revealed a strong male bias in all affected species; 89% of the grey-headed mollymawk carcasses were adult males (Ryan and Boix-Hinzen, 1999).
Grey-headed mollymawks are also killed in substantial numbers by pelagic southern blue-fin tuna (SBT) (Thunnus maccoyii) longline fisheries south of Africa (Ryan and Boix-Hinzen, 1998) and off western Australia (Gales et al., 1998). SBT longline fishing in the southern Indian Ocean is largely concentrated around the subtropical convergence (Ryan and Boix-Hinzen, 1998). This fishery reached its peak in the southern Indian Ocean during the mid 1980s and decreased to less than half during the early 1990s. Recently, during the mid 1990s, this fishery once again showed a dramatic increase to ca. 80% of the 1985 peak. This was mainly due to increased Korean longline fishing effort (Tuck and Polacheck, 1997).
Albatrosses exhibit an extreme k-selected breeding strategy. They display a low reproductive rate, high adult survival, delayed onset of breeding (up to 10 years) and very strong mate fidelity (Warham, 1996). Increased mortality due to longline fishing will thus have a complex effect on the demographics of a population that may not be immediately noticeable as a change in the number of birds attempting to breed annually (e.g. 50, 20). Studies on wandering albatrosses (Diomedea exulans) have shown that decreasing adult survival is accompanied by a decrease in the age at first breeding (10, 50), which could initially mask the signal. Also, seabird population trends are generally measured by the number of pairs attempting to breed each year. This measure is notoriously inadequate for measuring short term changes (i.e. <10 years) in population size of biennual breeding albatross populations (26, 14). However, as albatrosses have such low productivity and recruitment rates, population growth rate will take a very long time to stabilize after a perturbation (30–50 years in wandering albatrosses) (Moloney et al., 1994). As albatrosses display high site fidelity and breeding sites are limited and often distant from each other, longline fishing effort within the potential foraging ranges of these breeding populations and its resultant impacts may also vary markedly between populations. It is with this in mind that international conservation initiatives strongly urged research into the foraging ranges and ecology of affected albatross populations (e.g. 2, 13). The main objective of these initiatives is to determine the degree of spatial and temporal overlap of the foraging ranges of albatross populations with known longline fishing grounds and therefore the degree of exposure of these populations to the risk of incidental mortality.
Despite obvious threats from longline fishing operations within the potential foraging range of grey-headed mollymawks breeding at Marion Island, very little is known about their foraging ecology during the breeding season and their potential interaction with these fisheries. Current knowledge is restricted to a study of the diet of grey-headed mollymawks at the Prince Edward Islands during the chick rearing period (Hunter and Klages, 1989) and observations at sea (Abrams, 1985). These studies suggested that most foraging activity was concentrated mainly at the sub-Antarctic front and the polar frontal zone. In this study we: (1) present additional information on the foraging ecology of this population, (2) establish the level of spatial overlap with known fishing grounds while breeding, and (3) ascertain if there are any differences in the foraging behaviour of males and females, that may account for the high male-biased mortality in the local toothfish longline industry.
Section snippets
Methods
Approximately 7600 grey-headed mollymawk pairs breed annually in dense colonies on the southern coast of Marion Island (46°54′S, 37°45′E). Adults return to the colonies early in September, and eggs are laid during the middle of October. During the 1997/1998 season, the first chicks hatched between 14 and 21 December 1997 and the first chicks fledged during the last week of April 1998. Adults weigh between 3.0 and 3.7 kg with males being slightly heavier (Marchant and Higgins, 1990)
We
Foraging movements
During incubation, four foraging trips were followed using PTTs. One device stopped working after 5 days of an 18-day foraging trip (due to water entering the battery casing) and one device was lost at sea after 18 days of a 26-day trip. It is assumed that the latter bird lost its device at the beginning of its return flight, as it covered the return journey of 3500 km in 8 days at an average speed of 35 km h−1. This would leave little time for detours or extensive foraging activity. The other
Discussion
The results of this study have added greatly to our understanding of the foraging ecology of grey-headed mollymawks breeding at Marion Island. This in turn has allowed us to gain an understanding of both the nature and scale of interactions with longline fishing operations within their potential foraging range. Furthermore, a preliminary understanding of gender differences in the foraging strategy that could contribute towards sex-biased mortality in longline fishing operations was also
Acknowledgements
Financial and logistical support was received from the Department of Environmental Affairs and Tourism through the South African National Antarctic Programme, the Foundation for Research Development, the World Wide Fund for Nature- South Africa, the Charl van der Merwe Foundation, and the University of Cape Town. Thanks also to Michelle Greyling for helping with deployments and John McLinden for downloading and relaying positions of birds to us, while we were in the field. Martin Purves
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