have been observed to ingress to South African estuaries on a falling tide, swimming against the ebb tidal current (Harrison & Cooper 1991, Pattrick & Strydom 2014). In shallow microtidal estuaries where there is little or no vertical difference in tidal current speeds, the postflexion larvae may simply ‘ride’ the flood tide to gain entry and then move laterally into littoral areas at high tides to avoid the subsequent ebb tide (Beckley 1985, Whitfield 1989b, Strydom & Wooldridge 2005).
In addition to larvae of offshore‐spawning fishes, eggs and larvae of resident and anadromous species that spawn within the estuary or its freshwater tributaries are common. These species depend on within‐estuary processes to retain or disperse eggs and larvae, either to ensure they remain in favourable habitats within the estuary or to direct their dispersal from the estuary to offshore nurseries. For example, the atherinid Atherina breviceps attaches its fertilised eggs to the leaves of submerged aquatic macrophytes (Neira et al. 1988) and the gobiid Psammogobius knysnaensis attaches its eggs to submerged shells and rocks (Wasserman et al. 2017).
Some taxa of fishes that spawn in freshwaters exhibit amphidromy in which newly hatched larvae are rapidly advected downstream to estuaries and the ocean where they develop and grow before returning to freshwaters as advanced postflexion larvae or small juveniles, and where they reside to the adult stage (McDowall 2007). Some estuarine gobiid species (e.g. Caffrogobius gilchristi) have a synchronised larval hatching on the new moon, high tide so that the preflexion larvae are advected to sea (on the spring ebb tide), which is their primary nursery area, only to return to the estuarine environment as postflexion larvae or early juveniles (Whitfield 1989b, Strydom & Wooldridge 2005). While most amphidromous fishes reside in freshwaters of oceanic island ecosystems, others have evolved to reproduce and complete their life cycles on large island (e.g. New Zealand) and continental (e.g. Australia, Africa, South America) land masses (McDowell 2004). In such taxa, reproduction and recruitment are often associated with, and dependent on, estuaries. Gobiids (e.g. Sicyopterus japonicus, Rhinogobius spp.) galaxiids (e.g. Galaxias spp.) and the plecoglossid Plecoglossus altivelis are examples of amphidromous fishes that are estuary dependent. The dependence on estuaries by developing larvae of these fishes differs amongst taxa, but estuarine residence can constitute a substantial component of their life histories (Takahashi et al. 1998, Iida et al. 2008, Hickford & Schiel 2016, Hata & Otake 2019). In the case of galaxiids, there is evidence that, prior to entering estuaries, postlarvae may aggregate in embayments and coastal waters. These larval pools apparently sense and use cues in river plumes to facilitate pulsed entry to freshwater systems (Hickford & Schiel 2011). Engman (2017) has reported pulsed ingress of amphidromous gobiids to river‐estuaries in Puerto Rico.
3.3.1.4 Retention: estuarine features and processes
Resident species that spawn within estuaries may adopt behaviours that assure retention of eggs and larvae. For the pleuronectid Pseudopleuronectes americanus that spawns demersal eggs in western North Atlantic estuaries, pelagic, late‐stage larvae may be entrapped in small coves by vortices and slack water where they settle (Chant et al. 2000). On each flooding tide of the tidal cycle, new P. americanus larvae were delivered into the cove where weaker tidal energy favours settlement of larvae. In another example, the moronid Morone americana spawns demersal eggs above the salt front in freshwaters of tidal estuaries such as in Chesapeake Bay and its tidal tributaries where its larvae are aggregated and retained in the salt front by remaining at mid and bottom depths that favour retention (North & Houde 2001, 2006). Another example is the sparid Acanthopagrus butcheri, which spawns pelagic eggs in the Gippsland Lakes estuary, Australia (Jenkins et al. 2010, Williams et al. 2012, 2013). However, larvae of A. butcheri primarily occur near the salt fronts in the estuary's tributaries, aggregating there through passive advection in the higher‐salinity bottom waters after passive ingress into the tributaries. The larvae of the clupeid Gilchristella aestuaria are concentrated in the upper and middle reaches of permanently open South African estuaries and use tidal currents to maintain their position within these reaches (Melville‐Smith et al. 1981).
In the large and dynamic St Lawrence Estuary (Canada), vertical migrations cued by tides were demonstrated to be a mechanism utilised by clupeid Clupea harengus larvae to maintain location (Fortier & Leggett 1982, 1983). Behaviour of C. harengus larvae contrasted with that of an osmerid Mallotus villosus, whose larvae were surface‐oriented and subject to seaward advection. Vertical migrations cued to tides by larval C. harengus, and selection of mean depth near the estuary's null zone, could only partly compensate for seaward advection, but a cyclonic gyre was an additional feature and mechanism to retain the C. harengus larvae within the estuary. Additionally, vertical migrations of these larvae were correlated with movements of their zooplankton prey, a larval behaviour that served to retard seaward export (Fortier & Leggett 1983) in addition to ensuring good feeding conditions for the larvae.
The salt front is a common feature in mid‐ and high‐latitude estuaries, as is a well‐defined Estuarine Turbidity Maximum (ETM), often referred to as a maximum turbidity zone (MTZ) or entrapment zone (Schubel 1968, Sanford et al. 2001) that usually, but not always, may coincide with the freshwater–saltwater interface (Wolanski & Elliott 2015). These features and physics in the upper reaches of estuaries act to entrap or retain early‐life stages of estuary‐dependent fishes. ETM features are important nursery areas for larval fishes. Retention within an ETM zone may maintain larval fishes in habitats with relatively high concentrations of zooplankton (especially, the copepod Eurytemora spp. and mysids (Neomysis spp.) (Simenstad et al. 1994, Kimmerer et al. 1998, Roman et al. 2001, Islam et al. 2006, Suzuki et al. 2014) that serve as a prey resource for fish larvae.
The entrapment role of the ETM and effects on distribution are documented for the osmerid Osmerus mordax in the St Lawrence Estuary (Dodson et al. 1989, Laprise & Dodson 1989, Dauvin & Dodson 1990, Sirois & Dodson 2000). The daily movement of the ETM null zone and a general cyclonic circulation define a region in which O. mordax larvae remain despite mean down‐estuary velocities throughout the water column (Dodson et al. 1989). To assure retention, O. mordax larvae undertake vertical migrations, migrating closer to surface on flood tides (STST), using tidal flow to maintain their location in the ETM zone of the estuary (Laprise & Dodson 1989). Larvae of the gadid Microgadus tomcod in the same region of the St. Lawrence Estuary do not vertically migrate but, by maintaining their location deep in the water column, they are retained (Laprise & Dodson 1990).
In Chesapeake Bay and the San Francisco Bay Estuary, the anadromous moronid Morone saxatilis spawns up‐estuary of, but in proximity to, the salt front and ETM. The semi‐pelagic eggs and yolk‐sac larvae are retained by the hydrodynamic structure of the ETM region (North & Houde 2001, 2003, 2006, Bennett et al. 2002). Feeding‐stage larvae are retained in the ETM by vertical migratory behaviour, in tune with tides, and possibly by larvae tracking the vertical migrations of favoured zooplankton prey (Figure 3.11) (North & Houde 2003, 2006). Retention of M. saxatilis larvae is improved under conditions of moderately high freshwater discharge when the ETM and associated salt front are best developed (North & Houde 2001, Martino & Houde 2010) and the gravitational circulation and potential for retention are enhanced. Under exceptionally strong down‐estuary wind events and exceptionally high freshwater discharge, the ETM region can be disrupted, resulting in down‐estuary losses of M. saxatilis eggs and early‐stage larvae (North et al. 2005). Larvae and juveniles of the lateolabracid Lateolabrax japonicus find superior feeding conditions and, potentially, refuge from predation in the high‐turbidity ETM of the tidal Chikugo River, Japan (Shoji et al. 2006, Shoji & Tanaka 2007b). The salt front and ETM also may serve to retain larvae of other anadromous species within the freshwater, tidal portion of an estuary, for example the alosines Alosa aestivalis and A. pseudoharengus that spawn in freshwaters of the upper estuary and tributaries of Chesapeake Bay (Campfield & Houde 2011).