Scope and intent:

Previous studies have indicated the establishment of no take zones in Marine Parks often results in increased
abundance of target species which can in turn influence the community structure of other species by trophic
cascade effects. For example; changes in rates of predation, or competitive interactions. Target species may
also provide important ecological functions such as scavenging. The results of removing fishing pressure on a
single species can therefore have significant impacts on the wider ecology.

Zoobenthivorous fish such as mullet, whiting, bream and flathead made up 31% of the total recreational and
commercial harvest in Moreton Bay in 2005. The current study hypothesised that reduced fishing pressure on
these species following expansion of no take zones was likely to result in increased predation of zoobenthos.

The principal aim of the study was to compare variability in community structures in current protected areas
and open areas within Moreton Bay Marine Park with areas designated to become MNP’s following the rezoning process that was due to come into effect on 1st March 2009. The data collected was designed to form a
baseline from which the future effectiveness of any no take areas could be compared.
Predator and scavenging pressure was assessed at 15 sites, encompassing existing MNP’s, control (general
use) zones and areas earmarked to become MNP’s following re-zoning. Predation pressure was measured
using tethering experiments on a small burrowing Bivalve Mollusc; Paphies striata, and the estuarine shrimp
Paleamon serrefis. Scavenging pressure and variation in scavenging community was assessed using baited
underwater cameras. In addition, mud and blue swimmer crabs were trapped at 3 locations in each study
zone using pots or dillies. Individuals were frozen and their gut fullness and contents were examined.
Relative abundance was calculated by measuring the number and species of crabs caught per pot per hour.
1
Proportion of P. striata
predation.
0.5
0.4
0.3
0.2
0.1
0
Current Marine New Marine Park Control open area
Park
Fig. 1 (above): Mean predation of P. striata by zone in March 2009 (solid
bars) and July 2009 (striped bars.)
Fig. 2 (right): Mean catch rate of mud and blue swimmer crabs by zone.
Although differences in catch rates were not significant; (Levene’s test P=
0.137) three times as many mud crabs were caught in current MNP’s as
blue swimmer crabs. In new MNP’s the opposite was true.
K. Pitt, R. Connolly C. Beattie, D. Burfeind & D. Borombovits
Australian Rivers Institute, Griffith University.
September 2009.
Results:
Predation pressure:
Predation rates among P. striata and P. serrefis were similar and did not
differ significantly between current MNP, new MNP or control zones, but
was highly variable and did differ significantly between locations within
zones. There was also a clear seasonal impact on predation rates, (see Fig.
1.)
Scavenging pressure:
Similarly, scavenging pressure was highly variable and differed
significantly between locations within zones, but there was no significant
difference in scavenging pressure between current MNP, new MNP and
control zones. (See Fig. 3.)
The proportion of bait scavenged tended
to be higher in vegetated locations
compared to un-vegetated ones, although
not significantly so.
0.6
Mean catch rate (crabs pot-1 hour-1)

The effects of marine reserves on community
structure and ecological processes.
0.6
0.5
There was evidence to suggest
differentiation between scavenging
communities; the majority of scavenging
inside Marine Parks tended to be
undertaken by mud crabs. Whereas, in
control areas, the scavenging community
was dominated by blue swimmer crabs,
striped trumpeters and stingrays.
0.4
0.3
0.2
0.1
Crab population and diet:
0
mud crab
blue mud crab blue mud crab blue
swimmer
swimmer
swimmer
crab
crab
crab
Current Marine
Park
New Marine Park Control open area
Catch rates between zones appeared to
show greater numbers of mud crabs in
existing Marine Parks and a higher
abundance of blue swimmer crabs in
new Marine Parks; (see Fig. 2.) However,
analysis indicated the trend was not
statistically significant.
0.8
Proportion of bait consumed
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Coombabah McCoys Creek
Lake
2
Swan Bay
Tripcony
Bight
Willes Island
Blakesleys
Anchorage
Current Marine Park
Blue swimmer crabs tended to have twice as
much food in their guts when caught during the
daytime as those caught at night. Mud crabs
tended to have a roughly equal amount of gut
content whether caught during the day or at night.
Gut fullness differed significantly for both species
between zones. On average gut fullness was
around 11% in existing MNPs, 24% in new MNPs
and 17% in control areas.
Crustaceans and Bivalves were found to constitute
the majority of both species’ diets. The % volume
of Bivalves was significantly higher in blue
swimmer crabs than in mud crabs. Blue swimmer
crabs inside existing MNPs had a higher
proportion of plant matter in their gut. A
correspondingly lower proportion of mud crabs
inside existing MNPs had plant matter in their
guts compared to mud crabs caught in new MNPs
and control zones.
Lamb Island
Pannikin
East
Island/ Cobby
Cobby*
New Marine Park
Conclusions:
Point
Halloran
Pumicestone Canaipa South
Passage
Opposite
Couran
Elimbah Creek Lamb Island Victoria Point
North
Control open area
Fig. 3: Variation in mean proportion of bait consumed by
Predation pressure on both the bivalve Paphies striata and the barred
scavengers taken from baited remote underwater videos
(BRUV’s.) BRUV’s were deployed at random in Sep. 2008
estuarine shrimp, Paleamon serrefis, and scavenging pressure varied
(solid bars) and May 2009 (striped bars.) Variability
greatly among locations but did not vary among zones. Shrimp, such as
between locations within zones and seasonally is evident.
P. serrefis are thought to be preyed on by large carnivorous fish, with
large portunid crabs such as mud and blue swimmer crabs preying on
* Cobby Cobby was replaced by Pannikin Island in Apr.
bivalves and carrion. Given that large carnivorous fish and portunid crabs 2009 due to a limited sub tidal study area.
are targeted by commercial and recreational fishers, the lack of difference
in predation and scavenging pressure was unexpected, the original hypothesis was therefore not supported. Instead the results
suggested that both predation and scavenging processes may be resilient to the effects of fishing within Moreton Bay Marine Park.
The fact that predation and scavenging pressure varied greatly between locations within zones but not between zones suggests
there are as yet unknown factors operating over small spatial scales which may have a greater influence on these processes than
fishing pressure. There was a strong trend for scavenging communities to differ among zones. In particular, mud crabs were the
dominant scavengers within existing MNP’s and blue swimmer crabs were dominant in control areas.
There was indirect evidence of competitive displacement of blue swimmer crabs by mud crabs. The higher relative abundance of
mud crabs inside existing MNP’s suggested the removal of fishing pressure has allowed mud crabs to outcompete blue swimmer
crabs. In terms of dietary displacement, there was less evidence to support direct competition between species. Although both
species’ diets were relatively cosmopolitan, blue swimmer crabs tended to specialise in Bivalve predation, more so than mud
crabs.