reproduction, different life history modes, representing different migratory behaviours and spawning habitat selection, may be represented (i.e. bet‐hedging behaviour; the portfolio effect) (Secor 1999). In such taxa, spawning may occur in more than one habitat type (e.g. some sciaenids, engraulids and clupeids), in which some individuals or cohorts spawn in the estuary while other contingents may spawn in the coastal ocean or both (Able 2005).
Adult spawning behaviours and selection of specific spawning sites initiate the recruitment process. Many species spawn within the estuary itself, either in tidal brackish waters or in the estuary's freshwater headwaters and tributaries (e.g. salmonids, moronids, alosines). Shorelines of estuaries serve as spawning sites for many resident fishes, mostly small species that deposit demersal eggs on hard substrates, in seagrasses or in soft sediments (Able & Fahay 2010, Kraufvelin et al. 2018). Offshore spawning sites for species that spawn there must also be guided by homing behaviour, cueing on oceanographic features and genetic programming, which is especially notable in catadromous fishes (e.g. anguillid eels) but also for species that only travel modest distances to offshore spawning sites, e.g. the sciaenid Micropogonias undulatus, Baltic herring Clupea harengus membras) (Arula et al. 2014), the semi‐catadromous European pleuronectid Platichthys flesus (Amorim et al. 2016) or the centropomid Centropomus undecimalis that spawns in nearshore coastal waters of the Gulf of Mexico close to adult estuarine habitats (Adams et al. 2009). Poor selection of spawning sites or poor timing of spawning that fails to coincide with favourable conditions for egg and larvae survival (Harden‐Jones 1968, Cushing 1975, Secor 2015) has implications for recruitment success. For example, the consequences of misdirected drift of eggs and larvae (the ‘denatant drift’) related to poor selection of spawning sites can result in failure by early‐life stages to reach estuarine nurseries and favourable feeding areas and ultimately recruitment failure (Cushing 1975, 1990, Secor 2002, 2015).
3.2.1.1 Modes of reproduction
Estuaries can be diverse and complex habitats, offering a broad range of salinity, water quality, hydrographic characteristics and habitat types (Whitfield et al. 2022a). As such, it is not surprising that numerous reproductive modes have evolved and are represented by taxa using the estuary for reproduction. Species reproducing in estuaries and using them as nurseries range in size from small (e.g. fundulids, gobiids, clupeids, and engraulids) to the largest (some sharks, acipenserids) fishes (e.g. Blaber 2000, Elliott & Hemingway 2002, Able & Fahay 2010, Whitfield 2019). Habitat diversity in part explains the broad range of reproductive behaviours and habitat utilisation exhibited by adult spawners and early‐life stages (Able et al. 2022).
The modes of reproduction by fishes in estuaries have changed markedly since the Late Devonian when estuaries were dominated by live‐bearing taxa (Gess & Whitfield 2020). However, following a mass extinction event at the end of the Devonian, virtually all these estuary‐associated species were extirpated, and estuaries then became fully colonised by actinopterygians, a feature that has persisted into the Holocene. Therefore, modern estuaries are dominated by oviparous taxa, which contrasts to the earlier prevalence of viviparous and ovoviviparous species during the Late Devonian (Gess & Whitfield 2020).
Most marine bony fishes spawn pelagic eggs, but in estuaries there is increased incidence of taxa that spawn demersal eggs, presumably as an adaptation for retention in the spawning area (Pearcy and Richards 1962, Able 1978, Potts 1984, Elliott & Hemingway 2002, Able & Fahay 2010). Gobiids are classic examples of such fishes. The demersal‐spawning strategy apparently also applies to tidal and non‐tidal freshwaters as for Osmerus spp., Salmo salar and other salmonids. Many freshwater taxa with demersal eggs inhabit and reproduce in brackish waters of the northern Baltic Sea, for example percids, cyprinids and esocids. Fishes with demersal eggs include species that do not guard their eggs, those that do and those that bear live young (Balon 1990, Blaxter 1969). Estuarine fishes are represented in all these groups.
Species with pelagic eggs dominate the component of the estuarine fauna that originates from spawning in the ocean (Able and Fahay 2010, Whitfield 2019) and in some cases in estuaries (Schultz et al. 2000, Ribeiro et al. 2015). Some estuarine spawners have eggs that are slightly denser than water but can be suspended at modest current velocities, e.g. the moronid Morone saxatilis (Mansueti 1958) or at higher salinities as for the soleid Trinectes maculatus (Fahay 2007). Some variation can occur within the same or similar species. For example, the pleuronectid Platichthys flesus trachurus spawns pelagic eggs in deep waters of the Baltic Sea; however, in shallow coastal areas of the northern Baltic, and in the gulfs of Finland and Riga, there is a closely related, demersal‐spawning species Platichthys solemdali that produces smaller and heavier eggs, which sink in low salinities (5–7) and develop at the bottom of shallow banks (Solemdal 1967, Nissling et al. 2002, Florin & Höglund 2008, Momigliano et al. 2018). As in other marine teleosts in the Baltic Sea, reproductive success of the two Platichthys species depends on the ability of developing embryos or newly hatched larvae to float in the brackish, low‐saline water (Nissling et al. 2002).
Other estuarine species undergo embryonic development internally, as in viviparous and ovoviviparous species that are born live at relatively advanced stages of development (e.g. syngnathids, hippocampids, chondrichthyans). In the viviparous Zoarces viviparus (Zoarcidae) in the Baltic Sea and estuaries around the central and northern North Sea, females carry developing young for four to five months (Kristofferson & Pekkarinen 1975, Elliott & Griffiths 1986). In viviparous species, the development of embryos depends mainly on embryonic yolk reserves, with only a small maternal contribution to embryonic growth. In an upper‐estuarine gobiid from South Africa, Glossogobius callidus, precocial embryonic development results in the emergence of larvae that have already undergone flexion in the egg (Strydom & Neira 2006). This adaptation is thought to provide newly hatched larvae with stronger swimming abilities to prevent being lost by ebb‐tide flushing to the lower estuary or marine environment.
The modes of reproduction in estuarine fishes are strongly influenced by the spatial location of spawning sites. Some species, either residents or migrant spawners, spawn only in estuaries. Other species have population or stock components that spawn both coastally and within estuaries (Able & Fahay 2010). Adults of anadromous species have remarkable homing ability (philopatry) and return to their natal estuaries to spawn or undertake riverine spawning migrations (Secor 2015, Quinn 2018), which, for some taxa, is a singular event followed by death, i.e. semelparity, as in many salmonids (Figure 3.2), or for other taxa is characterised by potential for adults to return and undertake repeat spawning (i.e. iteroparity, as in acipenserids). This diversity in reproductive modes is mirrored in the variation in freshwater rearing duration (Figure 3.2).
Figure 3.2 Generalised life history of iteroparous and semelparous anadromous salmonids showing the central role of the estuary as a transition between the freshwater and ocean phases of the life cycle. a = sub‐yearling migrants, mid‐summer; b = sub‐yearling migrants, fall; c = yearling smolts; d = early spring migrants (recently hatched young or repeat migrants); e = kelts or returning veterans
(modified from Levings 2016).
In some instances, the location and timing of spawning vary amongst cohorts. Pomatomus saltatrix demonstrates cohort‐based differences of estuarine use and recruitment along the east coast of the USA (Wuenschel et al. 2012). Production of a spring‐spawned cohort occurred in both South Atlantic Bight and Middle Atlantic Bight habitats (south and north of Cape Hatteras, respectively), while a summer‐spawned cohort was limited to the Mid‐Atlantic Bight and south of Cape Hatteras. Estuarine
