influence dispersal outcomes (Codling et al. 2004). Near Georges Bank, in the Western North Atlantic, Werner et al. (1993) noted that swimming by fish larvae of only 0.3–1.0 cm s−1 could substantially enhance shoalward displacement. In a numerical model of the Florida coast, simulated larvae, swimming at only 1 cm s−1, achieved settlement rates on nearshore reefs 36–300% greater than passive larvae (Porch 1998). In a model of an Australian coral reef, a swimming speed of 10 cm s−1 by simulated, settlement‐stage larvae resulted in modelled recruitment levels higher than was possible if larvae were drifting passively (Wolanski and Kingsford 2014).
In a further example showing the likely importance of directional swimming, the larval recruitment process was modelled for the Ria Formosa, a coastal lagoon in Portugal with extensive seagrass beds (Teodosio et al. 2016, Baptista et al. 2020). Tidal currents in the lagoon created an apparent odour plume extending several kilometres offshore and alongshore in coastal waters. Several fish species, including the clupeid Sardina pilchardus, the engraulid Engraulis encrasicolus, and the sparid Diplodus sargus spawn near the coast and close to Ria Formosa. They entered the lagoon as postflexion larvae by swimming towards the lagoon and may use the estuary plume to orient their swimming through olfactory and taste senses. The modelled results of sensory response to odour and directional swimming suggested that, neglecting mortality, typically 30–50% of the directionally swimming postflexion larvae of S. pilchardus and E. encrasicolus could recruit to Ria Formosa, a level an order‐of‐magnitude higher than the fraction predicted without directional swimming.
3.3.1.3 Near‐ and within‐estuary transport processes
Once larvae and juveniles of offshore‐spawning, estuary‐associated fishes reach inshore areas, these habitats, including surf zones, may serve as holding areas for postflexion larvae that show varying degrees of dependence on estuaries as nurseries (Whitfield 1989a,b, Strydom 2003, Strydom & d’Hotman 2005). In South Africa, larvae have been shown to select micro‐habitat depressions with lower turbulence in surf‐zone waters (Watt‐Pringle & Strydom 2003) until there are opportunities for ingress via favourable alongshore flows and tidal currents into open estuaries or marine overwash events into temporary open–closed estuaries (Figure 3.8) (Cowley et al. 2001). Onshore winds were cited as the most important factor aiding ingress of larval fishes from the surf zone into an Argentine lagoon/estuary (Bruno et al. 2014).
In offshore‐spawning taxa, after offshore drift and dispersal, larvae reaching nearshore areas must adopt appropriate behaviours to ensure ingress to an estuary and, having achieved ingress, adopt behaviours that ensure up‐estuary transport and retention. Near the estuary mouth, it is evident that a combination of behaviours in response to estuary flows and chemical signals is utilised to gain entry. Sensory cues near mouths of estuaries may include those from light (phototaxis), gravity (geotaxis), sound (phonotaxis), currents (rheotaxis), magnetic fields (magnetotaxis) and several scalar factors (e.g. salinity, temperature, turbidity, chemical gradients) to direct larvae to the estuary mouth (e.g. Teodosio et al. 2016). It is quite certain that late‐stage postflexion larvae of many species have the ability to swim to an estuary in response to cues. However, despite considerable gains in knowledge of processes, the evidence to explain mechanisms often lacks clarity, may differ amongst species and is not always consistent across estuaries.
Postflexion larvae of estuary‐associated marine fish species dominate the early‐life stages present in high‐energy South African surf zones (Whitfield 1989a, Harris & Cyrus 1996, Strydom 2003). It was first suggested by Whitfield (1989a) that surf zones are an interim nursery area prior to entering final nursery habitats within estuaries. Once in the surf zone, postflexion larvae are able to move laterally along the surf zone (Strydom & D’Hotman 2005) until they reach an open estuary, where they enter the system (e.g. Whitfield 1989b, Harrison & Cooper 1991, Strydom & Wooldridge 2005, Pattrick & Strydom 2014). There are also indications that surf‐zone larval fish densities increase with increasing proximity to an estuary mouth (Whitfield 1989a, Harris & Cyrus 1996). However, this is not always the case and may depend on cohort availability in the surf zone at the time (Strydom 2003). This implies continuity in surf‐zone assemblages of mostly estuary‐associated species and an alongshore current‐mediated ‘conveyor belt’ supply of recruits along the surf zone until ingress to an estuary is possible. It is probable that ingress and recruitment success are not only tied to direct ocean–estuary connectivity but may well be linked in a sequence of ocean–surf‐zone estuary.
Once in shallow waters near or within the estuary, tides become an effective force or cue for transport of eggs and larvae (Epifanio & Garvine 2001). For some species, as they approach the relatively shallow waters of estuaries, selective tidal stream transport (STST) (Forward & Tankersley 2001) and other tidally mediated behaviours are frequently observed (Weinstein et al. 1980, Fortier and Leggett 1982, 1983, McCleave & Kleckner 1982, Rijnsdorp et al. 1985; McCleave & Wippelhauser 1987, Rowe & Epifanio 1994a, Bos et al. 1995, Bos 1999, Jager 1999, Bennett et al. 2002, Bolle et al. 2009). STST is a process in which larvae move near bottom to avoid being swept seaward by ebbing tides and move into the upper layers of the water column on flooding tides to promote advection into the estuary (Forward & Tankersley 2001). Within the estuary, waters are shallow and, by sensing the bottom, larvae can judge if the tide is rising or falling and use STST or other tidally mediated behaviours to insure retention or to move landward (Weinstein et al. 1980, Dodson et al. 1989, Loos & Perry 1991, Rowe & Epifanio 1994a, 1994b, Jager 1999, Forward et al. 1999, Schultz et al. 2000, 2003, Hare et al. 2005b). STST and other tidally induced vertical migrations and diel (or reverse diel) vertical migratory behaviours are effective for dispersing or maintaining larvae in the estuary, or particular zones within the estuary (Jager 1999, Jager & Mulder 1999).
In an early demonstration of tidally tuned vertical migrations, Weinstein et al. (1980) demonstrated that behaviours differ amongst larvae of species that ingressed into the tidal Cape Fear River, North Carolina. STST was utilised, accompanied by daily vertical migrations, by larvae of the sciaenid Leiostomus xanthurus and paralichthyid Paralichthys spp. Their larvae resided close to bottom during the day on ebbing tides but rose to the surface at night on flood tides, facilitating up‐estuary advection. In contrast, larvae of the sciaenid Micropogonias undulatus remained deep in the water column where residual bottom currents supported both retention and up‐estuary advection. Post‐yolk‐sac larvae of an estuary‐spawning sciaenid, Cynoscion regalis, utilised STST to insure retention and up‐estuary transport in the Delaware Bay, USA, but yolk‐sac larvae neither migrated vertically nor utilised STST (Rowe & Epifanio 1994a). The scope of tidal vertical migrations need not be extensive to be effective in ensuring retention, especially for systems with semi‐diurnal tides (Hill 1991).
Within estuaries in Europe, North America and Asia, amongst the clearest examples of STST are those for catadromous, glass‐stage anguillids Anguilla anguilla, A. rostrata and A japonica (Harrison et al. 2014). Glass eels, recently transformed from leptocephalus larvae, migrate from the ocean to coastal waters where sensory cues, including a magnetic compass (Cresci et al. 2017), guide them to estuary mouths where they utilise STST (Creutzberg 1958) to recruit to upper estuaries and eventually to freshwaters. McCleave & Kleckner (1982) documented the up‐estuary migration of A. rostrata glass eels in the Penobscot River estuary, Maine (USA), demonstrating that the eels combined endogenous rhythms and STST to migrate. The glass eels remained below the halocline and near bottom on ebb tide but rose to near the halocline on flooding tides, resulting in rapid up‐estuary transport. Upriver migration rates and use of STST by glass eels of A. anguilla in the Gironde River estuary (France) have been documented by Beaulaton & Castelnaud (2005). Similar behaviour by A. japonica glass eels was observed by Fukuda et al. (2016) in the Hamana Lake Estuary (Japan). McCleave & Wippelhauser (1987) and Wippelhauser & McCleave (1987) proposed that eels (and other fishes) using STST and vertical migrations may have a biological clock and argued that even imprecise timing behaviour would not greatly deter up‐estuary transport.
Not all fish larvae enter the estuary