variegatus (1.2–1.4 mm diameter). The recently hatched larvae of these two species are also relatively robust and large (4.8–5.5 mm and 4.2–5.2 mm, respectively) (Sakowicz 2003). A newly hatched F. heteroclitus is capable of entering the water column and even swimming at the surface, at least in the laboratory, while C. variegatus is incapable of these behaviours. The difference in these behaviours accounts for their different distributions in nature. For F. heteroclitus, its larvae are found in shallow depressions on the marsh surface and in the shallow waters of marsh pools and adjacent creeks (Able & Fahay 2010). For C. variegatus, larvae are only found in marsh pools. At the other extreme, eggs of salmonids that hatch in the gravel substrate where the eggs have been deposited (i.e. in a redd) may be resident there for months (Quinn 2018).
Figure 3.4 Eye migration during metamorphosis of settlement‐stage Paralichthys dentatus
(from Keefe & Able 1993).
The distribution of other species that spawn in the estuary varies in ways that may affect their transport into and out of estuaries. In one example, the moronid Morone americana spawns demersal eggs in estuaries on the east coast of North America that hatch into small (~3 mm) pelagic larvae with weak swimming ability. Larvae are passively transported to the estuarine salt front and estuarine turbidity maximum region where they are retained during early development (North & Houde 2001, Shoji et al. 2005b). In another example, the clupeid Gilchristella aestuaria spawns in the upper reaches of South African estuaries where the probability of estuarine retention for their pelagic eggs and weak‐swimming larvae is greatest (Talbot 1982).
Dispersal during the larval stage of estuary‐associated fishes was once considered to be largely via passive drift (Roberts 1997, Hare et al. 2002), but recent authors have noted that behaviours, including vertical and horizontal swimming, especially during late larval stages, have a strong influence on transport (Cowen et al. 1993, Leis et al. 1996, Stobutzki & Bellwood 1997, Hare et al. 2005a, 2005b). Dispersal during the recruitment process has been the emphasis of research on numerous species in many estuarine systems during recent decades. Recent evaluations of larval fish transport and navigation from offshore to the coast or estuaries have demonstrated that it is enabled by a suite of probable sensory cues (e.g. odour, sound, visual and geomagnetic cues) that become effective during ontogeny (Faillettaz et al. 2015, Teodosio et al. 2016, Rossi et al. 2019a), especially during and after the postflexion stage (Morais et al. 2017, Baptista et al. 2019, 2020) (Figure 3.5). It has been demonstrated that larvae and pelagic juveniles can use auditory cues (Montgomery et al. 2006), olfaction (James et al. 2008a) and other senses (Teodosio et al. 2016, Morais et al. 2017, Rossi et al. 2019a) for orientation towards estuary mouths using innate behaviours and infotaxis, an algorithm developed for turbulent odour plumes where searching movements by larvae are based on sporadic cues and partial information (Vergassola et al. 2007). Longshore currents may transport drifting larvae far from a natal or proximate estuary and directional swimming by postflexion larvae may be essential to ensure ingress to an estuary. Larvae unable to ingress to a proximate estuary may still ingress, but to an estuary downstream. Many examples documenting these transitions, and associated environmental conditions, are described for temperate estuaries (Boehlert & Mundy 1987, Allen & Barker 1990, Bruno et al. 2014, Ramos et al. 2017, Baptista et al. 2020). It is notable that larvae of many species enter estuaries at postflexion/metamorphic stages in North America (e.g. Figure in Able & Fahay 2010), South Africa (Strydom 2015), Europe (Bos et al. 1995, Jager 1999) and Australia (Miskiewicz 1986) when their sensory systems and swimming capabilities are still developing.
The swimming abilities of marine fish larvae, especially postflexion stages, may contribute importantly to dispersal (Fisher et al. 2000). In fact, in recent reviews (Leis 2006, Wuenschel & Able 2008), there is an indication that the larvae of many fishes are capable of swimming for long periods for much of the larval stage at speeds greater than ambient currents (see Section 3.3.1.2). The swimming capabilities of some larvae that use estuaries are impressive, as noted for postflexion and settlement stages of the sparids Diplodus capensis and Sarpa salpa (Pattrick & Strydom 2009) and Diplodus sargus (Baptista et al. 2019). In another example, the leptocephali of the congrid Conger oceanicus (69–117 mm Tl) and glass eels of the anguillid Anguilla rostrata (49–68 mm TL) were capable of performance in a laboratory flume that would allow them to swim the long distance from the Gulf Stream edge off the east coast of North America to the Little Egg Inlet (Wuenschel & Able 2008). This transit could be possible in ≈ 30 or 40 days from the edge of the continental shelf or the Gulf Stream edge for A. rostrata and ≈ 20 or 45 days from the shelf edge or Gulf Stream edge for C. oceanicus. Larvae of C. oceanicus entering this same inlet may arrive in even fewer days under conditions of faster growth (Correia et al. 2004).
Figure 3.5 Processes and cues supporting larval ingress into estuaries. (a) Estuarine (auditory, visual, odour) and navigational (geomagnetic, solar, stellar, coastal features) cues used by offshore fish larvae to detect estuaries and to navigate towards them prior to adopting active swimming for ingress
(modified from Teodosio et al. 2016, their figure 2).
Near the estuary mouth, larvae may adopt a suite of strategies to ingress (infotaxis). (b) Longshore currents (u) in coastal waters may transport drifting larvae far from a ‘natal’ or proximate estuary such that they are unable to utilise STST (Selective Tidal Stream Transport) to ingress at the postflexion stage. (c) Directional swimming by larvae may be essential to ensure ingress to some estuaries. Larvae unable to ingress to a ‘natal’ or proximate estuary may still ingress and recruit, but to another estuary.
In demersal species, settlement for many estuarine fishes often signals the end of highly dispersive egg (for some) and larval stages and initiation of a more localised juvenile stage. In flatfishes, dramatic eye migration occurs (Figure 3.4), along with the ability to bury in the sediment, as seen in the paralichthyid Paralichthys dentatus (Keefe & Able 1993). Development during settlement warrants understanding because these important morphological, physiological and behavioural transitions occur while fishes are undergoing habitat transitions (Moser 1981, Balon 1984, Chambers et al. 1988, Youson 1988, Levin 1991, Kaufman et al. 1992), and the transition is potentially associated with an increased risk of mortality (Able & Fahay 2010). This transition, often coupled with metamorphosis to a juvenile morphology, is not unique for estuarine fishes but is a common mode of ontogeny in many demersal species (Espinel‐Velasco et al. 2018). Metamorphosing and settling juveniles of estuary‐dependent fishes may also face osmoregulation challenges upon entering lower‐salinity waters in estuaries (Whitfield 2019).
In pelagic species that do not settle and in demersal settlers, ontogenetic shifts experienced by transforming individuals may be accompanied by changing food habits. This is evident, for example, based on ontogeny of the feeding apparatus in post‐settlement individuals from three foraging guilds (pelagic, generalist and benthic) of sciaenid fishes (Figure 3.6) in Chesapeake Bay (Deary et al. 2017). This research demonstrated that ontogenetic, developmental changes in the jaw and sensory capabilities were accompanied by shifts in feeding in which larvae adopted very different feeding habits and foods at settlement (Deary et al.