the system of larval and juvenile marine and catadromous fish ingress (Cyrus & Vivier 2006). This loss has impacted the abundance of coastal marine fish species that normally use the St Lucia system as a nursery area (Mann & Pradervand 2007).
3.6 Case studies
Case studies are presented to illustrate processes and mechanisms used by estuarine and estuarine‐associated fishes for dispersal from offshore spawning sites, ingress into estuaries, retention in estuarine nurseries and growth and survival during the recruitment process.
3.6.1 Pleuronectiformes
Mechanisms and processes controlling dispersal of pleuronectiform eggs and larvae from offshore spawning sites to estuarine nurseries are well researched but still not fully understood. The mechanisms are best described for economically valuable European species, Solea solea, S. senagalensis, Pleuronectes platessa and Platichthys flesus. All spawn in offshore waters and their larvae or juveniles ingress to estuarine nurseries where they settle and may spend from several weeks to two years before rejoining offshore spawning populations (Rijnsdorp et al. 1985, Van der Veer et al. 1998, Grioche et al. 2000, De Graaf et al. 2004, Bolle et al. 2009, Duffy‐Anderson et al. 2015). Excellent examples of the life‐stage transitions and dependence on connectivity amongst coastal seas, estuary ingress sites and estuaries are documented for egg, larval and juvenile stages of these estuary‐dependent pleuronectiforms (Ramos et al. 2010, Martinho et al. 2012, Primo et al. 2013, Duffy‐Anderson et al. 2015, Van der Veer et al. 2015).
Larvae of Solea solea in the Bay of Biscay are transported to nearshore and estuarine nurseries (Champalbert & Koutsikopoulos 1995). Upon approaching metamorphosis, probable tidal and clear diurnal behaviours are adopted that direct coastward transport. Koutsikopoulos et al. (1991) reported that most dispersal of larvae may be attributable to passive diffusion rather than directed transport, implying that most larvae perish offshore. Grioche et al. (2000) conducted research in the English Channel on transport of soleid S. solea and pleuronectid Platichthys flesus larvae, primarily documenting vertical distributions. The two species behaved differently; S. solea migrated vertically and a substantial fraction were predominantly located near bottom (<1 m off bottom) while P. flesus did not adopt vertical migratory behaviour until late in development. The behaviour of S. solea was proposed to be particularly effective in facilitating coastward transport.
Van der Veer et al. (1998) modelled transport of Pleuronectes platessa larvae from spawning sites in the southern North Sea to the Dutch coast, under the assumption that larvae were passive particles. In more recent 3D modelling, Bolle et al. (2009) modelled transport of P. platessa larvae from offshore to estuarine inlets, comparing results of passive transport with results expected if selective tidal stream transport (STST) were adopted by larvae when in depths <30 m. While STST improved outcomes, passive transport also successfully delivered a substantial fraction of larvae. In another modelling study (de Graaf et al. 2004), transport of particles resembling P. platessa or Platichthys flesus larvae was simulated from North Sea spawning sites to the Dutch coast. Larvae that adopted STST behaviour had substantially improved, successful transport. However, it is uncertain that larvae could employ STST in relatively deep offshore waters like those in the de Graaf et al. (2004) model. In the Baltic Sea, modelled drift of larval Platichthys solemdali assured dispersal to coastal nurseries from offshore if larvae remained at depths >10 m where currents are favourable for shoreward transport (Corell & Nissling 2019).
Results of diverse research on transport of larval Pleuronectiformes have demonstrated that mechanisms are reasonably well understood, but there are unexplained differences amongst studies that could arise from different hydrodynamic regimes (Duffy‐Anderson et al. 2015). For example, Burke et al. (1998) compared offshore behaviours of larvae of three Paralichthys species from North Carolina (USA) in which the larvae exhibited endogenous tidal and diel rhythms (possibly STST) that facilitated transport towards estuarine nurseries. In contrast, larvae of Paralichthys olivaceus in Yura Bay (Japan) did not display endogenous behaviours but mostly remained near bottom in a hydrodynamic environment promoting shoreward delivery (Burke et al. 1998). On the Pacific coast of the USA, larvae of Parophrys vetulus benefitted from offshore residual currents and Ekman transport to disperse shoreward, and then STST supported by endogenous behaviours to reach estuary mouths (Boehlert & Mundy 1987). Rooper et al. (2006) modelled coastal dispersal of P. vetulus larvae, concluding that passive drift alone in the offshore hydrodynamic regime might account for transport of many larval cohorts to the coast and mouths of estuaries.
The intensive research on European species, especially Pleuronectes platessa, Solea solea and Platichthys flesus, was amongst the first to document the use of STST by fish larvae as a mechanism to deliver them to the mouths of French, Dutch and German estuaries (e.g. Creutzberg 1958, Creutzberg et al. 1978, Rijnsdorp et al. 1985). In shallow coastal waters, near mouths of major estuaries (e.g. Dutch Wadden Sea), Rijnsdorp et al. (1985) reported that P. platessa larvae used STST to facilitate entry and, subsequently, up‐estuary transport. Upon entry to estuaries, STST may become increasingly important to ensure up‐estuary transport and retention of P. platessa and also P. flesus (e.g. Bos et al. 1995, Bos 1999, Jager 1999, Jager & Mulder 1999). Contrary to reports supporting the view that P. platessa depends on STST for ingress and up‐estuary transport, Bergman et al. (1989) argued that passive behaviour was sufficient to assure ingress and retention of P. platessa larvae in the Dutch Wadden Sea, also arguing that larvae may ingress, be flushed and re‐entrained into the estuary multiple times before settling. STST and passive ingress arguments both have merit if a combination of behaviours is adopted to achieve ingress.
Other flatfishes also exhibit STST and vertical swimming behaviours to facilitate transport. For Parophrys vetulus on the North American west coast, Boehlert & Mundy (1987) demonstrated that STST and decreasing salinity in ebbing bottom waters may cue larvae to prepare for entry to the estuary, and salinity also may be a cue to guide ingress and up‐estuary movement of metamorphosing larvae. Yamashita et al. (1996a) reported that Kareius (=Platichthys) bicoloratus larvae disperse to inshore and estuary areas from spawning sites in Sendai Bay, Japan. Its metamorphosing larvae exhibited behaviour indicative of STST that enabled migration to inshore and estuarine nurseries. Salinity was a strong force in guiding metamorphosing Platichthys flesus larvae to tidal freshwater habitats in the Elbe River, Germany, that serve as juvenile nurseries (Bos et al. 1995, Bos & Thiel 2006). Within the mouth of Chesapeake Bay (USA), Hare et al. (2005a) observed tidally mediated behaviour of larval Paralichthys dentatus and apparent STST, which supported net up‐estuary flux of the larvae. Settling larvae of Paralichthys olivaceus in Shijiki Bay (Japan) may utilise STST to enable shoreward transport (Fujii et al. 1989, Tanaka et al. 1989).
Growth and mortality of early‐life stages of estuary‐associated Pleuronectiformes are well researched. Recent reviews and syntheses have provided a good understanding of early‐life dynamics and implications for recruitment (Nash & Geffen 2012, 2015, Ciotti et al. 2014, Duffy‐Anderson et al. 2015, Van der Veer et al. 2015). Nash & Geffen (2015) reviewed age and growth of pleuronectiform early‐life stages and found that temperature was a major factor controlling growth rates of pelagic larvae and settled juveniles. Faster‐growing individuals of Pleuronectes platessa larvae may have a survival advantage acquired during the offshore pre‐settlement stage (Hovenkamp 1992). Growth of post‐settlement pleuronectiforms may vary more spatially than temporally, highlighting the importance of estuarine habitat in supporting their production (Van der Veer et al. 1994). There is evidence for density‐dependent growth of newly settled, young‐of‐the‐year juveniles of some, but not all, pleuronectiforms, and the importance of density‐dependent regulation via growth is variable amongst species (Nash & Geffen 2015). Evidence for density‐dependent regulation of mortality is reasonably compelling for settled juveniles of pleuronectiforms in estuaries (Van der Veer et al. 2015), although such mortality, attributable to predation, may be restricted to brief periods, as reported for P. platessa (Bergman