apparent that larval behaviours, particularly from attainment of the flexion stage and the associated formation of the caudal fin, play a key role in directing dispersal and assuring ingress to estuarine nurseries. Case studies at the end of this chapter (Section 3.6) provide examples of processes and illustrate how estuary‐dependent taxa use hydrodynamics, swimming and larval behaviour to ensure ingress and retention.
3.3.1.1 Offshore to estuary transport processes
Boehlert & Mundy (1988) discussed mechanisms that could deliver larvae shoreward and to the vicinity of estuaries, implicating behaviour as potentially important. Epifanio & Garvine (2001) reviewed processes implicated in the transport of passive larvae on continental shelves, identifying three major forcings: (i) wind stress, (ii) tides and (iii) density differences associated with the buoyant outflow from estuaries. They found that wind stress and buoyant flows were major determinants, but tidal effects on transport of larvae from the shelf to estuaries were minimal for two fishes (Brevoortia tyrannus, Pomatomus saltatrix) and a decapod (Callinectes sapidus). They argued that their conclusions are broadly applicable to eggs and larvae of many taxa that experience offshore transport. Buoyancy flows from estuaries and associated coastal currents and winds that induce downwelling (and Ekman transport) or upwelling had substantial effects on larval transport of their three focal species. Passive behaviours that depend on dispersal by winds and onshore flows also are important mechanisms to bring larvae to estuaries, e.g. as reported for Sillaginodes punctata (Jenkins et al. 1997) in Australian coastal waters.
Tides cannot be an effective force to transport larvae from distant offshore sites towards estuary mouths for three reasons, namely (i) by the time fish larvae develop to the postflexion stage, they may be far from an estuary mouth in a prevailing mean coastal current of 4–10 cm s−1, which is common worldwide (see review in Teodosio et al. 2016); (ii) field data reveal that larvae at sea generally remain in surface waters or they perform diel migrations, coming to the surface primarily at night (Forward et al. 1999, Garrido et al. 2009) in diel migrations not related to times of high and low tides and (iii) larvae do not have a pressure datum, that we know of, upon which to cue and thus may not be able to sense changes in water pressure due to tides. Put simply, early‐stage larvae do not know whether the tide is rising or falling.
To enter an estuary, postflexion larvae dispersed at sea must be advected into the coastal zone or near the estuary mouth and be able to detect the estuary and swim towards it. Detection cues can include odours originating from the estuary, estuarine soundscapes, salinity, water turbidity, temperature and magnetic North (Tosi et al. 1990, Kingsford et al. 2002, Strydom 2003, Bos & Thiel 2006, James et al. 2007a, Radford et al. 2012, Lillis et al. 2014, Teodosio et al. 2016, Cresci et al. 2017, Morais et al. 2017, Baptista et al. 2019, 2020). Cues and mechanisms that can guide larval migrations from offshore sites to estuaries are depicted in Figure 3.5. Preflexion larvae may use an innate or infotaxis strategy when away from estuarine cues. Upon detecting estuarine cues, postflexion larvae may use rheotaxis coupled with directional swimming along the estuarine‐cue concentration gradient (Figure 3.5a). In many cases, alongshore currents in coastal waters typically advect drifting larvae far from an estuary mouth such that the larvae cannot enter a specific estuary at postflexion by swimming or adoption of other behavioural mechanisms (Figure 3.5b). Directional swimming by larvae may be required to ensure recruitment to the estuary. If larvae are unable to recruit to a ‘natal’ or proximate estuary, they may still recruit to another estuary downstream (Figure 3.5c), potentially by adopting vertical swimming behaviours in alongshore currents and then tidal‐induced behaviours (e.g. selective tidal stream transport, STST) near estuary mouths. At the estuary mouth, larvae may use a suite of strategies to ingress (Teodosio et al. 2016).
3.3.1.2 Swimming as a transport mechanism
Vertical swimming behaviour is well known in early‐stage fish larvae and is executed by many taxa in offshore regions prior to entering estuarine nurseries. Diel vertical migrations, for example, are reported for offshore postflexion larvae of the clupeid Brevoortia tyrannus (Forward et al. 1999) or for pelagic larvae of the soleid Solea solea (Champalbert & Koutsikopoulos 1995). The diel vertical migrations appear to be endogenous behaviours that facilitate shoreward transport via depth regulation.
Swimming from offshore to the coast by postflexion larvae with developed fins provides a potential mechanism for transport to the coast. Postflexion fish larvae can swim at relative speeds several times their body length per second, at least for many late‐stage larvae of tropical and temperate fishes. Remarkably, they can swim at those speeds for hours or days (Bellwood & Fisher 2001, Clark et al. 2005, Leis 2006, Pattrick & Strydom 2009, Leis et al. 2013). As such, postflexion fish larvae of some species may swim to (or towards) estuaries, especially if they have cues to guide them. For example, swimming may be the mechanism used by the pomatomid Pomatomus saltatrix in the western North Atlantic (Hare & Cowen 1993) to transit from the Gulf Stream edge to coastal and estuarine nurseries on the US Atlantic coast.
Swimming is probably also the mechanism used by the glass‐eel stage of anguillid eels, Anguilla rostrata and A. anguilla, entering North American and European estuaries (Wuenschel & Able 2008) and A. japonica ingressing to Japanese estuaries (Tsukamoto 1990, Fukuda et al. 2016). Larvae of taxa from high latitudes (with lower temperatures) may have less swimming capability, but they too swim surprisingly well. Clark et al. (2005) conducted experiments on larvae that included the estuarine sciaenid Argyrosomus japonicus whose critical swimming speed (expressed as body lengths per second, BL s−1) increased from about 5 BL s−1 at 5 mm standard length to >10 BL s−1 at 10 mm SL. Furthermore, endurance, expressed as swimming distance, increased rapidly from a negligible distance for the smallest <5 mm larvae to ~15 km for 13 mm SL postflexion larvae (Figure 3.10).
Swimming ability of the warm‐temperate estuarine sciaenid Sciaenops ocellatus, tested in the laboratory, increased rapidly in early‐stage larvae (<7 mm TL) to about 10 cm s−1 (~14 BL s−1) at 7 mm TL and then more slowly later in development (Faria et al. 2009). In laboratory experiments, larvae of the temperate Diplodus sargus increased sustained swimming speed from about 5 BL s−1 at 5–10 mm TL to about 14 BL s−1 at 15 mm TL and 50 days posthatch (Baptista et al. 2019). The late‐stage larvae of two warm‐temperate sparid species, Sarpa salpa and Diplodus capensis (approximately 10–20 mm SL), had similar critical swimming speeds of 19 and 18 cm s−1, respectively, speeds that exceed mean nearshore current velocities near their coastal nurseries (Pattrick & Strydom 2009). In these examples, swimming durations (endurances) of larvae >10 mm in length can be expressed as days. Potential distances swum can be in the 10–100 km range, indicating that directed swimming from offshore by late‐stage larvae can partly account for larval ingress into estuaries. A recent analysis of several Mediterranean Sea species, including estuarine/coastal taxa (e.g. Mugilidae, Sparidae), confirms the substantial swimming ability of postflexion larvae and potential to direct their inshore dispersal (Rossi et al. 2019b).
