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.

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).

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 (Amatitlania nigrofasciata, Astyanax aeneus, Aequidens rivulatus and Notropis moralesi); 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.