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.S. Hindell et al. J. Exp. Mar. Biol. Ecol. 255 2000 153 –174
consistent amongst habitats. Atherinids and syngnathids were again the numerically dominant families of small fish; atherinids varied more with cage structure while syngnathids did not vary
statistically between cages, blocks locations within which a single replicate of each cage treatment was applied or habitats. Dietary analysis of caged A
. truttacea demonstrated the potential for this species to influence the assemblage structure of small fish through predation –
atherinids were consumed more frequently in unvegetated sand than seagrass, and syngnathids were consumed only in seagrass, where they are most abundant. Observations of significant cage
or predation effects depended strongly on the time at which sampling was undertaken. In the case of the atherinids, no predation or cage effects were observed during the first two sampling times,
but cage effects and predation effects strongly influenced abundances of fish during the third and fourth sampling times, respectively. Our study suggests that transient piscivorous fish may be
important in structuring assemblages of small fish in seagrass and unvegetated sand, and seagrass beds may provide a refuge to fishes. But the importance of habitat complexity and predation, in
relation to the potentially confounding effects of cage structure, depends strongly on the time at which treatments are sampled, and the periodicity and multiplicity of sampling should be
considered in future predation studies.
2000 Elsevier Science B.V. All rights reserved.
Keywords : Arripis truttacea; Australia; Unvegetated sand; Caging experiment; Heterozostera tasmanica;
Piscivory; Seagrass; Structural complexity; Temperate; Temporal variability
1. Introduction
Predation can be an important process structuring post-settlement assemblages of fish Choat, 1982, and one of the most abundant predators are other fishes. Correlative
studies often show that the abundance of piscivorous fish is negatively associated with the abundance of smaller prey fish Hixon, 1991; Bailey, 1994; Connell and Kingsford,
1997, 1998. Dietary studies complement correlative analyses by demonstrating the importance of particular suites of small fish in the diets of predatory fishes Hall et al.,
1990; Kingsford, 1992; Edgar and Shaw, 1995a; Connell and Kingsford, 1997; Connell, 1998. In concert, correlative studies and dietary analyses imply that predatory fish are
important determinants of the structure of small fish assemblages. However, few experimental studies have unequivocally concluded that fish predation is an important
contributor to variability in small fish assemblage structure Hixon, 1991.
Habitat structure is provided by both abiotic consolidated and unconsolidated sediments and rock and biotic coral, wood, oyster reef, submerged and emergent
vegetation elements, but vegetation has received most attention due to its wide distribution and because animal abundances in vegetation are generally greater than
alternative, usually unvegetated, areas nearby Heck and Crowder, 1991. In marine environments, the positive association of animals with structurally complex, vegetated
habitats is likely to be a reflection of interactions between habitat selection processes Edgar and Robertson, 1992; Levin and Hay, 1996, hydrodynamics and larval supply
Jenkins and Black, 1994; Hamer and Jenkins, 1996 and survival as a function of refuge provision and habitat complexity Choat, 1982; Orth et al., 1984; Orth, 1992. Predation
J .S. Hindell et al. J. Exp. Mar. Biol. Ecol. 255 2000 153 –174
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is an important source of mortality, and predation efficiency, as a function of detection, selection, pursuit and capture of prey Mattila, 1995, decreases with increasing habitat
complexity Choat, 1982; Stoner, 1982; Gotceitas et al., 1997. Thus, patterns in the abundance of fish across habitats which differ markedly in structure may reflect
differential predation pressure, i.e., the structure of the vegetation and the type of habitat complexity it generates determines the intensity and nature of predator–prey interac-
tions, and thereby affects the structuring capacity of predation Mattila, 1995.
Seagrasses are a common form of biogenic habitat in marine and estuarine systems worldwide Pollard, 1984; Bell and Pollard, 1989; Kemp, 1989, and compared with
alternative, usually unvegetated habitat, generally contain higher numbers of predatory and other fishes Heck et al., 1989; Edgar and Shaw, 1995a,b; Jenkins et al., 1997b. The
high, but temporally variable Heck et al., 1989, association of small fish with seagrass habitat has led to a paradigm which promotes the importance of seagrass beds in the
provision of nursery habitat for juvenile fishes Pollard, 1984; Bell and Pollard, 1989; Jenkins and Wheatley, 1998. Increased food availability Bell and Pollard, 1989 and
protection from environmental disturbance Kemp, 1989; Edgar, 1990 are two popular theories why seagrass beds contain high abundances of small fish. But, it is also
plausible that the structural complexity provided by aspects of the seagrass affects the efficacy or selectivity of predators Levin et al., 1997, and provides juvenile fish with a
refuge from predation Orth et al., 1984; Orth, 1992.
Previous studies suggest that broad-scale patterns in small fish assemblages may be influenced by variable larval supply Jenkins and Black, 1994; Jenkins et al., 1997a and,
micro-habitat selection, not predation, is the proximate cause for variability in the abundance of fauna within seagrass beds Bell et al., 1987; Edgar and Robertson, 1992;
Levin et al., 1997. However, many of these studies have manipulated predatory fish
2
over relatively small 1 m spatial scales Bell et al., 1987, which may reflect prey movements and behaviour more than predation effects Englund and Olsson, 1996;
Englund, 1997. Furthermore, while exclosure cages have been used extensively to manipulate abundances of predatory fish Steele, 1998; Levin et al., 1997; Kennelly,
1991, few studies have used enclosure cages to assess the role of predatory fish in structuring assemblages of fish in structurally diverse habitats. A more thorough
understanding of a the role of fish predation in structuring assemblages of small fish, and b the importance of seagrass beds in the provision of refuge from predation, will
be gained by conducting carefully designed experiments which manipulate predator abundance using controlled enclosure and exclosure caging experiments over similar
spatial and temporal scales in large plots of habitat seagrass and unvegetated sand that differ markedly in structural complexity.
The primary aim of our study was to investigate whether predatory fish influence small fish abundance in structured seagrass and unstructured unvegetated sand soft
sediment habitats by manipulating the presence of piscivorous fish using exclusion and enclosure cages. In our enclosure experiment we evaluated the impact of a single species
of predatory fish Arripis truttacea on assemblages of small fish over seagrass and unvegetated sand. Our results are discussed in relation to the importance of predatory
fish, cage effects, habitat characteristics and temporal variability in altering the assemblage structure of fishes.
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.S. Hindell et al. J. Exp. Mar. Biol. Ecol. 255 2000 153 –174
2. Materials and methods
