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4. Discussion
4.1. Effect of size of fish The first result of this experiment was that predation on panels varied according to the
type of fish accessing panels. Small elongate fish essentially toadfish rather than large deep-bodied fish caused changes in the structure of epibiotic assemblages. Their effect
was largely on oysters. There is substantial evidence demonstrating the effects of toadfish on the mortality and abundance of oysters in these experiments. In particular,
where an oyster had been removed due to predation by fish, the lower valve of the animal was still left attached to the panel, so that mortality could be estimated as a direct
effect Anderson and Connell, in press. Such direct effects of predation were not possible to observe for the percentage cover of algae and the abundances of the
gastropod Bembicium auratum. As found in previous work Anderson and Underwood, 1997 the cover of algae was negatively correlated with the abundance of oysters
r 5 2 0.32, t
5 2 2.44, P , 0.01, one-tailed test and the abundance of B. auratum
52
was positively correlated with the abundance of oysters r 5 0.50, t 5 4.13, P , 0.001,
52
one-tailed test. It is, therefore, likely that removal of oysters by fish indirectly caused decreases in recruitment of Bembicium and increases in the growth of algae. Further
manipulative experiments would be required to test for such indirect effects; fish could have preyed on gastropods.
Assemblages on patches protected from predation by toadfish cages with small mesh differed from those on patches primarily accessible to toadfish cages with large mesh
and all fish including toadfish open patches. The lack of difference between patches open to predation by all fish and patches open to predation only by toadfish indicated
that large deep-bodied fish made little difference. Oysters are found in the guts of toadfish D. Booth, unpublished data and Tetractenos spp. is the only fish in estuaries in
New South Wales that would be able to remove large oysters from panels inside cages with large mesh.
Because one type of predator was primarily responsible for alterations to assemblages, the design of future experiments and predictions of where predation is likely to be
important can be improved. The specific types of fish that were manipulated were based on observations of the sizes of predators at the study site see Section 2. Two previous
studies have tested experimentally the influences of specific types of fish on epibiota Ayling, 1981; Choat and Kingett, 1982. Particular predators were excluded by a
horizontal shield, which prevented predation by fish that orientate vertically to feed. These simple but ingenious experiments were based on information of the feeding
behaviour of the fish and highlight the importance of doing experiments to test hypotheses derived from known aspects of the biology of the predators and prey being
studied.
Variation in the abundance of large predatory fish is thought to cause important shifts in assemblages of epibiota Jones and Andrew, 1993. Moreover, it has been suggested
that because fishing targets large predatory fish Jennings and Polunin, 1996, it may have widespread effects on epibiota e.g. Hughes, 1994. Our results, and those of Choat
and Kingett 1982, warn that large predatory fish e.g. sparids may not always be
26 S
.D. Connell, M.J. Anderson J. Exp. Mar. Biol. Ecol. 241 1999 15 –29
important predators of epibiota. Most studies that have detected predation by fish have used one size of mesh which excludes both small and large predatory fish alike range of
mesh sizes 5 7.5–18.0 mm, average ¯ 11 mm; Foster, 1975; Russ, 1980; Keough, 1984; Menge et al., 1985, Breitberg, 1985; Sala, 1997. Without information on the relative
impacts of small versus large predatory fish, previous studies may have undervalued the importance of small predatory fish and overvalued that of large predatory fish.
4.2. Artefacts as alternative explanations of caging effects We did not detect consistent artefacts when comparing open panels with cage controls,
but the adequacy of partial cages in detecting artefacts can be problematic Wilson, 1991; Connell, 1997 and some discussion of potential artefacts is needed Peterson and
Black, 1994. We are confident that small predatory fish toadfish are responsible for differences in the structure of assemblages of epibiota particularly oysters among
caging treatments. If cages and not predators enhanced the abundance of oysters, we would not have observed equal densities of oysters between open panels and panels
inside cages with large mesh. Moreover, there is substantial evidence demonstrating that toadfish cause heavy mortality and alter subsequent abundances of oysters in these
experiments Anderson and Connell, in press.
The only other taxa affected by cages were algae, the snail Bembicium auratum and possibly Hexaminius sp. Shading by cages was unlikely to have reduced the cover of
algae inside cages; patches were already shaded in their face down position. The abundance of B
. auratum, a mobile and herbivorous gastropod, was negatively correlated with algal cover. Hence, artefacts associated with food availability were unlikely.
Demonstrable artefacts due to cages tend to cause barnacles to occur in fewer numbers inside cages Marshall et al., 1980; Schmidt and Warner, 1984, opposite to the results
we obtained for Hexaminius sp. In addition, the partial cages in our experimental design would have controlled for such artefacts as shading and the presence of mesh near
panels. Yet, none of the analyses designed to detect caging artefacts, through the use of partial cages, gave any significant results. It is, therefore, reasonable to conclude that
artefacts of cages were minimal.
4.3. Effect of size of patch It has long been recognised that predators are unlikely to respond in a similar way to
different sized patches of food e.g. Charnov, 1976; Hodges, 1985. There has been recent recognition that predatory fish are highly likely to respond differently to changes
in prey availability Werner et al., 1983; Kingsford, 1992; Wildhaber and Crowder, 1995. Moreover, prey that occur in larger aggregations have been shown to suffer
greater rates of fish predation Connell, 1998b suggesting that smaller aggregations of prey and patches of habitat may offer a refuge from predation e.g. Keough, 1984.
We postulated that predatory fish feed more intensely on larger patches of habitat and predicted that we would detect a greater impact of predators on larger sizes of panel.
This hypothesis was falsified convincingly by evidence showing the effect of predation was uniform across the different sized patches. It was not unusual, however, that patch
S .D. Connell, M.J. Anderson J. Exp. Mar. Biol. Ecol. 241 1999 15 –29
27
size alone had a large effect on assemblage structure; such effects have been well documented in this Anderson, 1998 and other assemblages of epibiota see review in
Connell and Keough, 1985. Note, however, that densities of the primary prey oysters did not differ on different-sized panels Table 2a. Investigation of the cause for
differences in assemblages on different patches has primarily concerned the processes of colonisation and competition Connell and Keough, 1985 rather than predation.
Consistent with the results obtained here and the suggestion that predators are more likely to respond to prey in larger numbers, we predict that predation would be greater
on prey in greater densities i.e. density-dependent predation but not on larger patches of prey whose density is similar to smaller patches as generally the case in this study.
Further experimental studies are required to test this prediction.
5. Conclusion
