Capítulo II 41
Fieldwork was carried out during four excursions, two in the dry season (July and
August 2004) and two in the rainy season (February and March 2005). In a five-day period
during each excursion, the fish species present among and under the submerged branches
of the vegetation were observed and collected for 10 meters along the margin of each of
the three stretches. The objective of the observations was to ascertain the sites where the
species were foraging. The specimens collected were used for checking taxonomic
identification and for diet analysis. Each day, the stretches were visited during the
morning, always during full moon and at low tide, for about one hour in each stretch, so
that all three stretches could be visited before the turn of the tide. During this period, we
made underwater observations, collections, and environmental measurements (dissolved
oxygen, temperature and salinity, by means of a water checker analyzer, HORIBA-U10).
The dive sessions were done in the first four days of each trip, by free-diving with a
half-face mask, snorkel, and dark-colored, insulating neoprene wetsuit and boots
("snorkeling" cf. SABINO, 1999), totaling approximately 13 hours of observations in the dry
season, and 6.5 hours in the rainy season. The observations were recorded with a PVC
plaque and eraser-pencil (SABINO, 1999); data on fishes swimming under the vegetation,
and their behavior were recorded. During a diving session, fish were collected with a small
hand net and fixed in 10% formalin. In March 2005, only collections were made, because
the mangrove was full of algae that float by any movement in the water, which made it
impossible to make observations.
On the fifth day of fieldwork, the fish were caught with a seine net (1 x 4 m; 2 mm
mesh). The specimens were fixed in 10% formalin. Collecting consisted of pulling the net
toward the shore, enclosing and suspending the vegetation, in short 2-meter stretches; the
collection was repeated in the same place until no more fish were caught. The same
procedure was repeated, moving upstream, along the entire 10-meter stretch worked.
The species collected were identified (FIGUEIREDO & MENEZES, 1980, 2000;
MENEZES & FIGUEIREDO, 1980, 1985) and vouchers are in the fish collections of the Museu
de Zoologia da Universidade de São Paulo (MZUSP 93928-93939). For each specimen the
standard length (SL) was noted before its gut was removed. Diets were analyzed separately
by season (dry and rainy), based on the stomach contents (for those species without a
differentiated stomach, the first third of the intestine was analyzed), and the food items
were identified to the lowest possible taxonomic category. Three methods were used:
frequency of occurrence, biovolume and numerical frequency. Frequency of occurrence
indicates the presence or absence of an item (HYSLOP, 1980), and is calculated from the