DOI: 10.5061/dryad.sqv9s4n2m
title: Seed consumption by small fish follows peak seed availability in a tropical dry forest river

description: 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.  
description: During 2015, we performed
 an intensive monthly sampling of fish in six locations along the Amacuzac
 River. Sampling was carried out in 200 m of river stretches, covering
 different types of habitats (rapids, parallel pools, backwaters and
 canals). The sampling effort was one hour. Fish were collected using cast
 nets of 2 m diameter and 20 mm mesh, and were kept cold for later analysis
 in the laboratory. We randomly selected 10-15 fish specimens per site from
 each species collected. Fish were dissected ventrally, leaving the
 visceral cavity exposed for extraction and for contents analysis by means
 of a stereoscopic microscope (Nikon SMZ-2T). Specimens were deposited in
 the fish collection (CICIB) at the Autonomous University of the State of
 Morelos (UAEM). The taxonomic identification of species was based on the
 criteria shown in Table S1, and with the help of the specialized staff at
 CICIB-UAEM. We determined the proportion of seeds with respect to total
 food items observed in stomachs using the numerical grid method (Lagler,
 1956; Windell & Bowen, 1978). Once we knew which fish species
 had seeds in their stomachs, we did a second field survey during the dry
 and rainy season of 2016, collecting only the four species identified in
 2015 (Table S2). The sampling protocol and objectives were similar to that
 of the previous year, and were based on the methodology used by
 Mejía-Mojica et al. (2012). We sampled five locations in the same area of
 previous years’ sampling (Figure 1). Between 15-30 specimens were taken
 per species and location. Seeds were identified to the genus level and
 classified according to known original dispersal syndromes (Table S3 and
 S4).  
description: In the data file we
 included information on the number of individuals with and without seeds
 in stomachs acording to: the year of field survey (2015, 2016); the season
 of fish sampling (rainy, dry); the fish species (<em>Amatitlania
 nigrofasciata</em>, <em>Astyanax aeneus</em>,
 <em>Aequidens rivulatus</em> and <em>Notropis
 moralesi</em>); the origin (exotic, native); and river
 location.  We
 also included information on weights and body sizes of the four fish
 species carrying seeds in the study.  You will find more information on
 the Supplementary Information file.