fish eggs/larvae).
Figure 2.11 Examples of fish species from each Feeding Mode Functional Group (FMFG, see Table 2.4 for more details) from southern African estuaries (after Whitfield 2019).
There appear to be very few examples of exclusive herbivory among the fishes found in estuaries, possibly due to their lack of a cellulase with which to digest the cell walls of plants (Kapoor et al. 1975). Hence, the designation of the herbivorous FMFG may be difficult, especially as most herbivorous fishes in estuaries also consume animal material and often small invertebrates associated with macrophytic plants and filamentous algae. Indeed, in most cases it would be almost impossible for a fish foraging on plant material not to also ingest small invertebrates associated with the vegetation. In some cases, the fish may be targeting the periphyton and epifauna associated with the plants, rather than the leaf blades on which these biota exist. An example of such a fish in estuaries is Diplodus capensis from southern Africa, which consumes wide spectrum of biota, especially aquatic macrophytes, filamentous algae and the associated epifaunal and periphyton (Coetzee 1986). Another species, Pelates octolineatus, consumes predominantly small crustaceans when <50 mm total length, but has an almost exclusively herbivorous diet at larger body sizes (Poh et al. 2018).
In some cases, there may be a temporal separation of food, e.g. Hyporhamphus capensis consumes mainly zooplankton in late winter and macrophytic plants in late summer (Coetzee 1981). At a finer scale, the congeneric Hyporhamphus melanochir consumes seagrass during the day, but benthic‐dwelling amphipods at night following their nocturnal vertical migration (Robertson & Klumpp 1983). There is the need for an energetic assessment of this type of feeding in that, although a large amount of plant material is ingested by these species, the relatively small amount of animal material may be more important to the fish because of the higher calorific value and assimilation rate. For example, although H. melanochir consumed three times more seagrass than crustaceans, these invertebrates were considered by Klumpp & Nichols (1983) to be an essential source of protein and be at least as important as seagrass in providing energy. While there may not be many true herbivores in estuaries, there are some, such as certain Hemiramphidae, Sparidae, Siganidae and Kyphosidae, in which their diet is overwhelmingly dominated by plant material (Nordlie & Kelso 1975). It is therefore proposed that the herbivore category be retained for further testing, especially as a means of comparing fishes in different systems.
Zooplanktivorous fishes are those that typically feed on small crustaceans in the water column, for example the diet of Anchoa mitchilli comprises mainly zooplankton but may also include fish eggs and larvae (Froese & Pauly 2006). Typical zooplanktivores found in estuaries include members of the Clupeidae and Engraulidae (Table 2.3). Zooplankton is also a major component of the diet of the larvae of marine fish species (Llopiz 2013) and most, if not all estuary‐associated fish species, are planktivorous during their larval life (Islam et al. 2006), but change to juvenile/adult diets with growth (Whitfield 1985, Strydom et al. 2014). The larvae of many anadromous, semi‐anadromous and estuarine‐resident fishes also prey predominantly on zooplankton (Costalago et al. 2016) and are therefore part of the zooplanktivorous guild at this stage of their life cycle. The majority of marine fishes that utilise estuaries, however, enter these systems as postlarvae and are therefore already at a stage where they have or are about to switch to their juvenile/adult diet (Strydom et al. 2014). In addition, there are examples of pelagic fish taxa such as Hilsa, Alosa and Brevoortia, which consume zooplankton during both the larval and juvenile stages (Blaber 1979). Juveniles and adults of several species of atherinid in south‐western Australia have been shown to consume large quantities of planktonic crustaceans (Prince et al. 1982).
Many bottom‐dwelling fishes feed on organisms associated with the substratum (zoobenthos), hence the terms zoobenthivore and benthophagous. Zoobenthos includes animals that live in the sediment (infauna), on the sediment (epifauna), or immediately above the sediment (hyperbenthos) and the term benthophagous can also include flora associated with the benthos. The term hyperbenthophagous fish species has also been identified, i.e. they are adapted to feeding on the hyperbenthos, i.e. those mobile organisms living just above the substratum (Sibert 1981). Thus, while hyperbenthophagous species could be regarded as a subgroup of the zoobenthos feeders, their prey sources will require different methods of capture and have thus been kept separate by Elliott et al. (2007). In general, many benthic/demersal fish species feed on fauna that are associated with the sediment. Since much of the zoobenthos (especially the epifauna) is sometimes in, sometimes on and sometimes above the sediment (e.g. many amphipods) according to the time of day, tidal regime, current flow rates, etc. (Read & Whitfield 1989, Rose et al. 2020), it is often difficult in practice to separate these two compartments. Furthermore, for most species, detailed feeding behaviour is unknown and likely to remain so in turbid estuaries.
While it is difficult to separate the zoobenthic components, the three subdivisions: zoobenthivore‐hyperbenthos, zoobenthivore‐epifauna, and zoobenthivore‐infauna created by Elliott et al. (2007) do accommodate any new information on fish diets that become available. For example, in recent decades, hyperbenthophagous fish species have been identified since their diet and method of feeding are centred on those mobile organisms living just above the estuary bottom. In addition, certain fish species have developed specialised feeding mechanisms to exploit infauna. For example, Pomadasys commersonnii and Lithognathus lithognathus both have protrusible mouthparts and employ a gill chamber pump action to extract mud prawns from their burrows (van der Elst 1988). Some fish species also deliberately target certain infauna or parts of their prey, e.g. members of the Gerridae in the Kosi Estuary, South Africa (Cyrus & Blaber 1982) and Soleidae in the Humber Estuary, UK (Marshall 1995) feed by cropping bivalve siphons. The possible impact of siphon nipping by fishes on bivalves has been investigated by Coen & Heck (1991) and shown to significantly reduce growth of the mollusc.
The piscivore category includes large carnivorous species that feed mainly on other fishes. Although estuaries provide shelter from major fish predators, several estuary‐associated fishes
