P .M. Spitzer et al. J. Exp. Mar. Biol. Ecol. 244 2000 67 –86
69
might play in influencing predator effectiveness in an open, marine system. They suggested that predator success would be inversely related to plant surface area, and
hypothesized that net benefit intake-cost to marine predators would be greatest at small density of seagrass Fig. 1. When comparing the mechanistic relationships for fresh
water closed and marine open vegetated environments Fig. 1, the omission of the relatively small growth rates at small density of vegetation in Heck and Orth’s model is
due to the fact that they did not hypothesize overexploitation at small densities Heck and Crowder, 1991. Their rationale for not expecting overexploitation was based on the
concept of marine environments as ‘open’ systems that allow for constant immigration of possible items of prey in the form of larvae, juveniles and adults. To date, very little
has been done to test Heck and Orth’s hypothesized relationship between complexity of habitat and the efficiency of actively foraging predators in marine environments.
Efficiency of predators may be assessed by many factors e.g. growth, strike rate, rate of ingestion, and selectivity of food. In the work described below, we chose growth-rate
as an indicator of predator efficiency primarily because of the ease of comparing our results with those of Crowder and Cooper 1979, 1982, but also because it is very
difficult to quantify many of the other measures of predator efficiency using field experiments.
The objective of this study was to assess the role of vegetation density on growth rates of juvenile pinfish, Lagodon rhomboides Linneaus, a common predator in seagrasses,
in various densities of seagrass. Specifically, we tested Heck and Orth’s 1980 hypothesis of a decrease in growth-rates with an increase in complexity of habitat in a
marine environment. We expected to find faster growth-rates in the smallest complexity of habitat and slower rates where complexity was largest. We also investigated the
possibility of overexploitation of prey in the smallest complexity of habitat, as was found by Crowder and Cooper 1982. Using an experimental field enclosure, we
explicitly tested the hypothesis that there was no difference in growth rates of pinfish among small, intermediate or large density seagrass beds.
2. Materials and methods
2.1. Field study site All experiments were conducted in Big Lagoon, Florida 308 18.59 N, 878 239 W
along the northern shore of the Gulf Islands National Seashore at Perdido Key Fig. 2. The extensive seagrass meadows found in Big Lagoon are dominated by Thalassia
testudinum, Halodule wrightii and Syringodium filiforme. All experiments occurred in Thalassia beds which paralleled the shore at | 1 m depth at low water. At the beginning
22
of the study, Thalassia densities were found to range between 0 and 133 shoots m
22 22
small, 177 and 267 shoots m intermediate and . 267 shoots m
large. The use of a single location with natural variation in density of vegetation at the same depth
minimized differences in physiochemical factors e.g. temperature, salinity among treatments.
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.M. Spitzer et al. J. Exp. Mar. Biol. Ecol. 244 2000 67 –86
Fig. 2. Study site within Big Lagoon, Florida.
2.2. Experimental predator Lagodon rhomboides hereafter referred to as Lagodon, the pinfish, occurs along the
Atlantic coast of North America from Cape Cod, Massachusetts to Yucatan, Mexico Caldwell, 1957; Muncy, 1984. Within the southern portion of its range, Lagodon is one
of the most abundant fishes in near shore waters Nelson, 1979; Stoner, 1979, and one of the most important epibenthic predators on amphipods in seagrass beds along the
lower east coast of the United States and in the Gulf of Mexico Stoner, 1982. Larval Lagodon enter estuaries as early as January, mature and then migrate into offshore
waters during the fall months, leaving only a minimal overwintering population in the estuary Hoss, 1974. Lagodon is a visual predator that actively pursues its prey Heck
and Orth, 1980, making the rate of capture susceptible to the effects of different densities of vegetation. Prey capture by Lagodon has been shown to be mediated by
seagrass biomass Nelson, 1979; Stoner, 1982. Lagodon is omnivorous as a juvenile with a diet consisting mainly of amphipods Stoner, 1979, 1982. As Lagodon increases
in size it becomes increasingly herbivorous. Since pinfish juveniles enter seagrass beds in the spring, field experiments began in June 1996 after juveniles reached a size of at
least 63 mm total length. Fish used to stock enclosures never exceeded 110 mm total length, since pinfish longer than 110 mm are mostly herbivorous Darnell, 1958; Carr
and Adams, 1973; Stoner, 1979.
2.3. Establishment of vegetation treatments In all experiments, enclosures 1 3 1 3 1 m were used to restrict pinfish to the target
P .M. Spitzer et al. J. Exp. Mar. Biol. Ecol. 244 2000 67 –86
71
density of seagrass. The enclosures were constructed of 1.91-cm polyethylene mesh that was cable-tied to a welded reinforcement bar frame. The cage tops were constructed of
1.91-cm polyethylene mesh cable-tied to 1.27-cm PVC piping to allow the tops to be opened for cleaning and sampling. The open bottom of the cage was surrounded by a
20-cm wide mesh apron to prevent burrowing animals from entering the cages. The enclosures were anchored by sinking the reinforcing bar ‘legs’ of the cages into the
sediment as well as sinking reinforcement bar stakes into the sediment along the bottom of the enclosures. Additional anchoring was provided by tying enclosures at opposing
corners to steel reinforcement bars driven into the sediment outside of the enclosures. Protocols for the five growth experiments conducted monthly from June to November
1996 are described below and summarized in Table 1.
Five replicate enclosures were deployed in each of the treatments: small, intermediate and large densities of vegetation. All treatments were in close proximity, with the large
density treatment being located furthest to the east and intermediate and small treatments more to the west, respectively. There was | 15 m between treatments. The initial
treatment positions in the naturally occurring densities of the seagrass bed were determined by haphazardly tossing a 10 cm 3 10 cm quadrat over the shoulder and
taking shoot counts to estimate density of vegetation until an area large enough to place all enclosures from a treatment was located. The procedure was repeated for each
treatment. Once the treatment was determined [0–3 shoots quadrat or mean plant
2 2
surface area 5 4687.0 cm plant surface area m 62025.3 S.E. for small density; 4–6
2
shoots quadrat or mean plant surface area 5 17 248.6 cm plant surface area
2
m 62514.0 S.E. for intermediate density; . 6 shoots quadrat or mean plant surface
2 2
area 5 27 011.1 cm plant surface area m 65845.0 S.E. for large density], the cages were deployed and prepared for stocking. Because additional fish and decapods could
prey upon the juvenile fish and or compete with them for food, the cages were seined prior to stocking, to remove any fish, crabs and shrimp larger than the cage mesh size.
The seine, 1 m wide by 1.3 m tall made of 3-mm mesh cable tied to PVC piping, was moved from one side of the cage to the other and then brought to the surface with the
base of the seine pressed against the side of the cage. The cage was considered ‘empty’ once the seine was brought to the surface three consecutive times without any organisms
larger than the cage mesh size.
2.4. Growth of pinfish Before beginning each experiment, Lagodon were collected with a 4.8-m otter trawl,
with 1.91-cm mesh wings and 0.64-cm cod end, which was towed for | 2-min durations in an area of Big Lagoon containing Thalassia. Trawling sites were well removed from
the study site. Prior to deployment, Lagodon were measured to the nearest millimetre TL, weighed to the nearest tenth of a gram wet weight, marked using a tagging
protocol described below and then placed into a designated enclosure. To increase precision, fish were always measured by the same person. Fish were also always
measured in the morning to minimize differences due to time of day. Following capture by trawl, pinfish were held in large coolers and kept cool with plastic bottles of ice. The
coolers also had portable aerators which provided additional oxygen for the pinfish.
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Time spent in the coolers was kept to a minimum in an attempt to reduce the amount of stress the pinfish experienced. Pinfish were removed from the coolers with a hand dipnet
one at a time, and were measured to ensure that fish were of the appropriate length the range of pinfish total lengths TL used in experiments was kept within 5–10 mm. The
size of the pinfish used during the initial experiment was determined by identifying the smallest fish size | 63 mm TL that could not escape through the mesh of the enclosure.
Subsequent intervals used the most abundant size found in the trawls. Mean initial total lengths TL6standard deviation S.D. were as follows for each of the intervals:
¯ ¯
x 5 72.4 mm TL64.0, n 5 150 June–July; x 5 93.3 mm TL66.0, n 5 90 July– ¯
¯ August; x 5 84.6 mm TL63.0, n 5 90 August–September; x 5 91.5 mm TL67.0,
¯ n 5 60 September–October; and x 5 90.1 mm TL64.0, n 5 45 October–November.
Lagodon population density within the enclosures was determined by estimates of population density in natural populations in the northern Gulf of Mexico Kip
Thompson, personal communication. Using a 4.8-m otter trawl, Thompson found natural pinfish populations in the Northern Gulf of Mexico in the Spring to be | 4.59
22 22
m . Similarly, Huh 1984 found annual pinfish populations to be 2.3 m
in shoalgrass Halodule and turtlegrass Thalassia. Since trawls are often only 30–50
effective Kjelson, 1977, Thompson’s numbers were doubled, resulting in the initial enclosure density June July of 10 pinfish per enclosure for each treatment. Each
subsequent study period had a smaller pinfish density, as determined by population estimates from trawls. This was done to simulate the natural population decline of
pinfish that occurs as the growing season progresses.
The tagging protocol chosen for all but the first interval Floy Tags E were used
during the first interval was that of liquid nitrogen branding Dr. Felicia Coleman, Florida State University, personal communication, since other tagging methods e.g.
Floy Tags E have been suggested to reduce the growth-rates of juvenile fish Serafy et
al., 1995, and have a fairly low return rate. The liquid nitrogen protocol utilized a metal template that was shaped into different numbers. Once a fish was measured and
weighed, the right side of its body was wiped with a paper towel and a numbered brand was removed from the liquid nitrogen and pressed against the pinfish for about 10 s.
While pressing, the brand was gently rocked to ensure an even mark on the pinfish. After 10 s, the brand was removed and the pinfish was placed in a large plastic bag filled
with water from Big Lagoon to await transport to the appropriate cage. Fish were maintained in bags for | 10 min to watch for signs of tagging stress mortality.
At the end of each 4-week interval, all pinfish were seined from each cage for growth measurements. Growth was recorded in wet weight to the nearest tenth of a gram and
total length to the nearest millimetre. Since caging sometimes leads to tail rot Kip Thompson, personal communication, measuring total length was not always possible.
Therefore a regression equation was used to predict the fish’s total length from standard
2
length measurements. The regression equation [TL 5 2 2.502 1 1.316 3 SL; r 5 0.99] was determined using standard length x as the independent variable and total
length y as the dependent variable. Data to create the regression were obtained by measuring at least 50 pinfish per trawling occasion from the natural population for total
length and for standard length. Subsampling of Lagodon occurred by trawling on each sampling date to extend the reliability of the regression equation.
P .M. Spitzer et al. J. Exp. Mar. Biol. Ecol. 244 2000 67 –86
73
After each experiment, the cages were removed from the treatment sites and cleaned to reduce effects of caging. Cleaning of the cages consisted of pulling the cages onto
shore and using wire brushes to remove barnacles, algae and other ‘fouling’ organisms. The cages were then relocated to new positions in the appropriate seagrass densities.
2.5. Benthic core analysis At the end of each experiment, benthic cores were taken from random positions within
each enclosure n 5 15 as well as from five random exterior locations per density treatment n 5 15. Samples from outside the enclosures n 5 15; five per treatment
were also taken prior to each interval. The benthic cores were taken to a depth of 10 cm using a 10.2-cm diameter PVC pipe and washed over a 0.5-mm sieve. The cores were
then analyzed in the lab for both benthic fauna and density of vegetation. Temperature, depth and salinity were also recorded at each density treatment at the beginning and end
of each 4-week interval.
2.6. Benthic fauna In the lab, samples were again sorted on a 0.5-mm sieve Lewis and Stoner, 1981 and
macroinvertebrates were extracted from the sample and identified to family level. After the fauna was identified, taxa were dried to a constant weight at 80–908C for at least 24
h, and then dry weights 0.0001 g accuracy were taken. Following weighing, the samples were placed in a muffle oven 5008C for 5 h and then reweighed 0.0001 g
accuracy. Although the cores were taken to 10 cm depth, pinfish do not have access to the deeper infauna burrowing bivalves and polychaetes, therefore these taxa were
eliminated as likely sources of food. The decapods primarily penaeid shrimp and crabs found in the cores that were judged to be too large for pinfish consumption, were also
eliminated as a possible source of food from subsequent analysis. Since amphipods are consistently reported to be a major constituent of juvenile pinfish diets Darnell, 1958;
Hansen, 1969; Carr and Adams, 1973; Young et al., 1976; Nelson, 1979; Stoner, 1979; Stoner, 1982; Stoner, 1983; Luczkovich, 1987, amphipod abundance was examined
among treatments for all five intervals. These data were used to compare both the abundance and AFDW of amphipods found within the cage to those found outside of the
cage. Amphipod abundance and AFDW were also examined for availability of food and for evidence of overexploitation.
2.7. Vegetation Vegetation was sampled using the benthic cores n 5 15; five per treatment and a
10 3 10 cm quadrat n 5 15; five per treatment at the beginning and the end of each 4-week interval to estimate leaf surface area, epiphyte coverage and to evaluate possible
caging effects. The vegetation in the cores was separated into above ground all shoots and loose leaves and below ground roots and rhizomes fractions. The number of
shoots and the number of loose blades were counted and recorded to verify the treatment density in each of the samples. The shoots were then divided into the number of blades
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present and the width and length of each blade from the shoot was measured using calipers none of the loose blades were measured. The data for total length and width
2
were used to estimate the amount of plant surface area m available both to predator and prey within each of the treatments.
The blades of seagrass within the core sample were also examined for the composition of epiphytes and the degree of coverage, since epiphytes are a possible food source for
pinfish and can provide amphipods and other items of prey with secondary structure, thereby increasing potential supply of food and complexity of habitat. One side of the
oldest blade from each shoot was covered with a 4 mm 3 4 mm grid and the type of epiphyte present at each intersection of the grid was recorded. After the composition of
epiphytes was measured, both sides of the blade were scraped of epiphytes using a razor blade and the epiphytes were then dried to a constant weight at 80–908C for at least 24
h, weighed, then placed in a muffle oven at 5008C for 5 h and reweighed. These data allowed a determination of the percentage of weight that can be attributed to calcareous
and non-calcareous epiphytes. Following all measurements of the blades, both the above ground and below ground materials were dried at 80–908C and then weighed.
Quadrat clippings were taken both inside and outside the enclosures n 5 30. The quadrat was randomly placed in the sample area and all above-ground biomass within
the quadrat was harvested at the sediment-water interface. Blades in the quadrat clippings were measured i.e. length and width for each blade, dried and weighed in the
same manner described above for benthic core vegetation. Length and width measure- ments for each blade were multiplied to obtain a blade surface area. The blade surface
2
areas were then added together to estimate vegetation surface area cm quadrat. The
2 2
surface area was then converted to vegetation surface area cm m . The data from the quadrat clippings surface area estimate and weight were compared with the data from
the cores to estimate loss from the coring process and allow corrected values to be calculated.
2.8. Statistical analysis For all study periods, growth in total length and weight of pinfish within each
replicate enclosure was averaged to generate enclosure means, and converted to growth per day to standardize among experiments. Enclosure means for growth were compared
among vegetation treatments by one way analysis of variance ANOVA using Microsoft Office Excel 7.0. All data were checked for normality and heteroscedasticity, using the
Kolmogorov–Smirnov test and checking variability about the group means respective- ly, prior to running ANOVAs. If either of the assumptions was violated, a nonparamet-
ric analysis Kruskal–Wallis was used.
A two-way analysis of variance two-way ANOVA using Sigmastat 2.0 was used to compare the ash free dry weights AFDW of fauna found in the benthic cores, plant
surface area, epiphyte coverage and epiphyte AFDW among treatments. The factors used for the two-way ANOVA were density of seagrass factor 1 and inside versus outside
the enclosures factor 2. Factor 1 was chosen to examine differences among treatments, whereas factor 2 was chosen to examine artifacts of caging. Significant differences
between samples inside the enclosure and samples outside the enclosures would be
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
