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2. Materials and methods
2.1. Study site Our study was carried out at Grand Scenic, which is situated on the southern shore of
the Geelong Arm of Port Phillip Bay Fig. 1. The currents in this region of Port Phillip
2 1
Bay are weak, generally much less than 17 cm s Rosenberg et al., 1992, and the area
is protected from the prevailing southerly winds. These largely sheltered conditions facilitate sedimentary processes that generate a substrate composed primarily of silty
sand Anon., 1973, rich in detritus J. Hindell, pers. obs.. Tides in this area are semidiurnal and the range is less than 1 m Jenkins et al., 1998. Grand Scenic contains
large contiguous subtidal beds of Heterozostera tasmanica, whose distribution is broken by patches of unvegetated sand. A variety of algae and small patches of rocky reef occur
sporadically throughout the beds of seagrass. One other species of seagrass, Zostera muelleri, Irmisch ex Ascherson, also occurs in this region of Port Phillip Bay, but its
distribution is largely confined to the intertidal.
2.2. Exclusion experiment
2
In our exclusion experiment, predatory fish were excluded from 16-m plots of
unvegetated sand and seagrass to test whether predatory fish influence the assemblage structure of small fish, and whether any observed predator effects are independent of
habitat complexity and time of sampling. A variety of predatory fishes, including pike-headed hardyheads, Kestratherina esox Klunzinger Atherinidae, Arripis truttacea,
yank flathead Platycephalus speculator Klunzinger, and rock flathead Platycephalus laevigatus Cuvier, were potentially excluded using cages, but only K
. esox and A. truttacea have been found to consume the small, early post-settlement stages of fish
sampled in the cage treatments at this location Hindell et al., 2000. Exclusion cages were constructed from 2.1 m long galvanised steel starpickets,
hammered into the substrate at each corner of a 4 3 4-m square plot. Black, 20-mm polypropylene netting of 1.5 m height, was placed around the perimeter of the plot,
2
enclosing an area of 16 m Fig. 2a. This mesh size was chosen because it is small enough to prevent the passage of predatory fish. The mesh was tightened at the top and
bottom of each cage with 5 mm nylon cord, and the bottom line was weighted to prevent fish from swimming between the bottom of the cage walls and the surface of the
substrate. The top of the cage walls exceeded the water level throughout the tidal range, however, the substrate within each cage treatment was always submerged. Partial cages
were constructed from the same materials and in the same dimensions as exclusion cages, except only half of each wall of the cage was attached Fig. 2b. Each partial cage
provided the structure of a cage but did not change the abundance of predatory fishes inside partial enclosures relative to uncaged areas J. Hindell unpublished data. These
cages were used to assess the role of cage structure per se in altering fish assemblages. Uncaged areas were unmanipulated 4 3 4-m plots of habitat.
Four locations blocks were selected randomly at Grand Scenic, i.e., by dividing this location into blocks of 50 3 50 m and using a random number generator to select each
J .S. Hindell et al. J. Exp. Mar. Biol. Ecol. 255 2000 153 –174
157
Fig. 1. Location of study site, Grand Scenic, in the Geelong arm of Port Phillip Bay. Insets: location of Port Phillip Bay in Australia and location of study region in Port Phillip Bay.
block. Within each location, the caging treatments exclosure cage, partial cage–cage control, and uncaged, were each applied to haphazardly chosen patches of unvegetated
sand and seagrass within randomly chosen locations blocks along the shore. Our
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.S. Hindell et al. J. Exp. Mar. Biol. Ecol. 255 2000 153 –174
Fig. 2. Design of a exclusion enclosure cages and b cage controls used to manipulate the abundance of predatory fish.
experiment was arranged in a completely randomised block design with ¯ 10 m between cage habitat treatments within blocks, and ¯ 50 m between blocks.
Following construction, cages were left for 1 week before sampling. Each cage was cleaned weekly to reduce the build up of drift algae which interferes with water
movement and may attenuate light Virnstein, 1978. Cages remained in the field for 1 month. Little algae grew on cage walls and the seagrass did not appear to be influenced
detrimentally by excessive sedimentation or overgrowth from epiphytes J. Hindell, pers. obs.. Recent studies have shown that there is no difference in the particle size
distribution, amounts of combustible organic matter, or composition of meiofauna between exclusion and partial cages J. Hindell, pers. obs..
2.3. Sampling small fish In our study, small fish refers to fish that are generally less than 5 cm in length. Most
of the small fish were juveniles. Only individuals from the family Syngnathidae, which were included in the small fish category, were commonly adults and their length often
exceeded 10 cm. The assemblages of small fish in each treatment were sampled on the
J .S. Hindell et al. J. Exp. Mar. Biol. Ecol. 255 2000 153 –174
159
same day during low tide using a modified beach seine net 4 m wide 3 1.5 m high 3 1.5 m deep 3 1-mm mesh. In this study, a pole was attached to each side of the net to aid in
hauling and foam floats and lead weights were attached to the top and bottom of the net, respectively. The net was drawn between two steel posts inside and at one end of the
cage, and hauled through to the opposite side of the cage by two people, one person holding each pole. Because pilot studies showed that roughly 90 of all small fish were
captured in the first haul of this type of net, regardless of habitat, only a single haul of the net was conducted in each experimental arena. Captured fish were anaesthetised in
benzocaine and preserved in 70 ethanol. This sampling procedure was repeated once weekly for four consecutive weeks. In the laboratory, fish were counted and identified to
species Gomon et al., 1994.
2.4. Enclosure experiment In our second experiment, enclosure cages, each containing juvenile Arripis truttacea
were added to the experimental design used in our exclosure experiment and the experiment was re-conducted using reconstructed cage treatments. Juvenile A
. truttacea, whose total length at the study location ranges between 10 and 15 cm J. Hindell, pers.
obs., are transient and gregarious, and commonly occur in shallow water over mosaics of seagrass and rocky reef interspersed with patches of unvegetated sand. A
. truttacea are perennial in this type of habitat, and at the time of year our study was conducted,
they feed voraciously on early post-settlement fishes, particularly atherinids Hindell et al., 2000. During winter and early spring, when small fishes are generally less abundant,
juvenile A . truttacea consume a range of pelagic invertebrates, the most common of
which are crustaceans of the order Mysidacea J. Hindell, pers. obs.. A . truttacea was
chosen to enclose in cages because they are robust to handling stress, easy to catch and maintain, and previous research has shown that their abundances are negatively related
to local abundances of juvenile atherinids and sillaginids Hindell et al., 2000. The structure and dimensions of enclosure cages were identical to the exclusion cages.
In our enclosure experiment, each cage type exclusion cage, enclosure cage, partial cage and uncaged was applied to haphazardly chosen unvegetated sand and seagrass plots
within each of four blocks. The block component of the enclosure experiment incorporated both temporal amongst weeks and spatial position along the shore
variability, and each block of the experiment orthogonal combination of cage and habitat was conducted independently and successively.
After the construction of each block, five juvenile Arripis truttacea each , 15 cm total length, approximately the ambient field density of this species at this location J.
Hindell, pers. obs, were added to each enclosure cage. A . truttacea were captured 2
days before being placed in enclosure cages and were maintained in 300-l flowing- seawater aquaria at the Queenscliff Marine Station. After 2 days of confinement, the A
. truttacea
, as well as the small fish assemblage, in each combination of habitat and cage, within a block, were sampled using the net and methods described earlier. Short periods
of enclosure of predatory fish were chosen to mimic the temporal patchiness of A .
truttacea in the field. All fish were anaesthetised and preserved in 70 ethanol. Small fish were counted and identified to family using Gomon et al. 1994.
160
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Table 1 Percent abundance of small fishes in each regime of cage C, exclusion cage; CC, cage control; PC, enclosure cage; UC, uncaged and habitat seagrass, unvegetated
a
sand, pooled across sampling times and blocks, for the enclosure and exclusion experiments
Species Enclosure experiment
Exclosure experiment Seagrass
Bare sand Seagrass
Bare sand C
CC PC
UC C
CC PC
UC C
CC UC
C CC
UC Atherinidae
Atherinasoma microstoma 40
14 –
– 40
14 21
17 –
– –
– –
– Leptatherina presbyteroides
– –
22 –
– –
– –
1 1
1 6
9 2
b b
Ketstratherina esox –
– –
– –
– –
– –
– –
– –
– Atherinid recruits
4 –
– 25
4 –
– –
93 86
15 13
12 3
Clinidae Heteroclinus perspicilatus
– –
– –
– 3
– 17
– –
– –
1 2
Gobiidae
b b
Favonigobius lateralis 4
– 6
25 –
– –
– –
– –
– –
–
b b
b b
Nesogobius sp. 1 2
– –
– –
– –
– –
– 6
– –
– Mugilidae
b b
Aldrichetta forsteri 2
– –
– –
– –
– –
– –
1 –
– Monacanthidae
b
Acanthaluteres spilomelanurus 4
– –
– 4
– –
– –
– –
– 1
1 Brachaluteres jacksonianus
– 3
6 –
– 3
– –
– –
– –
– –
b
Meuschenia freycinetti 2
– –
– 2
– –
– –
– –
– 1
4 Odacidae
b b
Neoodax balteatus –
– –
– –
– –
17 –
– –
– –
1 Platycephalidae
Neoplatycephalus aurimaculatus 2
– –
– –
– –
– –
– –
– –
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2000 153
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161
Pleuronectidae
b
Rhombosolea tapirina –
– 6
25 –
– –
– 1
2 24
– –
– Scorpaenidae
b
Gymnapistes marmoratus –
– –
– –
– –
– –
– –
– –
– Sillaginidae
b b
b
Sillaginodes punctata –
– –
– –
– –
– –
– 3
– 1
1 Syngnathidae
Lissocampus caudalis –
– –
– –
– –
– –
– –
– –
– Lissocampus runa
17 7
– 25
17 7
– –
– –
– –
– –
b
Stigmatopora nigra 2
3 –
– 2
3 –
17 –
1 1
1 1
2 Stigmatopora argus
25 58
61 –
25 57
63 17
3 8
33 65
48 52
Urocampus carinirostris 2
– –
– 2
– 5
– –
– –
– –
–
b
Vanacampus margaritifer –
– –
– –
– –
– –
– –
– –
– Vanacampus phillipi
4 10
– –
4 10
10 17
1 1
15 12
24 30
Total number of fish 56
16 18
4 52
29 19
6 1543
720 71
549 366
273 Mean number per cage
17 5
6 1
17 10
6 2
96 45
4 34
23 17
Number of species 8
3 5
4 8
7 4
6 10
10 8
12 12
13
a
All data are rounded to whole numbers.
b
Percent abundance , 0.5.
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.S. Hindell et al. J. Exp. Mar. Biol. Ecol. 255 2000 153 –174
The stomach of each re-captured Arripis truttacea was excised and the gut contents were categorised and counted. The dietary composition of A
. truttacea was described using percent frequency of occurrence F , percent mass M , and percent abundance
N Hyslop, 1980. 2.5. Statistical analysis
The exclusion experiment was analysed as a repeated measures time, three factor block, habitat and cage randomised blocks design. Habitat and cage were treated as
fixed factors. Block was treated as a random factor and time was the repeated factor. Raw data were logx 1 1 transformed where the assumptions of homogeneity of
variances and normality were not met. The assumption of sphericity was checked by the Greenhouse-Geisser G-G epsilon value e. The potential for sphericity to influence
our results was controlled by using the G-G adjusted probability P values, however, where the adjusted P value did not alter the significance of the un-adjusted P value, the
un-adjusted P value was used. A priori tests were used to determine how the levels of the cage effect varied. Where the number of a priori tests exceeded the degrees of
freedom df for the effect being tested, the significance level a was adjusted to control for the experimentwise Type I error rate by dividing the significance level for that test
0.05 by the number of comparisons in excess of the df for the effect being tested. This gave a critical level of a no. planned comparisons 2 df. Where interactions were found
between a main effect and time, separate one factor ANOVAs and a priori tests were conducted for each time to determine where the levels of the interacting main effect
varied.
The enclosure experiment was analysed as a three-factor block, habitat, cage randomised blocks design. Tests for assumptions and comparisons of main effects were
carried out as described for our exclosure experiment. All analyses were carried out
using SYSTAT statistical software Wilkinson et al., 1992.
3. Results
