Fish and Fisheries in Estuaries. Группа авторов

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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 using STST; indeed, larvae of several species 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

      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).

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