Changes in nekton community structure in north Brazilian intertidal mangrove
creeks were studied by setting block nets at high water in the wet season
1999. From a total of 47 species (27 families), the most abundant were:
Cathorops sp. (Ariidae), Colomesus psittacus (Tetraodontidae), Hexanematichthys herzbergii (Ariidae), Pseudauchenipterus nodosus (Auchenipteridae), the shrimp Fenneropenaeus subtilis (Penaeidae), Anchovia clupeoides (Engraulidae), Mugil spp. (Mugilidae), and Anableps anableps (Anablepidae).
Diversity H' was low (0.9). Mean values of fish density and biomass (± S.E.)
were 0.2 ± 0.1 fish m-3 and 2.6 ± 0.7 g m−3 or 1.4 ± 0.4 g m−2, respectively. The high proportion of juvenile fish
(87%) and shrimps point to the importance of intertidal mangrove creeks
for fish nurseries. The number of species was highest at spring tide-night
and lowest at neap tide-day. Evenness J' was lowest at spring tide-night
when more rare species entered and dominant species increased in
quantitative proportions. Four different species assemblages were identified
as being triggered by factor combinations such as “tide” and “time of day”:
spring tide-night, spring tide-day, neap tide-night and neap tide-day.
High-water level influenced the species assemblages, but salinity and water
temperature did not. Abundance of 14 species correlated positively with the
high-water level. Strong spring tides yielded extraordinary high nekton
catches, suggesting that high-water level is an important parameter in the
tidal migration of fish. Most fish entered the mangrove to feed. Predation
pressure was considered to be low because of the presence of only a few,
mainly juvenile predators. Beach seining in the subtidal parts at low water
revealed a high biomass (30 ± 14 g m−2). Larger fish withdraw to
depressions, small fish and juvenile shrimp were abundant near the shallow
banks, suggesting inter- and intraspecific splitting of the migration routes
when returning from the intertidal zone at ebb tide. To cover the entire
short-term range of tidal ichthyofauna responses, a sample design should
consider the spring/neap and the day/night cycle.