Seed consumption by small fish follows peak seed availability in a tropical dry forest river

Seed consumption and dispersal by fish has been more extensively described
in natural Neotropical large river systems of Amazonia, where ichthyochory
follows a seasonal gradient associated with a floodpulse that creates
long-lasting seasonally flood zones. It has been shown that it is
relevant in maintaining the plant community structure of
wetlands, but its effects on plant communities in seasonal dry forests are
largely unknown. The Amacuzac hydrological system, which runs
through one of the most extensive seasonal tropical dry forests of Central
Mexico, shows a marked climatic seasonality, which in turn determines a
transient increase in river flow and its potential to overflow, resulting
in a drag effect on banks, and in the river carrying three times more
seeds in the rainy than in the dry season. Another characteristic of the
system is the coexistence of native and non-native fish species, due to
the fact that the middle part of the river receives discharges from four
tributaries that cross important urban, industrial, and agricultural
districts as well as an extensive network of ornamental fish farms.
Therefore, we hypothesize that rates of seed consumption by fish would
correspondingly increase during rainy seasons, and that both native and
non-native fish would consume seeds in similar proportions. We evaluated
seed consumption by fish by (a) identifying and estimating the
frequency and number of fish species whose stomachs contained seeds, and
determining the percentage of each fishes’ diet corresponding to
seeds with respect to total food items, and (b) by evaluating if
there were seasonal differences in the consumption of intact seeds by fish
and between native vs. non-native species. We found two native species -
Astyanax aeneus and Notropis moralesi, and two non-native species -
Aequidens rivulatus and Amatitlania nigrofasciata containing seeds in
their stomachs. The number of individuals with seeds was significantly
higher in the rainy season than in the dry season, with a higher
proportion of non-native fishes carrying seeds, but only in the rainy
season. We found significant differences between the fish species in the
proportion of seeds consumed. Fishes follow the peak of seed availability
after a flood pulse released them from the soil seed banks. Thus, fishes
consume fruits/seeds on a seasonal basis, however in this case it is
explained by a different mechanism other than the timing of seed
production by trees. As with hydrochory in tropical dry forests, seed
consumption/dispersal by fish inhabiting rivers within this ecosystem has
been overlooked in the ecology and conservation literature. This evidence
is relevant for tropical dry forest ecosystem dynamics and management
strategies in riparian corridors.