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Variations of tadpole populations in two ponds at La Selva

David Wojonowski David Fletcher   David Lesley   Peter Ritson

Hypothesis:

A pond subjected to introduced fish will have decreased abundance and diversity of frog larvae.

Materials and Methods:

Two ponds at La Selva were sampled.  The experimental site was expected to have fish introduced by a recent flood.  The control site was not near the river and was expected to contain no fish.   Two sampling techniques were used.  The same two researchers obtained dip net samples at each site.  Dip net samples were obtained by dragging the dip nets over the bottom for one meter and ending the sample at the bank.  Subsequent samples were made at a distance of ten meters along the bank.  The two series of samples in each pond were made in opposite directions along the bank.  The other two researchers made Seine samples in the deeper water of each pond.  Each worker had a small seine and sampled from the bottom to the top of the vegetation.  Each researcher seined for one hour in each pond.  The seines were made of cloth with wooden poles. All workers vouchered morpho species for later identification in the laboratory.  Captured tadpoles, fish, and larger invertebrates were recorded.  Temperature of the air and water, maximum depth, and time of effort were recorded.  Bottle traps were left overnight in each pond. 

Site one is shown in Figure One.  Site Two is shown in Figure Two.
Figure one	Figure two

Results:

The pond near the river was designated site one.  It was next to trail marker STR4000.  A large hill prevented river water from reaching the site during the flood.  Frog larvae were captured in nearly every attempt by both methods.  No fish were captured.  The pond away from the river was designated site two.  It was downhill from trail marker CES 150.  This location had been inundated by river water within the last month.  Few frog larvae were captured.  Four species of fish were captured.  Few large invertebrates were seen. 

These results are presented in Table One.

    There were four species of fish captured from site 2.  Fish from the genus Astatheras, family Cichlidae, order Percomorpha (Spiny-rayed fish) were captured.  These fish were approximately 11 mm long, 2 mm in width, and a height of 2mm.  Basically these fish were brown-speckled in color.  Cichlids are generally bottom fish and show a great diversity below 100 meters in elevation.  Cichlids most feed upon insects and detritus although some are piscivorous.  Cichlids have strong spines in the dorsal, anal and pelvic fins.  The fish that we caught had a long dorsal fin, a round to pointed caudal fin, and an elongated pelvic fin.

    One fish from the genus Rivulus, family Rivulidae, and order Atherinomorpha (Crprinodonts and Silversides) was captured.  The approximate length was 50 mm, a height of 5mm, and a width of 5mm.  This fish was olive-colored.  This fish had a rather short and rounded dorsal fin posteriorly located on the dorsal side.  There was a short pelvic fin and the anal fin was directly opposite the dorsal fin.  The caudal fin was rounded and there was a spot on the body just anterior to the caudal fin.  Rivulus always swims near a grass or undergrowth refuge.  They mainly eat aquatic insects

    Fish from the genus Roeboides, family Characidae, order Ostariophysi (Tetras and Catfish) were captured.  These fish were approximately 14 mm in length, 3 mm in height, and 1 mm in width.  The color was an off- colored white with specks and almost transparent. These fish had a concave bulge on the body just posterior to the head.  The dorsal fin was in the middle of the back. There was a small adipose fin near a deeply forked tail. The anal fin extended from just posterior of the pectoral fin to almost the caudal fin.  These fish are typical of quiet waters.  These fish mainly feed of aquatic invertebrates and fish scales from other fish.

    Fish from the genus Poecilia, family Poecilidae, Order Atherinomorpha (Mollies) were captured.  The length ranged from 19 to 27 mm, the height varied from 2 to 4 mm, and the width of these fish were between 2 and 4mm.  The fish had a gray dorsal surface and a white belly.   The head of this fish is more triangular than roundish.  The dorsal fin begins more than halfway down the dorsal surface and is directly above the anal fin, which is approximately the same size as the dorsal fin.  The caudal fin is roundish with spots.   These are some of the most common freshwater fish in Costa Rica and are most abundant in slack waters of ponds and swamps

Conclusions:

The recent flood of the La Selva area introduced fish into some vernal pools but not others.  The distribution of frog larvae correlates negatively with the distribution of the fish.  This may indicate that predation has occurred.

Bibliography: 

Peces, Freshwater Fishes of Costa Rica,  William Bussing, Editorial de la Universidad de Costa Rica, 1998.