salinities. As diadromy has been used to imply transfer from seawater to freshwater or vice versa, the established diadromous terms anadromy and catadromy have been retained by Elliott et al. (2007) and Potter et al. (2015a) for species that undertake migrations between freshwaters and the sea, for reproduction (Figure 2.10). However, these authors also adopted the terms semi‐anadromous and semi‐catadromous for those few species whose landward or seaward migrations for spawning, respectively, stop within the estuary or other transitional water body (Figure 2.10).
In addition to anadromy and catadromy, the term amphidromy has been used (e.g. McDowall 1992). Myers (1949) defines an amphidromous strategy as ‘Diadromous fishes whose migration from fresh water to the sea, or vice versa, is not for the purpose of breeding but occurs regularly at some other definite stage of the life cycle’ (Figure 2.10). Although McDowall (1997) divided the amphidromy category into freshwater and marine amphidromous fish species, he later dropped marine amphidromy because of the apparent absence of this life cycle on a global scale (McDowall 2007).
Freshwater amphidromy is employed as a life‐history style by a large number of fish species and some crustaceans (Pattillo et al. 1997). Such aggregations have been referred to as ‘tismiche’. Examples of amphidromous postlarval fishes that have been identified from these aggregations at Tortuguero, Costa Rica, have included the Dormitator maculatus and Awaous tajasicae (Gilbert & Kelso 1971, Nordlie 1981, Winemiller & Ponwith 1998). Similarly, Keith (2003) extensively discusses the amphidromous gobiid fishes of Indo‐Pacific and Caribbean areas. After being spawned in freshwaters, the embryos drift seaward for a planktonic phase before returning to freshwaters for growth and reproduction, e.g. the genera Lentipes, Sicyopterus and Stenogobius in the Indo‐Pacific and Sicydium and Awaous in the Caribbean. Amphidromous species have also been recorded in temperate climates including members of the Cottidae (11–23 species) in Japan, Eleotridae in North and South America and New Zealand and Galaxiidae (11 species), Prototroctinae (2 species) and Retropinninae (4 species) in southern Australia and New Zealand (Nordlie 2012, Augspurger et al. 2017).
Gobies and other species that spawn in estuaries, followed by a marine larval phase and then return migration to the estuarine natal habitat as postlarvae or early juveniles, have been termed estuarine migrants (Whitfield 1999, Elliott et al. 2007, Figure 2.10). Such spawning events and return migrations have been observed in South African systems, where estuarine‐spawning gobies have eggs that hatch on the high tide and are carried out to sea on the ebb tide (Whitfield 1989). At the postlarval stage, these fishes then return to the estuarine environment to complete their life cycle, e.g. Caffrogobius gilchristi. The general lack of larvae of certain estuary‐associated gobies (e.g. Psammogobius knysnaensis) from temporarily open/closed systems in South Africa indicates that the marine larval phase may be obligatory for these species (Whitfield 1999).
Amongst estuarine species (Figure 2.10) there appear to be differences in life cycles between regions. For example, in southern Africa there are relatively few estuarine species conducting their entire life cycle within an estuary, although such solely estuarine taxa (e.g. clupeid Gilchristella aestuaria) may be locally abundant (Whitfield 2019). In contrast to South Africa having relatively few fully estuarine‐resident species, in North America there are several species that are found only in estuaries (e.g. see Able & Fahay 2010), and in south‐western Australian estuaries this category is represented by discrete estuarine populations of Atherinidae and Gobiidae (e.g. see Potter & Hyndes 1999). Furthermore, none of the above has a marine larval phase and many are predominantly found in the upper estuarine reaches throughout their entire life cycle.
In the UK, only two of the 97 species recorded amongst nearly 18 500 fish in the Severn Estuary were estuarine, and their contribution to the total number of individuals was only 0.7% (Potter et al. 1997). Egg and larval retention in the macrotidal Severn is less conducive to maintaining an estuarine life cycle than in the microtidal and often lagoonal estuaries in south‐western Australia and southern Africa where resident estuarine fish species are often abundant. This point was illustrated at a broader scale by Tweedley et al. (2016), who showed that estuarine species contributed on average only 8% of the total number of species and 21% of the number of individuals in eight macrotidal systems in northern Europe. In contrast, estuarine species contributed up to 83% of the species (average = 35%) and 100% of the individuals (average = 77%) in 25 microtidal systems across the Mediterranean, Americas, southern Africa and Australia.
Some estuarine species do have a marine dispersal phase, often larval, but this dispersal can also occur during the juvenile or adult life stages. In the tropics, this dispersal often takes place in the wet season when coastal salinities are lower (Albaret et al. 2004). Hence it is possible to use the term estuarine migrants for those estuarine species that use the adjoining marine (or freshwater) areas at some stage of their life cycle. However, it is necessary to separate species that are accidentally washed out of the estuary from those that have a well‐defined strategy for moving out and colonising nearby systems. Similarly, it is of value to separate the facultative use of estuaries, possibly with an element of opportunistic behaviour, from obligate estuarine residents, e.g. the clupeid Gilchristella aestuaria is abundant in most southern African estuaries but generally absent from the sea or inflowing rivers.
The recent recognition of estuarine & marine (E&M) and estuarine & freshwater species (E&F) categories (Potter et al. 2015a) is based on certain species having distinct breeding populations in both estuaries and the adjacent environment (Figure 2.10). Example of EM species include the atherinid Atherina breviceps from southern Africa and the plotosid catfish Cnidoglanis microcephalus in southern Australia, which are each represented by independent estuarine and marine spawning populations (Neira et al. 1988, Ayvazian et al. 1994). Similarly, Glossogobius callidus has independent estuarine and freshwater spawning populations (Whitfield 2019) and is therefore an example of an EF species.
Some estuary‐associated species display a range of traits that make categorization difficult. For example, Platichthys flesus spends most of its life within UK estuaries and then spawns in the coastal zone before the larvae use selective tidal stream transport to migrate into the estuary (Elliott & Hemingway 2002). There is thus a strong case to categorise this species as a marine estuarine‐opportunist. However, some authors (e.g. McDowall 1988) include P. flesus and other pleuronectiids in the catadromous category because they are found above the head of estuaries, despite the fact that there is no obligate freshwater phase in their life cycles. It is also debatable whether P. flesus is semi‐catadromous because it does not occupy rivers as a first choice habitat at any stage in its life cycle.
Another potentially semi‐catadromous species is Trinectes maculatus, which leaves fresh waters to move downstream into more saline waters followed by a move back upstream to the freshwater‐saltwater interface to settle and grow (Dovel et al. 1969). There are other examples from the north‐eastern USA that make categorizations difficult. Some species simultaneously use estuaries and the adjacent inner continental shelf as nurseries (Able 2005, Able et al. 2006). In others, the use of estuaries varies between cohorts such as that for Scophthalmus aquosus (Neuman & Able 2003) and Pomatomus saltatrix (Taylor & Able 2006) or between contingents within the same estuary as for Morone saxatilis (Secor et al. 2001) and M. americana (Kerr et al. 2009, Kerr & Secor 2010).
There is also evidence to suggest that P. flesus can be regarded as an estuarine migrant, i.e. spends most of its life within an estuary, but migrates between the estuarine and marine environments, and even freshwater areas as discussed above. Recent otolith microchemistry evidence from estuaries
