The effects of aquaculture on bottlenose dolphin (Tursiops sp.) ranging in Shark Bay, Western Australia

Abstract

The increasing presence of aquaculture in coastal waters calls for a better understanding of its environmental effects. Currently little information is available on the impact of shellfish farms on cetaceans. Here we compare long-term ranging patterns of adult female bottlenose dolphins (Tursiops sp. in Shark Bay, Western Australia) before and during full-scale pearl oyster farming operations, to determine if they were displaced. When the exact location of the oyster farm was determined, the dolphins decreased their use of that area after the farm was in place. Tracks of adult female dolphin movement near the oyster farm were compared to tracks of dolphin movement near an ecologically similar area where no oyster farm existed. Tracks near the oyster farm were less likely to enter the oyster farm itself than tracks near an ecologically similar location. This suggests that shellfish aquaculture could have a large impact on small cetaceans. The analytical techniques discussed apply broadly to aquatic and terrestrial animals.

Introduction

Aquaculture, the farming of finfish or shellfish, has grown 11% in the last decade, becoming the fastest growing industry in the world food economy (Newton, 2000). Aquaculture has the potential to reduce the amount of fish and shellfish taken from the ocean and to bring substantial income to an area; it produced 37.8 million metric tons of fish and shellfish and $55.7 billion USD in 2002 (Food and Agriculture Organization of the United Nations, 2003). However, farming of carnivorous fish that require food from the ocean still results in a net take of wild stocks (Naylor et al., 2000). Furthermore, if finfish or shellfish are farmed in a natural marine environment, concern arises over the effects of increased biodeposition from fecal and pseudofecal matter, introduction of new species, genetic mingling of wild and captive stock, antibiotics and medicines, as well as competition with, displacement of and interaction with other fauna (Fleming, 1994, Buschmann et al., 1996, Würsig and Gailey, 2002). The described impacts of shellfish farming primarily involve competition with local bivalves, deposition of organic matter, altered benthic communities, and introduction of exotic species (Mirto et al., 2000, Naylor et al., 2000, Naylor et al., 2001, Henderson et al., 2001, La Rosa et al., 2001).

Potential impacts on marine mammals as a result of aquaculture interaction include death or injury through entanglement in gear, displacement, altered food chain, disruption of migration pathways (for large cetaceans), and human intervention (marine mammals killed or relocated). Most of the literature to date has focused on otters and pinnipeds that prey on finfish and some shellfish, but there is a paucity of information on cetaceans and aquaculture (reviewed in Würsig and Gailey, 2002, Kemper et al., 2003). Unlike pinnipeds, cetaceans have not been reported to consume fish or shellfish out of farms, but have been known to get entangled in equipment, resulting in the damage of gear, release of fish, and self injury (Dans et al., 1997, Kemper and Gibbs, 2001, Crespo and Hall, 2002, Hall and Donovan, 2002). Displacement of cetaceans by aquaculture may also occur because they frequently share the same coastal habitat (Crespo and Hall, 2002, Würsig and Gailey, 2002, Markowitz et al., 2004).

Unlike finfish farms that are netted at the periphery, shellfish farms typically have open areas that are large enough to allow small cetaceans to swim through the farm. Nevertheless, cetaceans may still avoid shellfish farms because of ropes, equipment, human activities, water clarity, prey availability, or other habitat factors. In the last few years, several reports to management agencies (Mann and Janik, in litt.; Slooten et al., in litt.; Paton et al., in litt.) have highlighted the potential displacement of dolphins by oyster and mussel farming operations. For example, dusky dolphins (Lagenorhynchus obscurus) use a green-lipped mussel (Perna canaliculus) farm area less than nearby areas in Marlborough Sounds, New Zealand (Markowitz et al., 2004).

Bottlenose dolphins (Tursiops spp.) are appropriate and useful study animals in this case because of their world-wide distribution in tropical and temperate coastal waters. Because of their cosmopolitan distribution, the information gained in our study can be applied to management of shellfish farms world-wide and, because they share valuable coastal habitat with humans, bottlenose dolphins may be particularly affected by aquaculture. Variable coastal habitats may provide niches important for dolphin foraging or as a refuge from predators. For example, dolphins that specialize in habitat-specific foraging strategies (Mann and Sargeant, 2003) may be seriously affected if they are displaced from that area. Resident communities of dolphins are likely to be most affected and displacement could result in a population decline. Furthermore, the pressures of aquaculture on the dolphin population may add to existing anthropogenic pressures, such as boat traffic (see Chilvers et al., 2005 for an example of human impacts on marine mammals in Moreton Bay, Australia).

Even though these data are from only one bottlenose dolphin study site, it is appropriate to extrapolate to other areas. Shark Bay represents one of the longest running and extensive studies on small cetaceans; it is the only site where ranging information prior to a pearl oyster farm is available. Comparisons of Shark Bay to other sites show similar bottlenose dolphin social structure and behavior (Connor et al., 2000). Therefore, our study offers the best available test to date of the effects of an oyster farm on small cetacean ranging.

Marine mammals, including cetaceans, not only attract public interest, but are also protected in Australia under the Environmental Protection and Biological Conservation Act 1999. To manage a growing industry, more information on the effects of aquaculture on cetaceans is needed.

The aim of this paper is to determine if ranging patterns of bottlenose dolphins have been altered by an oyster farm in Shark Bay, Western Australia. To do this we analyzed their movements near the farm before and during its operation. We addressed the following questions: (1) do bottlenose dolphins change their use of an area once farming begins there; (2) do they move away from the farm; and (3) do they move around (but not through) the farm? We also make recommendations for future research and management. The analytical techniques discussed apply broadly to aquatic and terrestrial animals.

Shark Bay was designated in 1991 as a World Heritage Area for its extensive seagrass beds and dugong (Dugong dugon) population (United Nations Educational, Scientific, and Cultural Organization, 1991). Although Shark Bay is not a World Heritage Area because of the dolphins that inhabit it, the dolphins are a significant tourist draw to the area; currently four dolphins are hand fed daily and attract more than 100,000 tourists per year (Mann and Kemps, 2003). Feeding is regulated by the Western Australian Department of Conservation and Land Management. These four, and hundreds of other dolphins, have been part of a long-term research project since 1982 and have been continuously studied since 1984 (Connor and Smolker, 1985, Connor et al., 2000).