where the common juvenile fishes are consuming mainly mysids and shrimps (Mees & Jones 1997). Similarly, shrimps, mysids and swimming prawns are the dominant prey in the diet of several predatory fish species in the Scheldt Estuary. In contrast, there seem to be no bottom dwelling fish species in southern African estuaries that are exclusively hyperbenthophagic feeders. There are many species that feed on both benthic and hyperbenthic invertebrates (Blaber 1984, Marais 1984, Whitfield 1988), but none that fit neatly into the latter category only.
Using the FMFG classification outlined in this chapter, species that utilise similar food resources can be aggregated, thus providing a greater understanding of fish assemblage structure and trophic functioning in different estuaries. It also lays the foundations for examining the similarities and differences between estuarine fish assemblages in different parts of the world, which may share few or no species in common.
The review by Elliott et al. (2007) pointed to differences between the fish assemblage in different biogeographic areas that are worthy of further study. For example, the limited number of true estuarine residents in southern African estuaries (Whitfield 2019) and much higher proportion completing their life cycle in estuaries in Western Australia (Tweedley et al. 2016) are worthy of further scientific study. In terms of number of individuals, these species collectively make a substantial contribution to the ichthyofauna of many estuaries in southern Africa (Harrison 2005) and particularly so in south‐western Australia, where estuarine‐resident species can represent 93–99% of all fish (Hoeksema et al. 2009).
There is an indication that fish assemblages in adjacent marine systems in particular, and possibly freshwater habitats in certain regions, have a strong influence on the structure and diversity of estuarine communities (Albaret 1999). The repercussions of these links and origins for speciation within estuaries have not been well studied. Although estuaries in their current form are globally of recent origin, there are clear genetic differences between the marine and estuarine populations of certain species, e.g. Cnidoglanis macrocephalus (Watts & Johnson 2004). This also raises the question of the extent to which the speciation of fish can occur in estuarine environments, thereby enhancing the diversity of species within these systems. Unfortunately, the available evidence suggests that fish speciation in estuaries is relatively low (Whitfield 1994), especially when compared to marine and freshwater environments.
2.6 Fish functional groups and guild analyses
Functional groups and guilds have much in common and many authors have used the two terms more or less synonymously. However, according to Blondel (2003), processes or functions tend to be a functional group attribute whereas relationships between groups of species fall under the guild approach. Unfortunately, most ichthyologists have not distinguished clearly between functional groups and guilds in the literature, and thus we have accepted that these two terms are interchangeable for the purposes of this review. This synonymous use is epitomised by the term functional guild used by Franco et al. (2008a) below.
In recent years, the functional group or functional guild approach to analysing the structure and functioning of fish assemblages in estuaries has become widespread. Franco et al. (2008a) used this approach to analyse fish datasets from estuaries in the north‐east Atlantic, fjords in Scandinavia and lagoons in the Mediterranean. Based on functional guild analyses that focussed on the ways in which fish use estuaries, and the feeding mode and reproductive strategies of fishes in these systems, the above authors identified important characteristics of these assemblages. These were the importance of European estuaries for migration and providing nursery areas, the trophic dependence by fishes on detritivorous invertebrates associated with the hyperbenthos and infauna, and the high incidence of parental care by fish species that breed in these systems.
A study of selected Mediterranean estuarine lagoons highlighted habitat as a key driver of fish assemblage composition, amongst others. As expected, local features such as water area, habitat heterogeneity and salinity all played a role in affecting the different fish uses of these environments (Franco et al. 2008b). The lagoon fish assemblages were found to be more similar in terms of functional attributes than in taxonomic composition, thus reinforcing the shared functional aspects for fish species occupying these environments. In addition, latitude has a major influence on the variability in fish assemblage structure (França et al. 2009), particularly in terms of the feeding and reproductive characteristics of the different fish guilds at different latitudes (Franco et al. 2008b).
A similar comparative functional guild approach was used to assess the fish assemblages associated within and among nine estuarine systems along the Portuguese coast (França et al. 2009). Results indicated that small plant habitats (e.g. salt marsh and seagrass) within these estuaries support different fish assemblages and higher fish densities than the more widespread intertidal and subtidal soft habitats. Similarly, a guild analysis was used to examine the trophic structure of the Chesapeake Bay (USA) fish assemblage (Buchheister & Latour 2015). These authors determined that food structure within the 47 species was strongly influenced by habitat gradients, especially benthic to pelagic, and also to prey size. In addition, piscivorous fish species were more likely to show changes in trophic guild with ontogenetic development than benthivorous or other trophic guilds. This trend is repeated in other parts of the world (Whitfield 2019).
Other authors have used the guild approach to characterize the life‐history components of estuarine fish assemblages. Eick & Thiel (2014) determined that a large number of the 61 fish species recorded in the Elbe Estuary (Germany) belonged to marine estuarine‐opportunists and marine stragglers but that anadromous species dominated in terms of relative fish abundance. In the Amazon Estuary (Brazil), there was an increasing representation of marine guilds from the Guamá River mouth into the more marine Marajó Bay, but the trophic guilds underwent very little change along the same environmental gradient (Mourão et al. 2014).
Long‐term trends (2003–2010) in the fish assemblage of the Mondego Estuary (Portugal) were analysed from both a life‐history and trophic perspective (Nyitrai et al. 2012). In dry years, estuarine residents were the dominant group but in wet years the MEO species increased in abundance, possibly due to increased volumes of olfactory cues being carried from estuaries into the marine environment as may occur for Anguilla rostrata (Sullivan et al. 2006). When analysed by trophic groupings, the abundances of planktivorous and invertebrate consumers were higher in dry years, whilst the opposite trend occurred with omnivorous fishes. Although the balance between fish trophic groupings in South Africa changed according to zoogeographic region and estuarine typology, all estuaries were dominated by detritivores (Harrison & Whitfield 2012). More recently, Whitfield & Harrison (2020) have used the same comprehensive fish dataset to highlight the lack of redundancy in the various trophic groupings in temperate South African estuaries.
Functional fish guilds have also been used to document estuarine degradation due to anthropogenic pressures. For example, Fonseca et al. (2013) found that, in estuaries subject to chemical pollution, the abundance of marine estuarine‐opportunist species (in terms of life‐history categories) and numbers of piscivorous fishes (from a trophic perspective) were reduced. Indeed, a number of indices have been developed that are based on the functional attributes of fish guilds (e.g. fish community indices used for UK, European, South African and Australian estuaries) and this trend is likely to expand globally (e.g. Harrison & Whitfield 2004, Hallett et al. 2012).
A final example of the value of using a fish functional group approach to comparing fish assemblages in estuarine systems across different spatial and temporal scales is provided by the review of Gess & Whitfield (2020). These authors conducted a detailed analysis of the life‐history styles of fishes from a fossil southern African Devonian estuarine lake habitat and a modern estuarine lake on the subcontinent, and came to the surprising conclusion that assumed advanced forms of fish reproduction, such as ovovipary and vivipary, were more prevalent in estuaries in the Devonian than is the case in estuaries today. Ovipary is the overwhelmingly dominant breeding behaviour of modern estuary‐associated teleosts but the above fossil record suggests a high representation of probable ovoviparous fishes (e.g. coelacanths and placoderms) which are absent from Holocene estuaries. The Actinopterygii, which are dominant in modern estuaries around the world, were poorly represented in