Variability in the numbers of post-settlement King George whiting (Sillaginidae: Sillaginodes punctata, Cuvier) in relation to predation, habitat complexity and artificial cage structure
Introduction
Predation is thought to be a major determinant of the assemblage structure of fishes in marine ecosystems, and amongst the most common predators in marine systems are other fishes (Choat, 1982). The importance of predatory fishes has commonly been inferred from gut contents and the relative abundances of predatory fish in relation to their prey Hall et al., 1995, Connell and Kingsford, 1997, Hansson, 1998. In the past decade, manipulative experiments have demonstrated that predatory fishes influence assemblages of fish by altering survival and recruitment (Steele, 1999), densities of adults (Tupper and Boutilier, 1997), sizes and growth of juveniles (Levin et al., 1997), and habitat selection Gotceitas and Brown, 1993, Lindholm et al., 1999. Most research about predation on fish has been in coral reefs (see review by Hixon, 1991).
Structural complexity is thought to influence the potential for predatory fish to alter assemblages of fish. The shape and size of crevices and their location in relation to alternative habitats have received considerable attention Sale, 1991, Friedlander and Parrish, 1998, Steele, 1999. Algae and seagrasses also provide structural complexity in many marine systems Carr, 1994, Tupper and Boutilier, 1997, Gillanders and Kingsford, 1998. Amount Heck and Thoman, 1981, Gotceitas et al., 1997 or type (Stoner, 1982) of plants can influence numbers of fish Orth et al., 1984, Orth, 1992, efficiency of predators (Mattila, 1995) and choice of prey (Stoner, 1982). There has been little study of predatory fish in seagrass habitats, despite the often quoted importance of these vegetated areas as nurseries for juvenile fish (Connolly et al., 1999).
Seagrass beds generally have greater species diversity and larger numbers of fishes than adjacent unvegetated areas Orth, 1992, Butler and Jernakoff, 1999. This is thought to reflect the provision of refuge from biological and environmental perturbations, greater levels of food and more stable substrata Orth, 1992, Keough and Jenkins, 1995. Heck and Orth (1980) and Orth et al. (1984) suggested that the larger numbers of fish associated with seagrass may reflect reduced predation. Gotceitas et al. (1997) have shown that predation decreases (as the latency to capture increases) with increasing density of eelgrass. Bell et al., 1987, Bell et al., 1988, Bell and Pollard (1989) and Ferrell et al. (1993) presented an alternative model that processes influencing recruitment, particularly larval supply, are important in determining the initial, broad-scale variability in assemblages of fish amongst locations with seagrass. Following recruitment, variability in abundances of fish amongst alternative habitats is due to fish selecting particular regions within a seagrass bed which favours survival Bell and Westoby, 1986a, Bell and Westoby, 1986b—for example, areas that provide adequate levels of food or relief from environmental or biological perturbations.
Manipulative field experiments potentially provide the most rigorous and persuasive tests of hypotheses in predation studies (Raffaelli and Moller, 2000). Cages are commonly used to manipulate abundances of predatory fishes Doherty and Sale, 1985, Hall et al., 1990, Steele, 1996, Connell, 1997, Levin et al., 1997. The artificial structure used to exclude predators, however, may mask or mimic predation effects by altering abundances of fish Bell et al., 1985, Bohnsack et al., 1997, Carr and Hixon, 1997, Clarke and Aeby, 1998. Physical (e.g. particle size and organic composition of sediment) and biological (e.g. abundances of epifauna/meiofauna that are sources of food for small fish) attributes of the environment may also be altered by cage structure Virnstein, 1978, Hall et al., 1990. Partial cages (cage controls), which allow predatory fishes to forage in areas enclosed by cage structure, are necessary to separate predation effects from effects caused by artificial structure Virnstein, 1978, Steele, 1996, Connell, 1997. Research by ourselves (Hindell et al., in press), Schrijvers et al. (1998) and Mattila and Bonsdorff (1989) have shown that caging materials do not always alter meiofaunal abundances or sediment characteristics. Direct observations are needed, however, to determine whether caging materials attract fishes or differentially alter foraging by predatory fishes inside cage controls compared to uncaged areas (see Connell, 1997). Underwater video affords researchers an opportunity to quantify predatory fish whose temporal patchiness and transient nature often precludes the use of divers Burrows et al., 1994, Hixon and Carr, 1997, Morrisey et al., 1998.
In Port Phillip Bay, recruitment of juvenile King George whiting, Sillaginodes punctata, to seagrass beds varies considerably between locations Jenkins et al., 1997b, Jenkins et al., 1998, Jenkins and Wheatley, 1998. Larval supply and environmental disturbance (wave action) explain a significant amount of this broad-scale variability Jenkins and Black, 1994, Jenkins et al., 1997a. Hindell et al. (2000) have shown that sites with high numbers of predatory fish, such as Western Australian salmon (Arripis truttacea), correspond to sites where the recruitment of juvenile S. punctata is low. This pattern implies that predatory fish may influence abundances of small fish in seagrass beds.
In this study, we assessed whether (a) numbers of juvenile S. punctata varied between areas with and without predatory fish, and (b) patterns in numbers of S. punctata across caging treatments were consistent between seagrass and unvegetated sand, and at different locations. To answer these questions, we manipulated the numbers of predatory fish using cages in patches of seagrass and unvegetated sand at several locations in Port Phillip Bay. We also measured the numbers of piscivorous A. truttacea in seagrass and unvegetated sand at different locations, and amongst caging treatments to assess how the potential for predation varied.
Section snippets
Study sites
The caging experiments and predator surveys were carried out at three sites in Port Phillip Bay: Blairgowrie, Grand Scenic and Kilgour (Fig. 1). At each site, there are large contiguous beds of the seagrass Heterozostera tasmanica (Martens ex Ascherson) den Hartog, which are interspersed with patches of unvegetated sand and rocky reef in shallow (<3 m) water close to the shoreline. The currents around Grand Scenic and Kilgour are weak (≈10 cm s−1), but currents in the vicinity of Blairgowrie
Results
The numbers of S. punctata varied in a complex way between sites, habitats and caging treatments (Table 1). There was a significant three-way interaction between sites, caging and habitats, averaging data across times (Table 1, Fig. 3). The numbers of S. punctata were, on average, larger in unvegetated sand than seagrass at Kilgour, but smaller in unvegetated sand than seagrass at Blairgowrie (Fig. 3). There was no difference in the numbers of S. punctata between cage treatments in seagrass at
Discussion
Structural aspects of the environment, regardless of whether they are biogenic or not, often provide small fish with a refuge from predation Heck and Crowder, 1991, Beukers and Jones, 1997, and thereby influence patterns in survival and recruitment (Steele, 1999). In our study, larger numbers of juvenile S. punctata in uncaged areas of seagrass than unvegetated sand, and similar numbers of fish inside cages over unvegetated sand and any of the caging treatments in seagrass, implied that
Conclusion
Despite the attention given to determining the importance of predation in structuring assemblages of small fish that live in vegetated marine environments Bell and Pollard, 1989, Gotceitas et al., 1997, Rangeley and Kramer, 1998, the importance of predation by fish has remained controversial. For juveniles of S. punctata, predation strongly influences their inter-habitat distribution within a location, probably through behaviourally mediated antipredator measures rather than direct predation,
Acknowledgements
This manuscript was greatly improved by comments from A.J. Underwood, J. Mackie, R. Connolly, S. Connell and an anonymous reviewer. Thanks to M. Hendricks, L. McGrath, M. Wheatley and R. Watson for assistance in the field and at the research station. We gratefully acknowledge funding from the Fisheries Research and Development Corporation (1999/215), the Australian Research Council and a University of Melbourne Research Scholarship. Research was conducted using the facilities at the Queenscliff
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