162 J
.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
3.1. Exclusion experiment A diverse assemblage of small fish from 10 families and 22 species was sampled
throughout our study Table 1. Small fish were influenced both by manipulating predatory fish using cages, and by the habitat within which this manipulation was
conducted, but habitat and caging acted independently Table 2. There was significant variability in the abundance of small fish amongst cage types
Table 2 and Fig. 3. The abundance of fish in uncaged areas was significantly lower than either exclusion cages P 5 0.001 or cage controls P 5 0.012. Cage controls
contained significantly fewer fish than exclusion cages P 5 0.034. The abundance of small fish also varied significantly amongst habitats and times Table 2 and Fig. 3.
More fish were captured over unvegetated sand than seagrass, and the abundance of small fish, regardless of cage and habitat, varied significantly across sampling times
J .S. Hindell et al. J. Exp. Mar. Biol. Ecol. 255 2000 153 –174
163 Table 2
Three factor repeated measures analysis of variance comparing the numbers of small fish total, atherinids and syngnathids amongst blocks, habitats seagrass versus unvegetated sand and cage types exclusion cage, cage
a
control and uncaged n 5 24 Source
Num df Den df
Small fish Atherinids
Syngnathids Between subjects
Block B 3
6 0.141
0.806 0.015
Habitat H 1
3 0.048
0.378 ,
0.001
Cage C 2
6 0.002
, 0.001
0.039
B 3 H 3
6 0.688
0.157 0.877
B 3 C 6
6 0.911
0.781 0.725
H 3 C 2
6 0.299
0.116 0.764
Residual MS 6
1.966 1.108
1.017 Within Subjects
Time T 3
9 ,
0.001 0.004
0.119 T 3 B
9 18
0.999 0.608
0.133 T 3 H
3 9
0.061 0.115
0.120 T 3 C
6 18
0.057 0.010
0.489 T 3 B 3 H
9 18
0.051 0.523
0.534 T 3 B 3 C
18 18
0.262 0.911
0.204 T 3 H 3 C
6 18
0.076 0.411
0.333 Residual MS
18 0.646
1.634 0.444
a
The data table shows, for each group of small fish analysed, the probabilities associated with each of the terms in the model Source and the Residual MS. This information allows full reconstruction of the original
ANOVA table. Data were logx 1 1 transformed prior to analysis.
Table 2 and Fig. 3. Although the main effects block, habitat and cage acted independently, there were near significant interactions between time and habitat, time
and cage, and time and the habitat 3 block interaction Table 2 and Fig. 3. Atherinidae and Syngnathidae were the numerically dominant families of small fish in
this experiment Table 1; therefore, these two families of fish were analysed separately. Despite a trend suggesting otherwise, the abundance of atherinids did not vary
between habitats Table 2 and Fig. 3. Additionally, the abundance of atherinids varied inconsistently between cage treatments through time Table 2 and Fig. 3. During the
first and second sampling times, the abundance of atherinids did not vary between cage treatments df
, MS 5 0.761, P 5 0.427 and df , MS 5 0.876, P 5 0.378, respec-
2,18 2,18
tively. During the third sampling time, the abundance of atherinid recruits at Grand Scenic increased substantially J. Hindell, pers. obs.. This event corresponded with
significant variability in the abundance of atherinids, now predominantly juvenile fish, across cage treatments for the third and fourth sampling times df
, MS 5 14.319,
2,18
P , 0.001 and df , MS 5 13.194, P , 0.001, respectively. In the third sampling
2,18
time the abundance of atherinids did not vary significantly between exclusion cages and cage controls P 5 0.748, however, uncaged areas contained significantly fewer
atherinids than exclusion cages P , 0.001 and cage controls P , 0.001. In the final sampling time exclusion cages contained significantly more atherinids than cage controls
P 5 0.001 and uncaged areas P , 0.001. Cage controls and uncaged areas contained similar numbers of atherinids P 5 0.180.
164 J
.S. Hindell et al. J. Exp. Mar. Biol. Ecol. 255 2000 153 –174
Fig. 3. Mean abundance of small fish, atherinids and syngnathids in unvegetated and seagrass habitats during each sampling time in each caging treatment exclusion cage, cage control and uncaged.
Syngnathids showed a strong association with patches of seagrass; significantly more syngnathids occurred in seagrass than unvegetated sand Table 2 and Fig. 3.
Syngnathids also varied significantly between cage treatments Table 2, but this result was driven primarily by the much higher abundance of syngnathids in exclusion cages
compared to uncaged areas P 5 0.015, particularly in seagrass Fig. 3. The abundance of syngnathids did not vary significantly between partial cages and either exclusion
J .S. Hindell et al. J. Exp. Mar. Biol. Ecol. 255 2000 153 –174
165
cages P 5 0.294 or uncaged areas P 5 0.069, nor across times Table 2, Fig. 3. While the abundance of syngnathids varied with the position along the shore block
Table 2, we were more interested in the variance component and the subsequent reduction in residual variation associated with the block effect, than specific differences
between groups. Therefore, no further analysis was conducted for this effect.
3.2. Enclosure experiment The abundance of small fish did not vary between habitat types or blocks Table 3 and
Fig. 4a. However, the abundance of small fish did vary across cage treatments Table 3; uncaged areas contained significantly fewer fish than either exclusion cages P 5
0.001, cage controls P 5 0.005 and predator enclosures P 5 0.012 Fig. 4a. Small fish did not vary between cage controls and predator enclosures P 5 0.464; however,
there were significantly more small fish sampled from exclusion cages than predator enclosures P 5 0.021. The abundance of small fish almost varied significantly between
cage controls and exclusion cages P 5 0.058. When the level of significance was adjusted to control for the experimentwise Type I error rate, only the difference in the
abundance of small fish between exclusion cages and cages containing predators became non-significant.
As in the exclusion experiment, syngnathids and atherinids were the numerically dominant families of small fish in our enclosure experiment Table 1, and separate
analyses were conducted for each of these families. The atherinids showed similar patterns to the total fish in that the abundance of
atherinids did not vary with block or habitat Table 3 and Fig. 4b, but varied significantly across cage treatments Fig. 4b. Exclusion cages contained significantly
more atherinids than either cage controls P 5 0.029, predator enclosures P 5 0.018 or uncaged areas P 5 0.007. There was no significant difference in the abundance of
atherinids between uncaged areas and predator enclosures P 5 0.465 or cage controls P 5 0.292, nor between cage controls and predator enclosures P 5 0.722. After the
significance level was adjusted to control for Type I errors P 5 0.016, only a
Table 3 Three factor analysis of variance comparing the numbers of small fish total, atherinids and syngnathids
amongst blocks, habitats seagrass, unvegetated sand and cage types exclusion cage, cage control, enclosure
a
cage and uncaged n 5 24 Source
Num df Den df
Small fish Atherinids
Syngnathids Block B
2 6
0.587 0.084
0.121 Habitat H
1 2
0.212 0.783
0.121 Cage C
3 6
0.003 0.029
0.096 B 3 H
2 6
0.696 0.796
0.553 B 3 C
6 6
0.818 0.798
0.359 H 3 C
3 6
0.932 0.989
0.953 Residual MS
6 0.762
0.943 0.436
a
The data table shows, for each group of small fish analysed, the probabilities associated with each of the terms in the model Source and the Residual MS. This information allows full reconstruction of the original
ANOVA table. Data were logx 1 1 transformed prior to analysis.
166 J
.S. Hindell et al. J. Exp. Mar. Biol. Ecol. 255 2000 153 –174
Fig. 4. Mean abundance of a small fish, b atherinids and c syngnathids in the enclosure experiment for each cage treatment exclusion cage, uncaged, cage control, predator enclosure within unvegetated sand and
seagrass habitat pooled across blocks.
J .S. Hindell et al. J. Exp. Mar. Biol. Ecol. 255 2000 153 –174
167 Table 4
The percent abundance N , percent mass M , and percent frequency of occurrence F of dietary categories from stomach contents of Arripis truttacea n 5 15 enclosed in cages within unvegetated sand and seagrass
habitats
Prey items Bare sand
Seagrass N
M F
N M
F Fish
Atherinidae 5
12 13
3 2
7 Syngnathidae
– –
– 5
4 7
a
Unknown fish 3
– 13
– –
– Other
Crustaceans 92
87 60
89 93
40 Polychaetes
3 2
7 –
– –
a
Value , 0.5.
significant difference in the abundance of small fish between exclusion cages and predator cages existed, despite trends which suggest otherwise Fig. 4b.
The syngnathids in the enclosure experiment displayed similar patterns to those seen for syngnathids in the exclosure experiment Figs. 3 and 4c. Despite the trends Fig.
4c, the abundance of syngnathids did not vary statistically across cages, blocks or habitats Table 3 and Fig. 4c.
3.3. Dietary analysis of enclosed Arripis truttacea In seagrass habitats, 40 of Arripis truttacea contained no food compared with 53
of A . truttacea enclosed over unvegetated sand. Of the A. truttacea with stomachs
containing prey, crustaceans were the most common dietary item, and represented 92 and 87 abundance N and percent mass M , respectively. Teleost prey, which
included atherinids, syngnathids and unknown fish remains Table 4, appeared to be a more important component in the diets of A
. truttacea enclosed over unvegetated sand than seagrass Table 4. While atherinids were consumed in both unvegetated sand and
seagrass, syngnathids were consumed only in seagrass, the habitat within which they occur most commonly.
4. Discussion
