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contrasts were used were corrected using the Bonferroni correction of type I error rates. Statistical analyses were conducted using Statistical Analysis System SAS, 1993 and
power analyses were determined with SigmaStat Statistical Software SigmaStat Version 2.0, SigmaStat, 1997.
2.6. Benthic food resources To determine if juvenile shrimp and fish were capable of reducing food resources in
their artificial aquatic refuges, benthic meiofauna samples were obtained from two randomly chosen petri dishes within the high density fish and shrimp treatment
enclosures during the first experimental run. Two initial subsamples of benthic meiofauna were obtained three days prior to the initiation of the first experimental run
and after three days into the run. Two of the remaining enclosures without fish and shrimp were used as controls. The two subsamples obtained from each enclosure were
averaged providing two replicates for each treatment. Samples were collected just prior to the flooding tide by randomly choosing two petri dishes and pouring the contents
through a 1.0-mm mesh sieve into sealable plastic bags. The sieve allowed juvenile fish and shrimp to be separated from the sediments and returned to the petri dishes for the
continuation of the experiment. Plastic bags containing sediments were sealed and then placed into a cooler for transport to the laboratory.
Samples were rinsed with tap water through a 0.5-mm mesh sieve onto a 63-mm mesh sieve to retain meiofauna. After rinsing for approximately one minute the contents
retained on the 63-mm sieve were rinsed with filtered estuarine water into labeled glass bottles. Bottled samples were then stained with a solution of 10 buffered formalin and
rose bengal and stored until they could be analyzed.
Meiofauna were visually sorted from sediments and enumerated under a stereomicro- scope at 253 magnification. Organisms were enumerated to taxonomic groups cope-
pods-including nauplii, ostracods, and nematodes and stored in buffered formalin in microcentrifuge tubes.
Effects of juvenile fish and shrimp on numbers of meiofauna for each taxonomic group were determined by ANOVA using time initial and final and density control and
high density fish or shrimp treatments as main factors. A significant interaction between the two factors was used as an indication of a change in abundance due to the presence
of juvenile fish or shrimp.
3. Results
3.1. Direct competitive effects Juvenile shrimp exhibited intraspecific effects of reduced growth in the high density
treatments compared to controls Fig. 3, P ,0.05. However, no differences in growth were seen when predators were present Fig. 3. Reduced growth was also observed for
juvenile shrimp when interspecific competitor treatments for shrimp were compared to controls for shrimp Fig. 4, Table 3. However, when controls for juvenile shrimp were
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.E. Cross, A.E. Stiven J. Exp. Mar. Biol. Ecol. 242 1999 179 –199
21
Fig. 3. Mean growth in length mm day of juvenile shrimp. Error bars are 6S.E. P ,0.05 for
comparisons of treatments with low density control.
21
Fig. 4. Mean growth in dry mass g day of juvenile shrimp. Error bars are 6S.E. P ,0.01 for
comparisons of treatments with low density control. Table 3
a
Results of Anova for mean growth in dry weight of juvenile shrimp Source of variation
MS F
P Competitor
0.0341 16.0
0.004 Predation
0.0004 0.16
0.689 Competitor3Predation
0.0039 1.82
0.189
b
0 30 vs. 60 30 0.3133
14.73 0.002
c
0 30 vs. 60 301P 0.0134
6.32 0.055
d
60 30 vs. 60 301P 0.0036
7.70 0.611
b
Control 0 fish and 30 shrimp vs. 60 fish and 30 shrimp.
c
Control vs. 60 fish and 30 shrimp with predators.
d
Treatment with 60 fish and 30 shrimp vs. 60 fish and 30 shrimp with predators.
a
Significant P-values are in bold and P-values for a priori contrasts are adjusted using the Bonferroni correction for multiple contrasts. Number of replicates per treatment was 9. All treatment df51 and error
df527.
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189
Fig. 5. Mean proportion of juvenile shrimp surviving in the enclosures. Error bars are 6S.E.
compared with the competitor1predator treatment for juvenile shrimp 60 fish and 30 shrimp and predators, there was no difference in growth of shrimp Fig. 4. In addition,
there was no significant difference in growth between the competitor treatment with and without predators Table 3. The a priori contrasts also revealed no differences in
survival between treatments for juvenile shrimp Fig. 5.
Growth of juvenile fish did not differ between fish in the high density treatments and the controls Fig. 6. In addition, controls for juvenile fish showed no differences in
growth of juvenile fish compared with the other treatments to assess interspecific effects of shrimp on fish Fig. 6. Juvenile fish did not exhibit reduced survival in the presence
of juvenile shrimp in these experiments. Also, the presence of adult predators had no effect on the survival of juvenile fish in the enclosures Fig. 7.
There were no statistically significant reductions in final numbers of copepods, ostracods, nematodes, and copepod nauplii relative to controls for the fish or shrimp
treatments Fig. 8–11.
21
Fig. 6. Mean growth in length mm day of juvenile fish. Error bars are 6S.E.
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.E. Cross, A.E. Stiven J. Exp. Mar. Biol. Ecol. 242 1999 179 –199
Fig. 7. Mean proportion of juvenile fish surviving in the enclosures. Error bars are 6S.E.
Fig. 8. Mean numbers of copepods per petri dish at beginning and end of experiment. Error bars are 6S.E.
Fig. 9. Mean numbers of ostracods per petri dish at beginning and end of experiment. Error bars are 6S.E.
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191
Fig. 10. Mean numbers of nematodes per petri dish at beginning and end of experiment. Error bars are 6S.E.
Fig. 11. Mean numbers of copepod nauplii per petri dish at beginning and end of experiment. Error bars are 6S.E.
4. Discussion
