Abstract:
‘Infaunal haloes’ of either decreasing or increasing abundances of individual
soft-sediment species with distance from reefs have been suggested to be caused by
reef-associated predators. A large-scale mensurative experiment was used to investigate
the distribution of two size classes of macrofauna with distance from the reef edge
across three locations in northeastern New Zealand. The role of reef-associated
predators, the snapper (Pagrus auratus Sparidae) and rock lobster (Jasus edwardsii
Palinuridae), was investigated using established marine reserves at each location.
Consistent patterns were found in a few large-bodied fauna. The hermit crab Pagurus
novizelandiae occurred more frequently near the reef edge, whilst the heart urchin
Echinocardium cordatum and bivalve Dosinia subrosea were more abundant further
away from the reef. Dosinia subrosea and another bivalve, Myadora striata, exhibited
lower biomass at sites with higher densities of snapper and rock lobster. In contrast,
small-bodied macrofauna showed no consistent patterns with distance from the reef or
among sites with different predator populations.
It was hypothesised that predation was driving the distribution of large bivalves.
An experiment was done to investigate this model using D. subrosea. Equal densities of
this bivalve were established in plots either with or without cages at sites either inside or
outside of reserves. Significant predation was detected, but only inside reserves. Much
of this mortality could be specifically attributed to predation by large rock lobsters,
given the distinctive marks on the valves of dead D. subrosea.
Inside reserves, predators are not only more abundant but also larger. It was
hypothesised that different size classes of predators would result in different levels of
predation. Laboratory feeding experiments were used to investigate this model. Lobsters
of all sizes chose D. subrosea over the heavier shelled D. anus. Small lobsters chose to
prey on small D. subrosea and large lobsters more frequently chose larger prey. The
distributions of these two bivalve species at protected (large predators) and fished sites
(small predators) reflected the feeding choices observed in the laboratory.
Results suggested that rock lobster populations are capable, where their size
structure is not truncated by fishing pressure, of controlling population-level dynamics
of bivalve communities adjacent to reefs.