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).
Figure 3.11 Estuarine Turbidity Maximum region in Chesapeake Bay showing concentrations of eggs, yolk‐sac larvae and feeding‐stage larvae of striped bass Morone saxatilis and favoured prey, the copepod Eurytemora carrolleeae (=affinis). Contoured concentrations (number m−3) and isohalines are depicted. (a) Striped bass eggs. (b) Striped bass yolk‐sac larvae. (c) Striped bass larvae 5–10 mm. (d). Eurytemora adult females
(modified from North & Houde 2003, their figure 5).
In the salt front and ETM regions of two Chesapeake Bay tidal tributaries, Secor & Houde (1995) and Secor et al. (2017) documented up‐estuary advection of larval Morone saxatilis. Larvae originating from hatchery stocking, whose otoliths were chemically marked, were recaptured up‐estuary from stocking locations. STST is the probable mechanism controlling the observed distributions. In another example, Robichaud‐Leblanc et al. (1996) observed distributions of M. saxatilis preflexion larvae in the tidal portion of the Miramichi River Estuary (Canada) that indicated upriver displacement of larvae from the spawning site. Secor et al. (2017) argued that the potential strength of retention and up‐estuary dispersion, and the variable volume of habitat above the salt front, defined the favourable extent of the nursery region for M. saxatilis larvae and that this volume was directly dependent on the level of freshwater discharge.
Estuarine residence time and export/retention of fish larvae
Residence time (and its corollary the flushing rate) is a key metric that provides a measure of how long a passive particle might be retained in an estuary. An estuary is flushed both by river discharge and the tides as well as by internal density‐driven currents, wind‐driven currents, storm events and potentially Coriolis effects (Wolanski & Elliott 2015). Estimates of residence time applied to estuaries worldwide suggest that, except for the largest estuaries (e.g. Baltic Sea, Chesapeake Bay), the residence time of a well‐mixed estuary is seldom more than a week or two, and often it is a few days for small estuaries (Uncles et al. 2002). Hence, pelagic eggs and larval fishes of many species that spawn in the estuary could be swiftly exported seaward during their preflexion stage, before reaching sizes and developmental stages that would enable them to swim to settlement nurseries (Strydom & Wooldridge 2005).
Many estuaries are stratified and experience oceanic inflow along the bottom and riverine outflow to the ocean near the surface. The stratified LOICZ model (Swaney et al. 2011) can be applied to calculate residence time in these estuaries. Additionally, residence time has been calculated for many partially well‐mixed estuaries worldwide from field measurements of the water budgets, based on a salinity balance. In these estuaries, residence time increases with the mean spring tidal range and decreases with the distance from the estuary mouth to the tidal limit (Uncles et al. 2002).
Some water particles (hence also passive pelagic eggs and many preflexion stage larvae) that exit an estuary on the ebbing tide re‐enter the estuary at a later time on a rising tide (Chant et al. 2000, Strydom & Wooldridge 2005). The exposure time is defined as the time spent in the estuary and its seaward plume until a particle never re‐enters the estuary. The ratio between the number of particles re‐entering and the number of particles leaving is the return coefficient r, and it is <1. The return coefficient for passive particles can be estimated from measurements of the volume, salt and temperature fluxes across the estuary mouth (MacDonald 2006). Once exported from the estuary, a particle will only re‐enter if the coastal currents do not carry it away from the mouth. Thus, flushing of an estuary depends in part on the currents in the coastal ocean. Further, the flushing of water and eggs or larvae of fishes from an estuary is generally accelerated during river floods and oceanographic events such as upwelling, the passage of an oceanic eddy and storms (de Castro et al. 2006, Hinata 2006). A caveat, of course, is that the fish eggs and larvae truly are passive.
Stratification of estuarine waters with an upper low‐salinity or freshwater layer and a deeper saline layer may contribute to the probability of pelagic eggs or passive larvae being exported. Stratification can be a permanent feature throughout the year in some estuaries if freshwater run‐off is perennial and large (e.g. most fjords); it can be very short‐lived (i.e. a few days) if the freshwater discharge is large but short‐lived or episodic. In most estuaries, stratification changes seasonally with varying river flows (Vijith et al. 2009). Stratification nearly always diminishes the residence time of water in the surface layer. Accordingly, eggs or passive larvae residing near the surface are expected to be swiftly exported to the sea, despite density stratification and up‐estuary flow near bottom. In some cases, even estuarine‐resident larvae can be flushed from the upper reaches towards the ocean during high river flow through an estuary (Strydom et al. 2002).
The residence time of water in many estuaries is usually short (i.e. high flushing rate) compared to the pelagic larval duration (PLD) of eggs and preflexion larval stages in many fishes. Larvae are more likely to remain trapped in the estuary during all or part of their PLD in two types of estuaries, namely inverse estuaries and lagoons. In inverse estuaries in the tropics and semi‐tropics during the dry season, evaporation greatly exceeds freshwater inflow (Potter et al. 2010). A salinity maximum zone is formed and both oceanic and riverine water flow near the surface towards this zone (Wolanski 1986). There, this water downwells and spreads along the bottom both seaward and landward. The residence time in such estuaries can be several months and passive larval fishes can remain in or ingress to the estuary without a need for specific behaviours to prevent seaward export. Long tidal creeks in the tropics during the dry season also present a special case of a very long residence time of surface waters because these creeks behave as evaporation ponds, a process that is accelerated in the presence of extensive fringing saltmarshes or mangrove swamps (Wolanski & Elliott 2015). In such cases, ocean water is advected landward at the surface to replenish the water loss by evaporation. The high‐salinity water produced by evaporation downwells in the upper reaches of tidal creeks and is exported seaward along the bottom.
Another unique type of estuary that is more likely to retain fish larvae due to various entrapment mechanisms is the intermittently open–closed estuary type, i.e. lagoons, which are blocked at the mouth by a sand bar. The mouths are narrow and shallow, and flushing is often restricted to the rainy season during opening events. In lagoons, the narrow mouth measurably inhibits the propagation of the tides to the inner parts of these systems and the residence time is long, typically a few months between opening events (Whitfield et al. 2008, Newton et al. 2014, Whitfield 2019). Despite the narrow mouth and connecting channel restrictions in this type of estuary, ingress of larvae and early‐stage juveniles from the surf zone is robust and effective (Hall et al. 1987).
In other estuaries, fish larvae may remain confined in an estuary due to various entrainment mechanisms. In one example, larvae originate from eggs spawned in water that is trapped in estuarine embayments. Embayments can extend residence time in an estuary if the volume of water moving at the flood tide into the lateral embayment is small compared to the water volume in the embayment
