The salt marsh area of the Guadiana estuary, namely the Castro Marim area, was awarded Natural Reserve status in 1975. It is a very important area for biodiversity conservation at regional, national and international levels. This importance is mainly related to the presence of wetlands and coastal ecosystems (e.g. salt marsh creeks) that house a diverse fish fauna. In particular the two main creeks—Lezíria and Carrasqueira—are important sites of reproduction for many fish species of commercial value (Beja et al. 2007). More effective protection measures should be adopted as the area is highly affected by: (1) water obstruction and river fragmentation upstream, (2) local recreation activities and navigation, (3) pollution from wastewater emissions, and (4) heavy tourism (Videira et al. 2008). River flow in the Guadiana basin is controlled by numerous dams, which shift the natural flow regime, decrease natural variability (Morais et al. 2009), and induce changes in habitat availability by increasing sand deposition in saltmarshes (Garel et al. 2009). This area is also affected by invasive species (Chícharo et al. 2009) with potential negative effects on ichthyoplankton communities (Pereira et al. 2014). Moreover, the Guadiana saltmarshes are located in an area under Mediterranean climate influence and are classified as highly sensitive to global change. Studying fish larval communities in these impacted nursery areas with multidisciplinary approaches based on a detailed understanding of biota responses will be relevant to the management of local ecosystems and to larval fish ecology in general.

One of the most important processes for the environmental distribution of fish is habitat selection, defined as the non-random use of space resulting from the voluntary movements of organisms (Kramer et al. 1997). Habitat selection by fish is a response to a multiple factors, including abiotic conditions (temperature, salinity, water velocity, climatic conditions) and biotic conditions (food resources, predation risk and competition). Because the responses of fish larvae to these factors change during ontogenic development, their relative importance for habitat selection is also dynamic (Craig and Crowder 2000). In particular, the survival and distribution of early life stages of fish is mainly dependent on the local hydrological conditions associated with transport processes, climatic variability, prey and predator densities, and adult spawning (Franco-Gordo et al. 2002).

Tidal salt marsh habitats are vegetated wetlands periodically flooded by tides and renowned for their high productivity (Kneib 1997). They include irregularly flooded marsh areas, intertidal marsh creeks, sub-tidal marsh creeks and the bay marsh fringe (Routree and Able 1996). Salt marshes are recognized as important nurseries and feeding grounds for many fish species (Kneib 1997). Despite the variability of the abiotic factors in marsh habitats, most estuarine and coastal fishes can tolerate a wide range of physical conditions. Abiotic factors are likely to influence the habitat selection of fish in more subtle ways via effects on energy demands rather than physiological tolerances (Craig and Crowder 2000). In contrast, biotic factors, particularly food resources and predation risk, have a direct impact on the selection of these habitats by fishes. Also, there is evidence that salt marshes may be more profitable foraging areas than other habitats as they are a source of carbon, detritus, and primary/secondary production (Hettler 1989). Shallow and turbid waters associated with marshes also provide protection from predators during the critical life stages of fishes (Baltz et al. 1993).

The present study is the first to investigate the larval fish community in the Guadiana salt marsh area and the influence of environmental parameters on its occurrence. Veiga et al. (2006) analysed the Castro Marim salt marsh area in order to characterize communities of adult and juvenile fish. Other studies focusing on the population dynamics of fish larvae were only conducted in the main channel of the Guadiana estuary (Chícharo et al. 2000, Faria et al. 2006). This is a crucial first step for determining the sustainable carrying capacity of these ecosystems. We investigated spatial and temporal variations of the larval fish community in a salt marsh sub-tidal creek of the estuary and the effects of the environmental factors on larval fish occurrence using a decision tree (DT) model.

The present study is the first to investigate the larval fish community in the Guadiana salt marsh area and the influence of environmental parameters on its occurrence through a DT model. The model discloses the EF3 rule, i.e. eggs, food availability, flooding events and river flow as the main players in determining favourable conditions for larval fish occurrence in the lower marshes of the Guadiana estuary.

Despite the higher abundance of the three species (D. sargus, S. pilchardus and P. microps), we found lower diversity and abundance of the overall fish larvae community compared with previous surveys carried out in adjacent areas of the Guadiana estuary (Chícharo et al. 2000 and Faria et al. 2006). However, in these studies, captures exclusively near our sampling area had diversity and abundance values much closer to our observations. Chícharo et al. (2000) reported for the sampling point “Carrasqueira creek” (a marsh creek a few meters from our sampling site) the presence of 5 different species and an abundance peak of 143 larvae/100 m³ in March 2000. Values observed by Faria et al. (2006) for the lower estuary were slightly higher, with 19 different species and a maximum abundance of 285 larvae/100 m³ in June 2002. Diversity in the salt marsh creek agreed well with descriptions in the literature for other European marshes, where few species were found. Koutsogiannopoulou and Wilson (2007) observed 10 species over two years in Dublin Bay, Ireland, while Green et al. (2009) reported 14 species over one year on the Essex coastline, United Kingdom. An exception is Laffaille et al. (2000), who recorded 31 species in Mont Saint Michel Bay, France, but over a longer sampling period (three years). Abundance comparisons are harder, because abundance is expressed in different units and total abundances reflect values of different sampling designs and different sampling periods using dissimilar sampling gears. Yet, looking at total abundances normalized for a one-year period, capture values are significantly higher than ours. Koutsogiannopoulou and Wilson (2007) captured 3275 individuals, Green et al. (2009) 2821 individuals and Laffaille et al. (2000) 83333 individuals. These surveys exclusively aimed to capture juveniles and not larvae, as in the present study. Only Green et al. (2009) attempted capture of larvae and juveniles simultaneously, but they were present in short seasonal peaks.

Among ER, MJ, MS and MA, all the 9 identified species were reported in the lower estuary zones of Guadiana by Chícharo et al. (2000) and Faria et al. (2006). In addition, larval fish community composition in the salt marsh creek is comparable that reported by Veiga et al. (2006). Only three of the identified larvae species (S. vulgaris, B. luteum and Syngnathus abaster) were not reported by Veiga et al. (2006), suggesting that the salt marsh is mainly used during the early life cycle of these species. The larval fish assemblage composition differed from other European marshes in France (Laffaille et al. 2000), Ireland (Koutsogiannopoulou and Wilson 2007), and the United Kingdom (Green et al. 2009). However, the ER Pomatoschistus sp. was reported among the two most abundant species in all studies.

The occurrence of the MJ D. sargus as the dominant species is not surprising, because Sparidae is one of the most common families in estuaries and salt marsh assemblages worldwide (e.g. Gonçalves and Beldade 2000, Hindell and Jenkins 2004). Diplodus sargus spawn in coastal areas and after that eggs and larvae disperse at the scale of 100-200 km forced by marine currents, reaching nursery habitats such as estuaries (Franco et al. 2012). Also, the occurrence of the larvae of the MS S. pilchardus in the lower estuary is more related to oceanic circulation and water advection effects than to active habitat selection, i.e. the adults spawn in coastal zones and subsequently the eggs and larvae are transported from coastal into estuary zones by tidal current forces (Faria et al. 2006). The high occurrence of the ER P. microps is also not surprising, because it depends on estuarine habitats for its entire lifecycle. Furthermore, it is one of the most common species in estuaries and lagoons and along the shores in Europe (Laffaille et al. 2000, Koutsogiannopoulou and Wilson 2007, Green et al. 2009). While larval dispersal of ERs depends exclusively on the biology of the species (e.g. hatching date, pelagic larval duration and larval swimming behaviour), larval dispersal of MJs and MSs also depends on the local oceanographic features (e.g. directional currents).

There was no evidence of higher larvae occurrence inside the salt marsh creek than outside. A trend of higher abundances near the mouth of the creek than in more interior zones was evident (Fig. 2B). Larval fish did not exhibit a preference for vegetated habitats over sand bottoms in laboratory experiments (Gonçalves et al. 2012), a finding which corroborates these field observations. The relative contribution of the creek to the local population was estimated as 33% (number of larvae inside the creek [stations 1 to 5] / total number of larvae), slightly lower than the 51% estimated by Chícharo et al. (2000).

Several studies have reported that intertidal creeks provide important habitat for fishes (Hampel et al. 2003, Jin et al. 2007), but juveniles and adults generally dominate catches. Fishes at a more developed stage, with a higher swimming capacity, can more easily access tidal creeks and marsh habitats and remain there. It is only near settlement that fish larvae of many species become capable of swimming faster than the mean ambient current (Leis 2006). Particularly in the lower Guadiana estuary, strong currents are produced on the ebb (Garel et al. 2009), the period of our samplings. We assume that we failed to capture more larvae because the tidal current may have carried part of them out of the creek. We also accept that capture efforts have targeted smaller fishes than the ones that more regularly use salt marsh areas. Further surveys on this particular area should tests other fishing gears, such as seine nets, fixed traps and water pumps and target advanced larval phases or juvenile fish.

The ANOSIM indicated that winter and summer groups are more similar to each other, while autumn and spring groups are more dissimilar in terms of abundance and species composition. The similarity between summer and winter groups was mainly explained by species abundance (27.4 and 36.5 larvae/100 m³) rather than species composition (5 species observed in summer, 2 in winter, and only D. sargus was common to the two groups), while abundance (15.4 and 285.7 larvae/100 m³) and species composition (2 species observed in autumn and 9 in spring) both contributed to the differences between the autumn and spring groups.

The short period of our study (one year) limits conclusions about the seasonal changes on the community structure in the Guadiana estuary, but our results are correlated with the in situ observations performed by others in different years (Chícharo et al. 2000, Faria et al. 2006, Veiga et al. 2006). Overall, biodiversity in the salt marsh creek was relatively constant throughout the year, being dominated by the MJ D. sargus in spring, the MS S. pilchardus in autumn and winter, and the ER P. microps all the year. Peaks of total abundance were recorded during spring months, as observed by Chícharo et al. (2000), Faria et al. (2006) and Veiga et al. (2006). Abundance maxima in marsh areas further north in Europe were recorded later during the early autumn (Laffaille et al. 2000, Koutsogiannopoulou and Wilson 2007, Green et al. 2009). The highest abundance levels are coincident with periods of high zooplankton productivity along the Portuguese coast (Chícharo et al. 2006). The rapid decline observed in July seems to be more related to the spawning periods of fish species coinciding with an increase in predatory jellyfish abundance (Muha et al. 2012) rather than to the lack of zooplankton, which is still abundant during this month.

Periods of high fish eggs abundance typically overlap with peaks of larvae occurrence, especially for fishes with short hatching periods. However, the DT revealed that the peaks of fish eggs by themselves do not guarantee a high occurrence of fish larvae. This may be related to other factors, such as temperature and predation, which are the main causes of mortality in fish eggs (Riveiro et al. 2004).

The DT model also indicates that an increased density of potential prey might compensate for the low density of fish eggs. Among all potential prey, ciliates were the only ones that showed a positive correlation with the density of fish larvae. In general, marine fish larvae feed on copepods, but, they can consume other prey such as cladocerans, molluscs and ciliates (Chícharo et al. 2012). Looking at feeding preferences of the marine dominant species, only the MS S. pilchardus has been reported to feed on ciliates. According to Morote et al. (2010), S. pilchardus larvae<10 mm standard length feed mainly on the tintinnid Codonellopsis sp. and copepod nauplii with the ciliate Codonellopsis sp., accounting for 48% of their diet. Tintinnids were also the major prey items in the gut contents of S. pilchardus larvae collected off southern Portugal (Chícharo 1998). Sanchez-Velasco and Norbis (1997) reported that copepod nauplii, calanoid copepod eggs and cladoceran species were the dominant prey of the MJ D. sargus ranging between 2 to 7 mm total lengths. The ER P. microps is described as an opportunistic feeder that selects prey that are in sufficient quantities and are easily available within ecosystems (Antholz et al. 1991). Smaller P. microps analysed for gut contents were 35 mm average standard length (Mendonça et al. 2009); among the digested species, amphipods, harpacticoids, polychaetes, chironomids (larvae and pupae), and bivalve syphons. Based on the diet of these species, it is unlikely that D. sargus and P. microps contributed to the positive correlation between ciliates and larvae occurrence in the Guadiana estuary. The MS S. pilchardus seems to be the major beneficiary of the high abundances of ciliates observed in the study area.

The DT model explores associations between the density of fish larvae and the abiotic factors precipitation and river flow. Our findings corroborate the results of other studies, in which a positive correlation of rainfall and abundance of fishes was evident (Salen-Picard et al. 2002 and Whitfield et al. 2005). These associations might be due to other variables, including primary or secondary production that results from terrestrial nutrient addition (Meynecke et al. 2006). Salen-Pilcard et al. (2002) suggested that an increase in prey density after flooding events favoured different stages of the Solea solea life cycle, enhancing its population size for several years. Also, the model suggests that months with low precipitation combined with high-flow periods reduce the number of larval fish. Eggs and larvae appear to be flushed out as a result of the excessive amount of river flow that will potentially contribute to the loss of larvae from the estuary into the marine environment. Moderate freshwater discharge increases the overall primary production of estuarine systems that are vital for the nursery function of the lower reaches of estuaries (Dolbeth et al. 2008) and promotes the emergence of chemical cues used by larvae for orientation in estuarine zones (Costa et al. 2007). However, excessive supply can effectively alter the larval composition of estuarine resident and estuarine dependent marine fish species (Strydom et al. 2002). In terms of environmental factors that influence the occurrence of fish larvae, flooding events and river flow are particularly important, suggesting that the recruitment success and maintenance of fish diversity will require good water management that takes into account natural hydrography. It is important to maintain sufficient water flow during dry seasons to ensure the input of nutrients to the lower zones of the estuary, but a sudden increase in river flow must be avoided to prevent the advection of fish larvae out of the system.

Overall, this work indicated that, although the most common species use this habitat during their early stages of development, the density of fish larvae in the salt marsh zone of the Guadiana estuary is lower than in adjacent areas. This study resolves the influence of environmental factors on fish larvae density and the temporal and spatial occurrence of larvae in salt marsh habitats. The results highlight the value of restoration and conservation of salt marsh habitats, and emphasize the importance of flow-rate control in dry seasons that should be performed in parallel with fishery activities in the Guadiana region.