increasing instances of hypoxia and acidification and associated shifts in spawning phenology are likely to be major effects (Ojaveer & Kalejs 2005, Nye et al. 2009, 2014, Gillanders et al. 2011) as climate change continues to affect shallow estuarine systems (James et al. 2013, Wallace et al. 2014, Elliott et al. 2019). Related, naturally occurring weather events also can pose hazards to reproduction of estuary‐associated fishes (see Section 3.2.2.5). A recent review of climate‐induced effects of acidification on settlement and metamorphosis of fish early‐life stages indicated that, based on a relatively few estuary‐associated species, these key life‐history processes could be compromised, e.g. in the catadromous latid Lates calcarifer (Espinel‐Velasco et al. 2018) although effects might not be realised for many decades. Incidence of harmful algal blooms (HABs), noted above, that may disrupt and potentially reduce reproductive success of estuarine fishes is likely to increase in response to changing climate (Glibert 2016).
The influence of changing climate is evident in a 26‐year time series of larval ingress for fishes in a Mid‐Atlantic (USA) estuary, where the number of higher‐latitude species is now reduced and the number of lower‐latitude species has increased (Morson et al. 2019). There are documented northward shifts in distributions and fishery landings of adults of estuary‐associated species, for example the paralichthyid Paralichthys dentatus, on the US east coast (NEFSC 2016, 2019). Although shifts in distribution of the adult stock do not confirm changes in reproduction and/or recruitment, or that populations are migrating in response to climate change, the weight of evidence indicates that climate is a driving force that may threaten reproductive success and cause major shifts in spawning areas and seasons in some species.
3.5.8 Catastrophic events
Human‐caused catastrophes, although relatively uncommon, may reduce reproductive success of estuarine fish populations. The increasingly heavy use of coastlines and estuaries by humans has elevated the risk from hazards, both man‐made and natural (Elliott et al. 2014). Hazards that lead to catastrophes often are acute events, e.g. toxic spills that could kill early‐life stages of fishes or, alternatively, catastrophes may evolve from chronic stressors that have long‐term impacts on habitat and severely reduce reproductive capacity. Amongst the best documented events are those caused by spills of oil in the coastal ocean and estuaries. One of the largest known catastrophes of human origin, the Deepwater Horizon oil spill in the Gulf of Mexico, did not result in any clear effects of the oil on marsh fish population abundance (Able et al. 2015). Nor was it possible to confidently understand negative effects on individuals versus populations, indicating critical knowledge gaps (Fodrie et al. 2014). However, there were clear changes in gene expression and associated gill immunochemistry (Whitehead et al. 2012) that could be indicative of stress on future reproduction. A similar difficulty in interpreting catastrophic effects occurred when trying to evaluate the response of estuarine fishes to Hurricane Sandy in Barnegat Bay (USA) in the Mid‐Atlantic (Valenti et al. 2020).
In southern Africa, the loss of Lake St Lucia as a fish nursery area, particularly for estuary‐associated marine fish species, primarily due to freshwater deprivation and the subsequent evaporation of more than 90% of the surface area of this 35 000 ha estuarine lake ranks as a major catastrophe for coastal fishes (Cyrus et al. 2010). The prolonged closure of the St Lucia Estuary mouth due to human‐induced, inadequate river inflow to the lake has deprived 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 &
