P .M. Spitzer et al. J. Exp. Mar. Biol. Ecol. 244 2000 67 –86
75
expected if enclosure artifacts were present. If significant differences were found P value 0.05, Tukey’s multiple comparison test was run, as suggested by Day and
Quinn 1989, to determine which treatments differed significantly. Amphipod AFDWs
22
were expressed as g AFDW m to estimate availability of food within the enclosures.
Plant surface area was calculated from leaf length and width measurements and
2
converted to surface area m to estimate the plant surface area within and outside each enclosure. To determine the accuracy of the core sample vegetation estimates which
could result in chopped off leaves, core estimates and quadrat clipping estimates both
22
converted to g dry weight m were compared during the first four intervals. If
differences were found, the cored plant surface area estimates were adjusted upward accordingly and plant surface area analysis was repeated for the new estimates.
3. Results
3.1. Environmental data Due to the close proximity of the treatments , 15 m between treatments, physical
factors were not expected to differ significantly among treatments. As expected, monthly measurements of temperature, salinity and water depth varied only slightly among the
treatments, with mean differences among treatments less than 0.24 ppt salinity, 0.088C temperature and 0.04 m depth. Water temperatures ranged from 17.2 to 31.48C
throughout the course of the experiment and salinity ranged from 19.7 to 34.5 ppt.
3.2. Vegetation To determine the effectiveness of the coring process in estimating surface area of
vegetation, quadrat clip surface area estimates were compared to core surface area estimates. Since estimates of vegetation for clips were consistently greater than estimates
of vegetation for cores, it was concluded that cores underestimated surface area. Therefore, these estimates were corrected for percentages lost in the coring process.
Corrections were made by 1 calculating a percentage core estimate clip estimate, 2 subtracting that percentage from 100 and 3 multiplying the core estimate by
100 1 percentage found in step 2. These corrected values were used to compare surface area of vegetation among treatments.
Vegetation treatments Table 1 were significantly different in surface area of plants ANOVA: P , 0.001, df 5 14 for all periods except November ANOVA: P 5 0.310,
df 5 14, when the seasonal dieoff of the seagrass bed occurred. Multiple comparison tests demonstrated that all pairwise comparisons showed significant differences at
0.10 . P . 0.05 except during the month of October, when all pairwise comparisons were significant at P , 0.05. Epiphyte coverage showed no significant differences
ANOVA: P . 0.05, df 5 14 among treatments, except during July ANOVA: P 5 0.0242, df 5 14 when small density treatments showed significantly greater epiphyte
biomass than intermediate and large density treatments.
The four main categories of epiphytes found in the cores of vegetation were
76 P
.M. Spitzer et al. J. Exp. Mar. Biol. Ecol. 244 2000 67 –86 Table 1
Experimental protocol for all five intervals I, II, III, IV, V of the field growth comparison experiments in
a
various small, intermediate and large Thalassia densities Interval
Interval No. of pinfish
Small Intermediate
Large dates
per density
density density
enclosure ]
] ]
I 6 13 96–7 10 96
10 n55, x54264.0,
n55, x514 439.6, n55, x524 533.0,
S.E.52426.7, S.E.54417.4,
S.E.57505.2, b5118.5638.3
b5285.1652.3 b5580.8669.3
] ]
] II
7 28 96–8 22 96 6
n55, x53975.3, n55, x514 212.6,
n55, x521 283.3, S.E.51675.2,
S.E.52579.3, S.E.52008.2,
b594.4618.0 b5358.0628.6
b5403.7629.5 ]
] ]
III 8 26 96–9 17 96
6 n55, x53895.3,
n55, x512 747.5, n55, x515 141.8,
S.E.52845.1, S.E.53372.5,
S.E.52544.9, b5154.4676.0
b5449.0667.8 b5589.3664.2
] ]
] IV
9 24 96–10 17 96 4
n54, x51687.1, n54, x513 087.7,
n52, x515 426.3, S.E.5854.1,
S.E.52106.3, S.E.54251.3,
b551.7613.9 b5453.5654.8
b5484.9620.8 ]
] ]
V 10 19 96–11 17 96
3 n53, x52877.3,
n53, x57588.3, n53, x54081.3,
S.E.51432.5, S.E.51164.8,
S.E.51304.8, b528.8619.7
b5163.0673.6 b538.9620.0
a 2
2
] n, number of replicate enclosures stocked with juvenile pinfish; x, average plant surface area cm m ;
2
S.E., standard error; b, average biomass of vegetation g dry weight m 6S.E. of biomass.
filamentous brown, calcareous, filamentous red, and bare 5 no epiphyte present. Brown algae were present throughout all five intervals, while calcareous and filamentous
red epiphytes increased as the summer proceeded. During all five intervals, over 75 of the blades were classified as bare, presumably as a result of grazing.
Epiphyte AFDW Table 2 showed no trends between small and intermediate treatments, but large density AFDW was lower during all intervals. However, no
significant differences were found among treatments ANOVA: P . 0.05, df 5 14. 3.3. Pinfish growth
Mean daily rates of growth and mean weight gain in all treatments were positive with none of the recovered fish losing weight. The average recapture rate of tagged fish was
53 Table 3. Low recovery rates were attributed to escape rather than tagging mortality since there were very few signs of stress such as tail rot or enlarged pupils in
Table 2
2
Comparison among treatments of epiphyte AFDW g AFDW m 6S.E. for each interval Interval
Interval dates Small density
Intermediate density Large density
I 6 13 96–7 10 96
2.65e2466.47e25 1.9e246 2.45e25
1.82e2463.0e25 II
7 28 96–8 22 96 1.8e2466.02e25
1.1e2465.51e25 6.8e2463.34e25
III 8 26 96–9 17 96
1.43e2467.98e25 1.52e2467.71e25
3.91e2461.55e25 IV
9 24 96–10 17 96 1.8e2468.93e25
1.64e2462.85e25 1.51e2461.93e25
V 10 19 96–11 17 96
1.11e2462.24e25 1.33e2461.97e25
3.73e2563.73e25
P .M. Spitzer et al. J. Exp. Mar. Biol. Ecol. 244 2000 67 –86
77 Table 3
Recapture rates of tagged fish for all five intervals I, II, III, IV, V of the experiments in various small,
a
intermediate and large density treatments Interval
Tagged Small
Intermediate Large
Overall Recapture
Rate Recapture
Rate Recapture
Rate I
50 18
0.36 26
0.52 23
0.46 0.45
II 30
21 0.70
17 0.57
14 0.47
0.58 III
30 20
0.67 23
0.77 15
0.50 0.64
b
IV 20
3 0.15
4 0.25
1 0.05
0.13 V
15 6
0.4 7
0.47 7
0.47 0.44
Experiment recap-
ture rate: 0.53
a
Tagged, number of fish tagged and deployed per treatment; recapture, number of fish recovered from the enclosures per treatment; rate, recapture tagged per treatment; overall, total number of fish recovered for the
interval total number of fish tagged per interval; experiment recapture rate, total number of fish recaptured for all intervals see note total number of fish tagged for all intervals.
b
Note: fish from interval IV were not included in the experiment recapture rate since Tropical storm Josephine lifted the cages slightly allowing many fish to escape.
any of the recovered fish and no carcasses were recovered. The standard length total length regression equation was used to estimate total length for only eight of the 205 fish
recovered due to slight tail rot. Treatment means indicate that pinfish grew faster and gained more weight in the small
density treatment than in either the intermediate or large density treatments Figs. 3a and 3b, although pinfish total lengths were not significantly different ANOVA: P . 0.05,
df 5 14 among vegetation treatments. However, a clear trend of decreasing growth rates with increasing vegetation density was seen for all intervals except November Fig. 3a.
There was no significant effect ANOVA: P . 0.05, df 5 14 of vegetation treatment on pinfish weight gain either, although a clear trend of decreasing weight gain with
increasing vegetation density can be seen in two of the five intervals Fig. 3b. The power of the ANOVAs was uniformly low, and ranged from 0.0494 to 0.0503 for growth
to 0.0494 to 0.0601 for weight with a 5 0.05. Thus, there is a very considerable probability of making a Type II error.
Due to the potential error involved in measuring live fish and the low power of the statistical tests, we investigated further. We determined the probability of obtaining
results in which the same treatment rankings were obtained in four of five July, August, September and October and two of five July and August months for total length and
weight gain, respectively, as seen in our study Fig. 3. We used simple probability theory to estimate the probability of obtaining the ranking we saw most frequently
small . intermediate . large when all possible combinations are considered equally likely. Using the following equation:
n
x n 2x
Px 5 ? p
? 1 2 p
s d
s s
x where n is the number of trials, x is the number of successes and p
is the probability of
s
success in our case: 1 6, we calculated the probability of obtaining all three densities
78 P
.M. Spitzer et al. J. Exp. Mar. Biol. Ecol. 244 2000 67 –86
Fig. 3. Mean growth rates 6S.E. of pinfish recovered from the field enclosures in various vegetation densities during all five intervals. a Mean daily growth in TL mm; b mean daily weight gain g wet
weight. P-values for one-way ANOVA are given above each month.
in the same sequence by chance for four of the five growth intervals to be 0.003 and two of the five weight intervals to be 0.16. If we eliminate the November interval, the time
when seasonal seagrass dieoff occurred from consideration, and examine the probability of randomly obtaining the same ranking of treatments for four of the four growth
P .M. Spitzer et al. J. Exp. Mar. Biol. Ecol. 244 2000 67 –86
79
intervals and two of the four weight intervals, we find the probabilities of obtaining these results to be 0.0008 and 0.12 for growth and weight, respectively.
3.4. Benthic fauna Ash free dry weights AFDW of macrofauna found within the benthic cores indicate
that more food was available for the fish placed in the large density treatment, since the AFDW mean was greater in the large density treatment during all intervals Fig. 4.
Although the large density treatment had a greater AFDW mean than the other treatments during four out of five intervals, these differences were not significant
2ANOVA: P .0.05, df529, except during the October interval 2ANOVA: P 50.0083, df529. In October, both the intermediate and large density treatments, as well as the
small and large density treatments, differed significantly Tukey test: P ,0.05.
No caging effects were evident when comparing faunal samples from inside the enclosure with those from outside the enclosure Table 4. Abundance of amphipods
was examined among treatments for all five intervals Fig. 5. The normality assumption failed and amphipod abundance was examined using a Kruskal–Wallis test. Amphipod
abundance significantly increased with an increase in density of vegetation Kruskal– Wallis: P .0.05, df52 in three of the five intervals with abundance in the large density
treatment being significantly greater than the abundance in the small density treatment. October and November also showed an increase in abundance with increasing density of
vegetation, although not a significant one Kruskal–Wallis: P 50.153 and P 50.430 respectively, df52. Amphipod AFDWs also showed a positive relationship with
densities of vegetation Fig. 6 during all five intervals, however these differences were
2
Fig. 4. Comparison of mean faunal biomass g AFDW m 6S.E. among treatments. P-values for one-way ANOVA tests are above each month.
80 P
.M. Spitzer et al. J. Exp. Mar. Biol. Ecol. 244 2000 67 –86 Table 4
Results of testing used to examine for caging effects. Data from cores taken both inside and outside of the enclosures were compared
Aspect examined Test used
P value Degrees of
freedom Faunal abundance
2ANOVA .0.05
29 Fauna AFDW
2ANOVA .0.05
29 Amphipod abundance
Mann–Whitney .0.05
N A Amphipod AFDW
Mann–Whitney .0.05
N A Vegetation surface
ANOVA .0.05
14 area
Epiphyte coverage ANOVA
.0.05 14
Epiphyte AFDW ANOVA
.0.05 14
not significant in four of the five intervals Kruskal–Wallis: P .0.05, df52. August was the only interval that showed significant differences Kruskal–Wallis: P ,0.001,
df52 in amphipod AFDW, with the greatest AFDW present in the large density treatment.
2
Fig. 5. A comparison of amphipod abundance number m 6S.E. among treatments for each of the five intervals. P-values for Kruskal–Wallis test are given above each month.
P .M. Spitzer et al. J. Exp. Mar. Biol. Ecol. 244 2000 67 –86
81
2
Fig. 6. A comparison of mean amphipod biomass g AFDW m 6S.E. among treatments for each of the five intervals. P-values for Kruskal–Wallis test are given above each month.
4. Discussion
