TROPHIC FISH STRUCTURE ALONG THE BEACHES OF THE BRAZILIAN AMAZON RIVER

Purpose: This study analyzes fish communities' diet and trophic structure on beaches along the Amazon River. Methods: 41 beaches were studied, and fish were collected with 12x3m trawl nets and 5mm mesh between nodes. The stomachs were analyzed using the frequency of occurrence and points method. The dietary index was calculated using the preponderance index (PI). Using IP values, the following trophic categories were established: detritivore, hematophagous, herbivorous, benthic invertivore, pelagic invertivore, terrestrial invertivore, scavenger, and piscivore. Species richness and abundance of fish in trophic categories were related to the longitudinal variation of the sampled sites through linear regression 1,166 stomachs from 119 species were analyzed. Results and discussion: The abundance and richness of detritivore species and the richness of scavengers showed a negative relationship throughout the floodplain, while the number of aquatic invertivore species showed a positive relationship . Implications of the research: The existing differences in beach water quality create a longitudinal conductivity gradient, transparency, and suspended material, gradually altering the environment and composition of the ichthyofauna and the trophic structure along the Amazon River. Originality


INTRODUCTION
During the drought, the floodplain is a complex environment with numerous mesohabitats that only appear in this period.The beaches represent approximately half of the banks of the great rivers of the Amazon plain.Their waters are home to a very diverse and poorly studied aquatic biota, with fish being the most varied and abundant vertebrate group in this environment (Sleen & Albert, 2017;Duponchelle et al., 2021).Duarte et al. (2019) and Silva et al. (2020) studied the ichthyofauna of beaches and showed critical environments for small fish.Their shallow waters can constitute a refuge environment against catfish and other large fish predators.
These changes along the course of waterways directly affect the aquatic community, changing the environment's quality and the availability of resources and influencing the distribution and composition of macroinvertebrates and fish species (Doretto et al., 2020).There are some studies on the trophic structure of fish assemblages in different environments such as lakes, rapids, streams, macrophyte banks, beaches, and reservoirs (Claro-Jr, 2003;Silva et al., 2020;Jacobs et al., 2021;Melack & Coe, 2021;Nonato et al., 2021;Paula et al., 2021;Silva et al., 2021;Souza et al., 2021), showing a great diversity of diets and food resources.Vannote et al. (1980) proposed the Rio Continuum Theory (TRC), stating a longitudinal gradient of rivers' physicochemical and geomorphological characteristics in the upstreamdownstream direction.
It is necessary to study fish assemblages and their ecological function to propose new protected areas for conservation.However, this research aimed to characterize the trophic structure of beach fish in a longitudinal gradient of the Amazon River in Brazilian territory.Ecological studies relating fish trophic structure in a large-scale longitudinal gradient are scarce in the Amazon River floodplain, making this article essential for understanding Amazonian systems.

MATERIAL AND METHODS
We sampled 41 beaches along the floodplain of the Amazon River (Figure 1).We took the samples in September and October, and the fish were collected using beach seine nets, measuring 12x3m and 5mm mesh between opposing nodes.The determination of the trophic structure of the ichthyofauna was made by studying the diet.We used two methods to analyze stomach content (occurrence frequency method and relative volume method) according to Hyslop (1980).The analysis of stomach contents was preceded by a visual assessment of the stomach's degree of repletion (DR), assigning values between zero and 100%.
Under a stereomicroscope, all stomach contents were placed in a Petri dish, where the items were separated by type and identified at the most accurate taxonomic level.The frequency of food items was obtained by the number of times a given thing occurred about all stomachs analyzed with food.We estimated the relative abundance of food items by assigning percentage values corresponding to the volume of each content item so that the total food found in each stomach is equivalent to 100% (Hyslop 1980).We calculated the relative magnitude of each item in the stomach contents from the relative abundance values of each item multiplied by the DR of the stomach.
For ranking the essential food items, the Food Importance Index (FII) was used, proposed by Kawakami and Vazzoler (1980).As the FII ranges from 0 to 1, items with values greater than 0.5 were considered dominant, and the species fell into one of the following trophic categories: Detritivores (diet composed of debris, mud, and sand), Hematophages (blood diet), Herbivores (plant and algae diet), Benthic Invertivores (diet composed of invertebrates that inhabit the deep or benthic region), Pelagic Invertivores (diet based on invertebrates that inhabit the water column), Terrestrial Invertivores (diet of invertebrates that inhabit the terrestrial environment or treetops), Necrophages (diet made up of animal corpses) and Piscivores (diet made up of fish).We used simple linear regression analyses to verify whether the trophic structure presents a longitudinal variation along the Amazon River (Zar, 1999).It used the longitude value (downstream) of the beaches as an independent variable and the values of fish abundance and species richness for each trophic category on the beaches of the Amazon River as dependent variables.The findings are presented in graphs and tables, as recommended by Nascimento-e-Silva (2021; 2023).

RESULTS
We captured 119 fish species and 1,166 stomachs analyzed, of which 147 were empty.The main species that make up the trophic category of detritivores are from the Prochilodontidae, and Curimatidae families, in addition to some Loricariidae, such as the bodo (Lipossarcus pardalis) and Sturizoma rostratum, some Doradidae, such as Hassar orestis and Nemadoras trimaculatus; a species of Characidae, Prodontocharax sp.; and a species of Cichlid, Apistogramma bitaeniata.Many invertebrates are associated with sediments, such as insect larvae, microcrustaceans, protozoa, bacteria, and algae.These species ingest large amounts of waste using the organic material present.
Detritivores had the fourth-highest abundance on the beaches, with 720 specimens, 8.38% of the total.The number of individuals ranged between zero and 152 specimens on the beaches.The most abundant species among detritivores were Hemidoras stenopeltis, with 150 specimens on a single beach.The richness of detritivores on the beaches was 20 species, approximately 16% (Figure 2).Detritivore richness ranged from zero to seven species on the beaches.
The main species of fish that make up the trophic category of hematophagous are from the Trichomycteridae family.Trichomycteridae is fish popularly known as candiru.The species of this family have spines in the opercular region that help fix other fish's gills.The main species found on the beaches were Trichomycterus hasemani, Trichomycterus johnsoni, and Vandellia cirrhosa.The total abundance of hematophagous fish on sandy beaches was 15 specimens, only 0.18% of the total (Figure 2).
The trophic category of herbivores were fish species that presented their diet based on seeds and roots of aquatic herbaceous plants and filamentous algae.The main herbivorous species found on the beaches of the Amazon River were from the Anostomidae family: aracus (Abramites hypselonotus and Rhytiodus microlepis), and Characidae: piaba (Moenkhausia jamesi) and pacu (Myleus setiger).
On the beaches, herbivores had 106 specimens, just over 1.00% of the sampled fish.Among the herbivores, the most abundant species was Moenkhausia jamesi, with nine specimens on the same beach.The richness of herbivores on the beaches of the Amazon River was 12 species, making up little more than 10% of the total (Figure 2).
The benthic invertivorous fish are presented as the main items in their diet shrimp, microcrustaceans, and insects that live and develop in the deep or benthic region of the beaches.In this category, the most consumed insects were Ephemeroptera nymphs, Megaloptera larvae, Trichoptera, Hemiptera, and Diptera larvae pupae, such as Chironomidae, which were very frequent.Of the shrimp, the most consumed were the Macrobrachium genus, which is very common on floodplain beaches and white-water systems.The main fish species in this trophic category were the piabas, Aphyocharax alburnus; the sarapós, Eigenmannia trilineata; the acarás, Acarichthys heckelii and the Doradidae Trachydoras nattereri and Trachydoras stendachneri that fed mainly on insects; hake, Plagioscion squamosissimus, which fed on shrimp and sardines, Rhinosardinia amazon, which provided on microcrustaceans, Ostracoda.The benthic invertivores category was the most abundant group on all the Amazon River beaches, with 4,802 individuals, more than 50% of the fish caught.Trachydoras stendachneri was the most abundant species among benthic invertivores, with 803 individuals on a single beach.This group presented the most extraordinary richness among the trophic categories, with 37 species, corresponding to approximately one-third of the species collected on the beaches of the Amazon River (Figure 2).
The category of pelagic invertivores included fish whose diet was based on invertebrates that lived in the pelagic area of the beaches.The main prey consumed in this category were larvae and pupae of Chaoboridae (Diptera) and planktonic microcrustaceans such as Cladoceras and Copepodos (Cyclopoida and Calanoida).The main fish species in this trophic category were: the pufferfish (Colomesus asellus -Tetraodontidae), which fed on large amounts of plankton, and the acará (Chetobranchus semifasciatus -Cichlidae).This trophic category presented a total abundance of 1,429 individuals on the beaches, representing 16.64% of the fish collected in this environment.The most abundant species in this trophic category was Colomesus asellus, with 986 individuals on a single beach.The total richness was seven species, accounting for little more than 5.00% of the species on the beach of the Amazon River (Figure 2).
Terrestrial invertivores were fish species that had their diet based on insects and other invertebrates living in terrestrial environments or treetops.They accidentally fall to the water's surface and are consumed by fish.The main species that made up this category belong to the Characidae family: Ctenobrycon hauxwellianus, which fed mainly on ants; Thoracocharax stellatus, and Triportheus albus, which fed primarily on insects such as Orthoptera, Coleoptera, and Hymenoptera.On the beaches, the terrestrial invertivores presented a total abundance of 301 individuals, about 5.00% of the fish collected in this environment.The most abundant 6 species in the terrestrial invertivores category was Thoracocharax stellatus, with 118 specimens on the same beach.The total richness was ten species, approximately 8.50% (Figure 2).
Scavengers were fish in their diet composed of fish remains, pieces and carcasses, and terrestrial insects, ingested with sediment and sand.The physical appearance of the stomach contents was amorphous similar to decaying organic material.The species that made up the food category of scavengers are from the Pimelodidae family: Pimelodus aff.altissimus and Pimelodus sp.
The total abundance of scavenger fish was 855 individuals, approximately 10% of the fish collected, and 1.68% of the species richness contained on the beaches of the Amazon River (Figure 2).Pimelodus aff.altissimus showed greater abundance, with 323 specimens on the same beach.
Piscivores were species that fed on fish and scales.Some species provided on whole fish corresponding to up to 30% of their size, such as the dogfish (Acestrorhynchus falcatus, Hydrolycus scomberoides, and Raphyodon vulpinus), the thrushfish (Hoplias malabaricus).
Others with more than one individual in their stomachs, such as some catfish (Ageneiosus sp., Sorubimichthys planiceps, and Sorubim lima).Other species of piscivores have the habit of tearing their prey, as is the case of piranhas (Pygocentrus nattereri, Serrasalmus calmni, Serrasalmus rhombeus).Other fish in this group presented their diets based on the scales and fins of other fish, such as Henonemus punctatus and Ochmacanthus off.reinhardtii and Roeboides affinis.On the beaches, piscivores presented a total abundance of 356 individuals, making 4.14%.Roeboides affinis was the most abundant species of piscivores, with 164 specimens on P17 beach.The full richness of piscivorous fish was 27 species, 22.68% (Figure 2).The trophic structure showed a longitudinal relationship along the floodplain beaches of the Amazon River.The species richness of the trophic categories of detritivores, pelagic invertivores, and scavengers showed significant associations with beach length.The richness of the other classes did not establish a substantial relationship along the beaches of the floodplain of the Amazon River (Table 1).The number of scavenger and scavenger species increases longitudinally along the shores of the Amazon River.However, the number of pelagic invertivore species decreases (Figure 3).
Investigating a linear relationship between the abundance of individuals in the trophic categories and the length of the beaches, we noticed that most types did not show a significant relationship.Only the trophic sort of detritivores showed a meaningful relationship, showing a decrease in individuals along the beaches of the Amazon River (Figure 4).

DISCUSSION
The diversity of food resources present in the stomach contents of fish reflects their great trophic adaptation, together with the wide availability of resources in tropical environments (Heig et al., 2018;Costa & Angelini, 2020;Neves et al., 2021).Despite this, the trophic category of hematophagous showed excellent food specificity, showing a separate group, generally classified as piscivores or carnivores (Resende, 2000;Spotte et al., 2001;Araújo-Wang et al., 2019).However, hematophagous can be considered parasites of other fish, as they hardly kill their host, a food resource, which is a larger fish (large catfish).Experiments on candirus' behavior and feeding strategy showed that the first attack is the gills trying to reach the main artery.However, if the infestation is large, some pierce the nostrils, leading to the host's death (Zuanon & Sazima, 2004).
Determining trophic structures is subjective and mainly based on diet and food preference.But it can provide crucial additional information on species ecology, such as the origin of the food item (Claro-Jr et al., 2004;Arantes et al., 2018;Castello et al., 2018), the behavior in obtaining food (Zuanon & Sazima, 2004;Campos-Cardoso et al., 2019;Huby et al., 2019) and the quality of the environment (Andrade et al., 2019;Duarte et al., 2019;Ribeiro-Brasil et al., 2020).The trophic structure of fish was assembled from information on the diet, food strategy, and spatial niche of prey in the coastal regions of the beaches, thus trying to understand this environment better on the shores of the Amazon River floodplain.Although the beaches are considered poor in food resources, compared to the flooded forest, (Duarte et al., 2019) showed that the beaches are rich in fish species and with a great diversity of items found in the stomach contents of the ichthyofauna.
The main food item used by fish on the beaches of the Amazon River were benthic invertebrates, including insects and crustaceans, an important food source in an environment of low productivity (Duarte et al., 2019).Such food items were also crucial in the feeding of some fish species in other domains such as reservoirs (Feitosa & Resende, 2020), flooded forests (Lima et al., 2020;Mendes-Júnior et al., 2020;Capitani et al., 2021), little rivers and streams (Barros et al., 2017;Cardoso & Couceiro, 2017;Benone et al., 2020).10 Decápods proved to be the main food item for some fish species on the beaches of the Amazon River, such as hake (Plagioscion squamosissimus and Plagioscion auratus) and cat sardine (Lycengraulis grossidens).Few fish species exploit this food resource, even though it is abundant on the beaches.Competition with other piscivorous fish may cause Plagioscion spp.and L. grossidens to use shrimp instead of fish (Pereira et al., 2017).
Zooplankton is a vital food resource in the diet of fish (Campos-Silva, 2021) and consuming these food items presents different strategies for predation in the water column.Some fish can visualize the zooplankton and choose the prey chasing until the attack (Gray, 2021), such as Colomessus asellus (Tetraodontidae) and other cichlids.While other species filter large amounts of water and retain prey in the gill tracks such as Hypophthalmus spp.(Pimelodidae) and C, macropomum young fishes.Fish that adopt the first strategy usually have medium-sized stomachs but relatively large food items.On the other hand, filter feeders commonly have full stomachs and a high diversity of microcrustacean species (Albuquerque et al., 2019;López-Rodríguez et al., 2019).
Exploitation by terrestrial insects was low on beaches compared to flooded forests, where this food item represents one of the essential sources in the fish diet (Claro-Jr et al., 2004;Silva et al., 2021).The absence of trees and shrubs makes terrestrial insects less abundant and, thus, a scarce resource on beaches.Eventually, ants, beetles, and butterflies fall into the water, creating surface vibrations and attracting some fish species, such as Triportheus albus, Thoracocharax stellatus, and Moenkhausia sp., which quickly capture these insects.The superior position of these species' mouths helps capture prey and food items that fall to the water surface (Siqueira-Souza, 2017;Cardoso, 2019;Evans et al., 2021).
Periphytic algae associated with macrophytes are the energy sources of carbon for fish in the Central Amazon (Leite et al., 2002;Arantes et al., 2019).In other Amazonian environments, fish in the trophic category of herbivores are more numerous (Rejas, 2018) than on beaches, where only 10% of species exploit this resource.It was expected that fish consuming algae and macrophytes would be more abundant in a shallow environment with great light and nutrients available for primary production.The minor exploitation of this resource can be caused by the low nutritional composition of algae and macrophytes concerning other food items and the fact that only some animals can digest cellulose (Gerking, 1994).However, some species on beaches, such as Schizodon fasciatus and Rhytiodus microlepis are specialists in this food resource (Rejas, 2018).
For herbivorous fish, fruits, and seeds are the essential items in the diet compared to algae and macrophytes (Arante et al., 2018).They can be explained by the fact that the fruits have high nutritional values (Waldhoff et al., 1996;Arantes et al., 2019), allowing the fish to accumulate reserves and present better development (Roubach & Saint-Paul, 1994;Val & Oliveira, 2021).However, no fruit or seeds from the flooded forest were found in the stomach contents of the ichthyofauna on the beaches of the Amazon River due to the time of year.
Piscivores was the second group with the highest number of species collected on the beaches of the Amazon River, being a trophic category present in experimental fisheries in all types of environments (Rejas, 2018;Arantes et al., 2019;Duante et al., 2019).The predator/prey relationship can explain the high number and species of this trophic category can be defined by the predator/prey relationship.The supply of prey plays an essential role in the fish community, functioning as a regulator of population size (Barbosa et al., 2018).
The piscivorous fish on the floodplain beaches concentrate their prey among the small characids and engraulids that live on the sandbanks, seeking refuge and food.In other floodplain environments, piscivores show the same behavior, showing that they are foraging species moving between the basin's habitats (Barbosa et al., 2018).
Detritivores are abundant in tropical environments, especially in the floodplain.Due to the significant contribution and accumulation of organic material from the decomposition of plant remains.Other studies on fishing and trophic structure of the tropical ichthyofauna found a large abundance of detritivorous fish (Arantes et al., 2018;Rejas, 2018;Goulding et al., 2019;Farago et al., 2020).Forming shoals can make this group of fishes very sampled, as enormous shoals of detritivores were found on the beaches, mainly Prodontocharax sp. and Hemidoras stenopeltis.
Studies on variations in the fish distribution along a gradient are common (Cox Fernandez et al., 2004;Couto et al., 2018;Arantes et al., 2019;Seabra et al., 2021).However, only some present their implications for the trophic structure of ichthyofauna (Claro-Jr et al., 2004;Arantes et al., 2019).There is expected to be a variation in the form of the ichthyofauna, which influences these communities' functioning.Therefore, the floodplain beaches showed differences in the richness and abundance of trophic categories along the floodplain of the Amazon River.
The Detritivores category presented a significant relationship between the number of species and the abundance of individuals along the floodplain beaches, explained by the amount of sediment inversely proportional to the longitude.The farther away from the mouth, the higher the conductivity value, changing negatively along the longitudinal gradient of the Amazon River.As conductivity is correlated to suspended material, we can say that white water is diluted along the floodplain.This leads to a decrease in the number of species and abundance of fish exploring particulate organic material and sediment.
However, this gradual decrease in conductivity and suspended material implies increased water visibility and transparency, giving some species advantages in capturing their food (Arantes et al., 2019).Water transparency is essential for predatory zooplankton species, as many use vision to identify, chase and capture their prey (Gerking, 1994;Gray, 2021).It happened with the trophic category of pelagic invertivores that significantly increased the number of species on beaches along the floodplain of the Amazon River.
In the case of scavengers, the significant decrease in species, which ingested dead material or state of decomposition as the main item, is explained by the correlation between longitude and conductivity.This relationship makes the Amazon River less productive downstream and less organic material available in the environment.However, this meaningful relationship between scavengers and longitude is explained by the geographic distribution of the species Pimelodus aff.altissimus and Pimelodus sp.P. altissimus is widely distributed in the Amazon basin.However, its most extraordinary occurrence is to the west of this basin, where the holotype of this species was recorded (Ribeiro & Lucena, 2006;Formiga et al., 2021).
The absence of a significant relationship in the analyses between longitude and trophic categories of benthic and piscivorous invertivores is due to the ample supply of these resources on all beaches of the Amazon River.It causes many species to continuously explore aquatic insects and other fish in these beach environments (Duarte et al. 2019).However, the low occurrence of hematophagous fish possibly meant that the relationships between these trophic categories and their longitudinal distribution needed to be more present.The abundance of fish (or potential hosts) has no longitudinal relationship (Zuanon et al., 2008).Then the food supply for hematophagous was also not affected, and thus there was no change in this trophic category.The number of species and abundance of fish in the trophic category of terrestrial invertivores remained constant along the floodplain, justified by the low occurrence of this food resource, which makes it impossible for fish to exploit it more frequently.
Herbivores from the beaches of the Amazon River showed a longitudinal relationship corroborating the Continuous River Theory-TRC (Vannote et al., 1980).In TRC, the system's primary productivity increases along the river as the light input and the increase in the river width provide favorable conditions for the development of algae and macrophytes that grow, occupying the margins of the aquatic system.Miranda et al. 2019 found increased herbivores and planktivores along the river.They showed that the transformation from a lotic to a lentic environment increased the primary production and abundance of herbivores and planktivores compared to other food categories.Arantes et al. (2018) identified an inverse trend between the number of autochthonous herbivores and flooded forests in floodplain lakes in central Amazonia.Claro-Jr et al. (2004) showed a direct relationship between the amount of flooded forest and the feeding of some fish species, indicating that the preservation of the floodplain is essential for the conservation of fishery resources, which are responsible for the primary source of animal protein for the Amazon population (Goulding et al., 2003;Begossi et al., 2018).
The seasonality of tropical environments is one of the causes of significant influence on changes in the trophic structure of fish (Duarte et al., 2018, Arantes et al., 2019).Some fish perform lateral migrations moving from lakes to main rivers during the dry season.These changes alter the proportions of some trophic categories, such as piscivores, that increase their density inside lakes in the dry period (Duarte et al., 2018).In the dry period, the food web is dominated by fish with different life strategies: species that leave lakes due to increased predation, others due to changes in the limnological condition of the environment, or even fish that anticipate maturation and spawning.Therefore, significant local differences in fish assemblages increase the complexity of food webs, affecting the proportion of all trophic groups (Duarte et al., 2018).
Fish migrations along the river, such as large catfish and fish of the Prochilodontidae family (Prochilodus spp.and Semaprochilodus spp.), can affect the trophic structure in tropical environments (Duarte et al., 2018).First, with the migration of algivores/detritivores, there may be changes in secondary production from one place to another, resulting in competition and influencing the dynamics of the primary output in the systems that receive the migrants (Duarte et al., 2018).Second, the resident piscivores will have a nutritional contribution with the arrival of emigrants, increasing their populations and altering resident prey populations.Finally, the appearance of large piscivores can increase the mortality of specific fish populations in the ecosystem that receives the migrants, directly or indirectly altering the dynamics between predator/prey and the local trophic structure (Duarte et al., 2018).

Figure 1 .
Figure 1.Collection points on beaches of the Amazon River.Source: Prepared by the authors.

Figure 2 .
Figure 2. Relative frequency of species richness and abundance of fish caught on the beaches of the Amazon River.Source: Data collected by the authors.

Figure 3 .
Figure 3. Relationship between species richness of the trophic categories and the longitude of the beaches of the Amazon River.Source: Data collected by the authors.

Figure 4 .
Figure 4. Relationship between the abundance of fish in the trophic categories and the longitude of the beaches of the Amazon River.Source: Data collected by the authors.

Table 1 .
The simple linear regression analysis results were applied to species richness and fish abundance data for trophic categories x longitude.Regressions (dependent var.x independent var.) (Values in bold indicate significant relationships.)Source: Data collected by the authors.