that the life history of P. flesus is highly plastic, with some populations spawning in freshwater and others in coastal waters (estuarine and marine) of varying salinity (Daverat et al. 2012). The apparent diversity of habitats used by this species during its life cycle, including early larval ontogeny, makes it difficult to place this species in a particular category.
A similar problematic and well‐known species in terms of life history estuarine use is the latid Lates calcarifer. Although the life cycle generally is semi‐catadromous in much of Australia and Asia, there is strong evidence that its biology and life‐history differ within its distributional range, suggesting significant intraspecific variation (Grey 1987). This is particularly the case in the Fly River system of Papua New Guinea, where, for example, the coastal and estuarine populations form the majority of spawners, with little input from middle and upper Fly River fish (Blaber et al. 2009). Hence, variations across the range can make categorization of such species within a particular guild difficult.
The combination of all freshwater species into a single group (McHugh 1967) was rejected by Elliott et al. (2007) in order to separate those freshwater species which often move or migrate into estuaries (freshwater estuarine‐opportunist) from those freshwater species which incidentally occur in these systems, possibly as the result of large freshwater flows or river flooding (freshwater straggler). In all geographic areas, there are freshwater species that are found in estuaries either in small or larger numbers, but the inclusion of any species in the estuarine fauna relies on its degree of association with and penetration into the estuary. Hence, a freshwater straggler is analogous to a marine straggler, but these enter the estuary from opposite ends.
It is important to note that not all freshwater fishes extend only into the low salinity upper reaches; some species, e.g. Oreochromis mossambicus can tolerate high salinities and be found in the upper, middle and lower reaches of estuaries when conditions are suitable (Whitfield & Blaber 1979). Indeed, this species often occurs in large numbers in subtropical estuarine lakes and lagoons where it is able to construct breeding nests and spawn successfully (Ellender et al. 2008), even under hypersaline conditions (Vivier et al. 2010). In north‐eastern South America many freshwater species of several families extend well into the lower reaches of estuaries during the wet season (Blaber & Barletta 2016).
2.4.2 Feeding Mode Functional Group (FMFG)
The overall aim of the approach described here is to help understand, explain and eventually use in a management context, the functioning of estuaries and especially the use of these areas, many of which have been perturbed by human activities (Jackson et al. 2001, Warwick et al. 2018). Hence, it is particularly important to determine the food webs, predator‐prey relationships and carrying capacity of estuaries, and the effects on these as a result of anthropogenic, hydromorphological and chemical modifications. This requires an understanding of the feeding mode of fish species when they are within the estuary, irrespective of foraging mode while they are in any adjacent habitat. For example, Hostens and Mees (1999) analysed feeding guilds in an estuarine environment and found that, while certain fish species depended on the hyperbenthos (i.e. the mobile forms on or just above substratum) in the estuary, they take other prey when outside the estuary. Also, a comparison of large fish predator diets in an estuary found that dietary diversity was greater there than on the adjacent inner continental shelf (Able et al. 2017). Therefore, in the approach used here the Feeding Mode Functional Group (FMFG) relates to the diet of a species while in the estuary, acknowledging that, given the often opportunistic nature of feeding by most fishes associated with these systems (e.g. Nemerson & Able 2004), some taxa will be difficult to assign to a particular group. When it is not possible to assign a species because of a very wide set of feeding preferences, that species should be regarded as miscellaneous/opportunistic, hence the inclusion of this category in Table 2.3.
Essentially, the FMFG classification of Elliott et al. (2007) is a trophic guild system designed to allow fish species that utilise similar food resources to be aggregated. The classification draws upon other fish feeding guilds such as that developed by Elliott & Dewailly (1995) for European fishes, as well as a standardised table of fish food items provided in FishBase (Froese & Pauly 2006) and identifies seven broad categories, viz. detritivore, herbivore, omnivore, zooplanktivore, zoobenthivore, piscivore and miscellaneous/opportunist (Table 2.3, Figure 2.11).
The detritivore category includes those species that feed on decaying organic (plant or animal) matter along with the associated bacteria and fungi, sometimes referred to as benthic floc. Although most detritivores also consume benthic microalgae (microphytobenthos) (Whitfield & Blaber 1978), it is often unclear whether the detritus/benthic floc or microphytobenthos is the targeted food source. Mugilids, for example, feed on organic matter and diatoms off the substratum and have a muscular gizzard‐like stomach designed for physical grinding (using ingested sediment) of this algal and detrital material (Cardona 2016). Similarly, the term iliophagous (sediment‐feeding, sensu Bowen 1979) can be used as an indication that fish are ingesting the sediment, together with its detritus, microphytobenthos and small associated fauna, e.g. meiofauna. In view of this, and to prevent the creation of a large number of terms, the detritivore category has been extended from previous analyses (e.g. Elliott & Dewailly 1995, Potter & Hyndes 1999) to include those fish that consume microphytobenthos, detritus, sediment and small associated fauna.
The herbivore category includes fishes that consume plant material and although few, if any, estuarine‐associated species are reported to ingest only phytoplankton, the subcategory herbivore‐phytoplankton is included to accommodate phytoplanktivorous fishes should new information on fish diets become available. Hajisamae et al. (2003), for example, reported that phytoplankton dominated the diet of the clupeid Anodontostoma chacunda in the Johor Strait estuarine system in Singapore. FishBase also includes phytoplankton as a subset of the plant food category (Froese & Pauly 2006) and, in the assessment by Elliott et al. (2007), the herbivore category accommodates those fishes that have a mainly herbivorous diet comprising large plants or phytoplankton (Table 2.3).
Table 2.3 Feeding Mode Functional Group (FMFG) (modified from Elliott et al. 2007).
| Category | Definition | Examples | |
|---|---|---|---|
| Cool/warm temperate | Subtropical/tropical | ||
| Herbivore (HV) | Grazing predominantly on living macroalgal and macrophyte material or phytoplankton. | Sarpa salpa (Sparidae) | Coptodon rendalli (Cichlidae) |
| Omnivore (OV) | Feeding predominantly on filamentous algae, macrophytes, periphyton, epifauna and infauna. | Diplodus sargus (Sparidae) Rhabdosargus holubi (Sparidae) Hyporhamphus capensis (Hemiramphidae) | Chanos chanos (Chanidae) Arrhamphus sclerolepis (Hemiramphidae) Siganus vermiculatus (Siganidae) |
| Detritivore (DV) | Feeding predominantly on benthic detritus, microphytobenthos and associated meiofauna. | Mugil curema (Mugilidae) Chelon richardsonii (Mugilidae) | Mugil cephalus (Mugilidae) Oreochromis mossambicus (Cichlidae) |
| Zooplanktivore (ZP) |
Feeding predominantly on zooplankton (e.g.
|
