include members of the Carangidae, Elopidae, Gadidae, Pomatomidae and Sciaenidae (Table 2.3). While piscivores feed primarily on fishes, they will also consume other prey items. In the estuarine Lake St Lucia, for example, swimming prawns (Penaeidae) form a substantial component of the diet of both Argyrosomus japonicus and Elops machnata at certain times (Whitfield & Blaber 1978). There is also a trend for piscivory within this category to increase with increasing body size (Juanes et al. 2001, Nemerson & Able 2003). In recent years the role of small piscivorous predators has become more obvious, including in Australia (Baker & Sheaves 2005) and North America (Able et al. 2007, Musumeci et al. 2014).
In addition to finfish, FishBase includes cephalopods (squid and cuttlefish) within the nekton food category (Froese & Pauly 2006). FishBase also contains a category ‘other’ which includes food items such as reptiles, birds and mammals that are typically consumed by apex predators such as sharks. Apex fish predators are generally absent from estuaries although Carcharhinus leucas enters large estuarine bays, lakes and rivers. In South Africa, however, the individuals found in estuaries are usually juveniles whose diet is dominated by fishes (D'Aubrey 1971).
Those species that fit into the ‘other’ category should be relatively consistent in their prey/food choice in a range of estuaries, but this is not always the case (Able et al. 2017). Given the nature of feeding by many estuary‐associated species, however, there is a need for an opportunist category, which is separate from the omnivore category and used for those species that are opportunist but not necessarily omnivorous. Examples of estuarine omnivores include Diplodus capensis from southern Africa (Coetzee 1986), Acanthopagrus butcheri in south‐western Australian estuaries (Sarre et al. 2000, Chuwen et al. 2007) and Lagodon rhomboides from North American east coast estuaries (Stoner 1980, Darcy 1985). Both species ingest a wide spectrum of animal and plant food items, especially aquatic macrophytes, filamentous algae and the associated epifauna and periphyton. Although these species are opportunistic in that the proportions of major dietary categories consumed varies greatly among estuaries, this opportunism should not influence the primary FMFG categorization which is driven at a higher level of resolution.
An opportunist species is likely to feed on very different prey/food in different estuaries depending on food availability, whereas omnivores will probably have a dietary mix of plant and animal material in different areas. The term opportunistic has been used by Elliott et al. (2007) to indicate both the feeding behaviour and food preferences of fish, rather than food preferences alone. It is increasingly observed that many estuarine fishes take almost any suitably sized prey that they encounter and should thus be regarded as opportunists. For example, the flounder P. flesus may take infaunal benthos, epifaunal shrimps or zooplankton, depending on prey availability and size of the individual fish (Costa & Elliott 1991).
Thus, while the EUFG reflects the migratory behaviour and physiological tolerances of fishes, the FMFG reflects the feeding behaviour and their body structure as an adaptation to feeding on particular prey, i.e. their ecotrophomorphology or ecomorphology (Wootton 1990). For example, streamlined clupeids inhabiting the water column have relatively large terminal mouths and feed on zooplankton and therefore have a very different ecomorphology to that of demersal sparids, which have subterminal mouths and prey on hyperbenthic crustaceans using protrusion, suction and pivot foraging methods.
The southern African sparid R. holubi is an omnivore that always ingests both plants and animals where both of those food sources are available (Blaber 1974). Where aquatic macrophytes are unavailable, it will actively search for alternative plant material, and then, for example, feed on filamentous algae growing on the stems of Phragmites reeds as in the Mhlanga Estuary (Whitfield 1980b). In the Swartvlei system, the diet of R. holubi was dominated by both pondweed Stuckenia and associated epifauna, but when the macrophytes disappeared from the littoral zone it consumed the filamentous algal mats that replaced the Stuckenia and the epifauna associated with the sediments (Whitfield 1984). When the algal mats also disappeared, this sparid became restricted to a carnivorous diet (epifauna associated with the sediments) and its body condition, as reflected in the length/weight relationship, declined. Thus, while juvenile R. holubi may survive on a carnivorous diet, they require an omnivorous diet to thrive and select both plant and animal material when available.
The FMFG classification of Elliott et al. (2007) provides a method of grouping fishes according to common broad dietary categories. However, it is becoming increasingly evident that many estuarine‐associated fishes are opportunistic in that they switch their diet beyond their ‘normal’ spectrum, as and when opportunities arise. Such opportunistic feeding behaviour sometimes makes it difficult to assign a species to a particular feeding category. While most estuarine‐associated fishes are to a certain extent opportunistic, Elliott et al. (2007) proposed that a FMFG classification should be based on the normal or preferred diet of a species. An opportunistic category was created by the above authors, however, to accommodate situations where a species cannot be placed in one of the more conventional FMFG categories. This opportunistic category should not be confused with the omnivore category, which includes those species with a regular, varied diet of both plant and animal material.
The diets of most estuary‐associated fish species also undergo ontogenetic shifts. During their larval life most fishes are planktivorous but switch to juvenile diets at lengths that vary according to the taxa (Blaber & Whitfield 1977, Strydom et al. 2014). Some species such as Mugil cephalus retain the same feeding mode through the juvenile and/or adult life stages (Blaber 1976). The types of food consumed by certain other species, however, change markedly with growth. For example, Gadus morhua is a planktonic feeder as a larva, benthophagous as a 0+ juvenile, then consumes hyperbenthic crustaceans such as mysids and pericarid shrimp, before becoming a piscivore when mature (Costa & Elliott 1991). Thus, this group of species will move from one feeding category to another during their lives. Where possible, ontogenetic changes in diet should be accounted for by allocating the diets of the various size classes to the appropriate FMFG category. In practice, however, this may be difficult and an alternative may be to allocate a FMFG category to a particular species based on the diet of the predominant size classes or life cycle stage (e.g. mostly juveniles) present within an estuary.
While it is recognised that estuaries form an important function for a number of fish species, many of the species found in estuaries are marine stragglers and not dependent on these environments. Furthermore, many diadromous species such as all lampreys and certain salmonids and anguillid eels do not feed while passing through the estuary, especially the mature individuals on their final spawning migrations (Froese & Pauly 2006). It is recommended by Elliott et al. (2007) that a feeding guild analysis should be restricted to the dominant Estuarine Use Functional Groups (EUFG) occupying a particular system, i.e. those fish species that are common and forage mainly within the estuary. For example, marine stragglers, catadromous and anadromous groups should not be included in a FMFG analysis.
2.4.3 Reproductive Mode Functional Group (RMFG)
The use of reproductive guilds for fishes in general was first proposed by Balon (1975). Elliott et al. (2007) have adopted an independent approach that relates specifically to estuarine fishes. In particular, the definition of a species' mode of reproduction as a guild is related to a fundamental understanding of both its reproductive behaviour and the functioning of estuaries. In the case of estuarine spawners, it is aimed at determining firstly, the importance of the estuary as a breeding area and, secondly, the strategies employed by these species in response to environmental conditions, especially the hydrodynamic regime of transitional waters (Potter et al. 1993). In particular, the strategies employed by certain estuarine spawners to retain their young within an estuary are especially important (Tweedley et al. 2016). Where the flushing rate through an estuary is too high, and thus the hydrographic residence or water retention time too short, the successful development of pelagic eggs or larvae may be compromised (Elliott & Dewailly 1995, Nordlie 2000, Strydom et
