The ichthyofauna can be described and classified through the functional attributes of organisms, mainly based on the trophic level, reproductive strategy or use of the environment (Elliott et al. 2007). The functional attributes divide the species into guilds, defined as groups of species that exploit the same class of environmental resources in a similar way (Root 1967). The guild approach allows a better understanding of the ecology and role of the biota in the ecosystem (Elliott et al. 2007). It may help identify overexploited resources through changes in the composition of the food web (Garrison and Link 2000) and of the energy flows in the system (Harrison and Whitfield 2008). The guild approach also helps to understand the effects of climate changes on the structure and composition of fish fauna (Feyrer et al. 2015).

Trophic and estuarine use guilds have been widely applied to understand the structure and functioning of aquatic ecosystems, the movement pattern between environments and their use as feeding, breeding or development grounds (Elliott et al. 2007). Estuarine use guilds reflect migratory patterns and physiological adaptations of species that explore the area throughout their life cycle or part of it (Elliott et al. 2007). Trophic guilds are useful in the comprehension of the feeding habits of a species (Elliott et al. 2007). Its ecological relationships and the energy flows (Paiva et al. 2008) may reflect the possible strategies for avoiding competition or for optimizing the consumption of available resources (Angel and Ojeda 2001).

Estuaries are important transitional environments for the movement of the ichthyofauna between the continental basins and the ocean (Ray 2005). As an ecotone, estuaries link marine and freshwater ecosystems (Gray and Elliott 2009), and persistent environmental fluctuations place considerable physiological demands on the species inhabiting the area (Elliott and Quintino 2007). Many species are dependent on estuarine environments; several marine species are considered visitors and explore estuarine habitats during their ontogenetic development, evidencing the relationship with coastal environments (Able 2005). Therefore, defining the relationships between species and their functional roles within communities is critical for understanding the dynamics of the ecosystem and fundamental for the implementation of ecosystem-based fisheries management (Buchheister and Latour 2015).

The Brazilian coast hosts large estuarine complexes along the 187 km of the coast of Pernambuco, and several areas are considered of great environmental importance (CPRH 2010). The variety of habitats, along with the complexity of interactions within the fish community and the migratory nature of many species, hampers the assessment of the overall condition of the area (Vasconcelos Filho et al. 2003).

Using the ecological guilds approach, this study describes the composition and structure of the fish fauna along a tropical estuarine complex in order to identify and explain the main patterns of seasonal and spatial variations in assemblage composition. The study also discusses the importance of the use of the ecological guilds approach to assess the effects of multiple anthropogenic pressures on the structure and functioning of fish communities in tropical estuaries.

Overall, species composition of the IIC was similar to that of the fish fauna typically found in other tropical estuaries (Paiva et al. 2008, Mourão et al. 2014). The observed species richness was close to the estimated richness, indicating that sampling was satisfactory, thanks to the concomitant implementation of active and passive fishing gear. Sampling is known to affect catch composition, especially its diversity (Magurran and McGill 2011), but the use of different gears provides the best estimate of structure (Kwak and Peterson 2007, Mourão et al. 2014) and diversity of fish assemblages (Mérigot et al. 2016). Different gears use different capture processes, mainly based on fish behaviour (Huse et al. 1999). In this study, the use of multiple gears was necessary, considering the differential characteristics of each environment sampled and the fact that fish species explore different habitats of a given environment differently. By exploring multiple gears in different habitats, we improved the estimation of biodiversity, thus providing as wide a variety of guilds as possible.

PCO analysis and PERMANOVA showed spatial differences in the estuarine use functional group between the estuarine and coastal areas, but temporal variations were not evidenced. Spatial segregation processes were observed in other tropical estuaries (Mourão et al. 2014, Loureiro et al. 2016) and may be related to differences in the life cycle and in species tolerance to diverse environmental stresses. Temporal changes in the composition of estuarine fish communities were not observed in the IIC, as reported in other tropical estuaries (Castillo-Rivera et al. 2002, Mendoza et al. 2009).

In the estuary, migrant species predominated in richness and biomass, and estuarine species in abundance. The high richness and biomass of marine species in the estuary can be attributed to the permanent connection between the estuarine area and the Atlantic Ocean throughout the year (Medeiros and Kjerfve 1993), allowing an uninterrupted connectivity with the marine ecosystem (Vasconcelos et al. 2015). Migratory species are of great importance in connected systems, such as estuaries and the adjacent marine area (Harrison and Whitfield 2008). In addition, the IIC is considered a system with high biodiversity and primary and secondary productivity (Vasconcelos Filho et al. 2010, Mérigot et al. 2016). The positive effect of primary productivity on species richness allows larger populations to persist, thereby reducing extinction risk and supporting a higher diversity of niche specialists (Tittensor et al. 2010). According to Vasconcelos Filho and Oliveira (1999), marine species of the IIC are mostly juveniles, some of which are of commercial value. The high abundance of estuarine species within the estuary of the IIC was mainly due to gobiids. Mérigot et al. (2016) analysed the diversity of fish communities in estuarine complexes in Brazil and revealed differences between assemblages from Itapissuma, especially due to the relatively high abundance of some species of Gobiidae. The high abundance of gobiids in tropical estuaries may be partly due to their prolonged larval duration (Shen and Tzeng 2008), closely linked to the mainly muddy substrate and thus restricting their migrations to the sea (Vasconcelos Filho and Oliveira 1999).

In the coastal environment of the IIC, the marine stragglers predominated in richness, abundance and biomass in all periods. However, the percentages of resident (estuarine) and dependent (marine migrant) species were also high, thus confirming the dependence between the estuary and coast of the IIC. The connection between continental and marine environments is an essential characteristic, as marine species are important exporters of energy to the adjacent coastal areas (Vasconcelos Filho et al. 2009). Also, the coast of the IIC offers favourable conditions for the development of the marine fish fauna as protection and food resource (Medeiros et al. 2001).

In relation to the feeding guild approach, our findings emphasized that the substrate of the IIC is of extreme importance for the high productivity in the system (CPRH 2010), contributing to the high occurrence of species with feeding habits associated with the substrate (i.e. zoobenthivores and detritivores). The high availability of organic rich detritus in mangroves may increase the feeding opportunities for detritivores (Kuo et al. 1999), and can be considered the main trophic contribution factor for the estuarine fish fauna (Paiva et al. 2008). In north Brazil, Loureiro et al. (2016) observed that fish assemblage was strongly associated with substrates composed of organic matter. The high richness of zoobenthivores in the estuarine area of the IIC can be attributed to the great abundance of available benthic fauna (Silva 2013). Benthos is one of the structuring elements of the food web and plays an important role in the system dynamics (Herman et al. 1999), transferring energy to fishes in estuarine environments (Buchheister and Latour 2015). Detrivores dominated in abundance and biomass mainly due to large supply of organic matter and detritus in the IIC (Vasconcelos Filho et al. 2009, 2010), which support estuarine trophic webs (Hoffman et al. 2008). The estuarine organic material of the IIC originates from various rivers (Eskinazi-Lessa et al. 1999). The river discharge, sediment resuspension, mangrove litter, waste input, terrestrial runoff and atmospheric input are sources of nutrients in the IIC estuary (Medeiros 1991). The highest proportion of detritus usually occurs in environments with great amounts of organic matter. Detritus is consumed, constituting a link between organic production and animal nutrition, and increasing the efficiency of the energy transfer between the trophic levels (Qasim and Sankaranarayanan 1972).

The large supply of zoobenthic fauna (Silva 2013) and the sandy substrate along the coast (Almeida and Manso 2011) favour the high species richness and abundance of zoobenthivores in the IIC coastal area. Benthophagous fish are highly associated with sandy substrates (Loureiro et al. 2016). The dominance of piscivores in biomass is mainly due to large carangids, which benefit from a high supply of food in the coastal area. Carangids are visual, active predators that spend a great part of their time on the reef searching for prey (Cervigón 1972): they feed on fish and also consume benthic prey to complement their diets (Moreno-Sánchez et al. 2016).

Estuaries are dynamic ecosystems subject to notable variability of environmental conditions, and their fish assemblages show within-estuary seasonal and spatial variations, so taking into account this variability should further clarify trait patterns and drivers of estuarine fish (Henriques et al. 2017). The IIC is an important ecosystem for several species that inhabit or visit the area, mainly associated with the substrate. However, coastal areas are exposed to multiple anthropogenic pressures (Blaber and Barletta 2016) that can alter the structure and function of the fish community (Baptista et al. 2015). The anthropogenic stresses and climate changes may facilitate or inhibite the processing of detritus and consequently cause dramatic shifts in species composition, which are often long-lasting and difficult to reverse (Ooi and Chong 2011). The increase in human impacts could significantly affect the topology and functioning of the food web by altering stabilizing elements of the network and decreasing the diversity of trophic flows that ensures the resilience of the trophic structure (Lobry et al. 2008). From the point of view of ecosystem management, it is necessary to identify and understand the biotic and abiotic effects on the distribution of fish fauna as a precursor for the management and monitoring of coastal environments (Pichler et al. 2017).