& Elliott 2004).
The assignment of functional reproductive guilds facilitate comparisons between the reproductive biology of estuarine‐resident species, most of which are hypothesised to require egg and larval retention strategies within estuaries, and MEO species in which egg and larval development occurs in the marine environment (Able & Fahay 2010). An analysis of reproductive guilds is especially important when considering the mechanisms for successful reproduction within an estuary and an analysis of strategies for maintaining the young stages within the estuary, thus preventing washout (Melville‐Smith et al. 1981, Houde et al. 2022). However, if it is selectively advantageous for the offspring of some estuarine spawners to become widely dispersed in the sea, this would be facilitated by the release of pelagic eggs or larvae on the spring high tide (Whitfield 1989). Elevated river flow would perform a similar function for spawning amphidromous fish species trying to maximize marine dispersal for the eggs and larvae. Similarly, the underlying basis for the apparent necessity for most MEO and MED species to spawn at sea rather than in estuaries is an area of research that has yet to be properly explored.
The fishes found in estuaries can be allocated to one of three functional reproductive groups: viviparous, ovoviviparous and oviparous, although it is valuable to subdivide the latter based on the reproductive strategy of the species (Table 2.4). Analysis of these guilds should be restricted to the dominant species that spawn in estuaries and MEO/MED species that spawn at sea. Most estuary‐associated fish species are seaborne spawners and have notable strategies to ensure estuarine recruitment on both a spatial and temporal basis. A few southern African MEO/MED species (e.g. the sparid Acanthopagrus vagus) have been recorded spawning in the mouth region of marine dominated estuaries (Garratt 1993), but generally the marine taxa have extended spawning periods (Whitfield 2019) and release their eggs in the sea relatively close to the coast/estuaries (Wallace 1975). However, other species may spawn offshore and occur as larvae at great distances from an estuary (Houde et al. 2022).
Table 2.4 Reproductive Mode Functional Group (RMFG) (modified from Elliott et al. 2007).
| Category | Definition | Examples | ||
|---|---|---|---|---|
| Cool/warm temperate | Subtropical/tropical | |||
| Viviparous (V) | Species in which the female produces live progeny | Zoarces viviparus (Zoarcidae) Clinus superciliosus (Clinidae) | Gambusia affinis (Poeciliidae) Carcharhinus leucas (Carcharhinidae) | |
| Ovoviviparous (W) | Species producing an egg case in which the young develop | Squalus acanthias (Squalidae) | Myliobatis aquila (Myliobatidae) | |
| Oviparous (O) | Producing eggs that are liberated into the surrounding waters. The species in this category can be subdivided into the following five guilds: | |||
| Op | Species producing pelagic eggs that remain suspended in the water column | Platichthys flesus (Pleuronectidae) Pomatomus saltatrix (Pomatomidae) | Mugil cephalus (Mugilidae) Rhabdosargus sarba (Sparidae) | |
| Ob | Species that produce eggs which settle on the substratum | Osmerus eperlanus (Osmeridae) Alosa sapidissima (Clupeidae) | Nematalosa erebi (Clupeidae) Atherinomorus duodecimalis (Atherinidae) | |
| Ov | Species that produce adhesive eggs that are attached to substrata and/or vegetation | Atherina breviceps (Atherinidae) Menidia beryllina (Atherinopsidae) | Croilia mossambica (Gobiidae) Atherinops affinis (Atherinopsidae) | |
| Og | Species in which one or the other parent guards their eggs externally, e.g. in a nest | Gasterosteus aculeatus (Gasterosteidae) Cnidoglanis macrocephalus (Plotosidae) | Oreochromis mossambicus (Cichlidae) Periophthalmus argentilineatus (Gobiidae) | |
| Os | Species that shed their eggs and then protect them for a period in a part of their body, e.g. brood pouch or mouth, where they develop into a post‐larva or juvenile and then are released into the surrounding waters | Galeichthys feliceps (Ariidae) Ostorhinchus rueppellii (Apogonidae) | Hippichthys heptagonus (Syngnathidae) Hippichthys spicifer (Syngnathidae) | |
The classical strategy of retaining a brood in a location with the highest level of protection is reflected by viviparous species. Examples of this strategy in estuaries range from species such as the viviparous Zoarces viviparous and Mustelus canis in temperate waters to Carcharhinus leucas and Pristis zijsron in tropical and subtropical areas (Table 2.4). In a previous use of this guild, brooding species such as the syngnathids (pipefishes and seahorses) in which the male has a ventral brood chamber, were also grouped with viviparous or ovoviviparous (Elliott & Dewailly 1995). However, for consistency, and given that this reproductive mode represents a similar strategy to other brooders, such as mouth‐brooding cichlids, apogonids and ariids, the brooders of fertilised eggs and young have been grouped under the guild subcategory Os (Table 2.4).
In many estuaries, the dominant group of spawners are oviparous MEO/MED which breed at sea, followed by an onshore migration of postflexion larvae and early juveniles (Wallace & van der Elst 1975, Boehlert & Mundy 1988, Able & Fahay 2010). Most of this group are serial spawners that release large numbers of pelagic eggs, which are fertilised within the water column during mass spawning aggregations (Wallace 1975). It is becoming obvious that a number of species that spawn at sea, such as the anguillid eels, are capable of swimming across the continental shelf into estuaries (Wuenschel & Able 2008). However, the immigration of larvae and postlarvae into large, well‐flushed estuaries of the northern hemisphere takes place mainly by using passive and/or selective tidal stream transport (STST) for entry to and retention within these systems (Weinstein et al. 1980, Fortier & Leggett 1982). For example, the pelagic spawner Platichthys flesus spawns on the coast, which is followed by STST that enables the larvae and early juveniles to migrate into their estuarine nursery area. A similar pattern is evident for Paralichthys dentatus in estuaries along the east coast of the North America (Keefe & Able 1993, Hare et al. 2005). Even in the microtidal estuaries of South Africa, southern Australia and New Zealand, where the two‐layered circulation pattern is less pronounced or absent during much of the year, the larvae and juveniles of some marine species enter these systems on the flood tide and are retained by rapidly settling along the banks or on the bottom where water movements are reduced (Beckley 1985, Roper 1986, Neira & Potter 1994).
As indicated in Table 2.4 and Houde et al. (2022), several reproductive strategies ensure that eggs and young are retained within an estuary. For example, egg and brood protection by mouth, pouches
