in mortality under low‐flow, low productivity conditions (Strydom et al. 2002). In Chesapeake Bay, low freshwater discharges are associated with down‐estuary advection of larval moronids (North & Houde 2001, North et al. 2005), while, counter‐intuitively, moderately high flows are associated with retention. Highest flows may result in flushing or washout of Morone saxatilis eggs and larvae in Chesapeake Bay and its tributaries (Rutherford et al. 1997, North et al. 2005).
3.3 Early‐life stages and recruitment dynamics
Numerous biotic and abiotic processes act to enhance or diminish recruitment levels of estuary‐dependent and ‐associated fishes. Important processes are those generally recognised for marine fishes (Houde 2016) although the life histories of estuary‐dependent and ‐associated species add complexity to recruitment pathways. Key processes addressed in this chapter that contribute to variability in survival and production of early‐life stages of estuary‐dependent and ‐associated fishes are highlighted in the text box.
Key processes contributing to variable survival of early‐life stages of estuary‐dependent/associated fishes
1 Selection of spawning sites by adults.
2 Coastal circulation processes that disperse eggs and larvae of offshore‐spawning species.
3 Advective processes that transport larval stages towards estuaries and support connectivity.
4 Estuarine hydrography that entrains or retains larvae within estuaries.
5 Behaviour of larvae and juveniles, including swimming ability, that facilitates recruitment into estuaries.
6 Trophodynamic processes that support growth and nutrition of early‐life stages.
7 Predation and other processes that impose mortality during early life.
8 Effects of fishing on recruitment variability.
The role of connectivity and mechanisms that support it to assure recruitment success is increasingly recognised for its importance in guiding the recruitment process of marine fishes (Pineda et al. 2007, White et al. 2019) and are especially important for estuary‐dependent and ‐associated species, which have early‐life and juvenile stages dependent on processes operating in complex coastal‐estuarine ecosystems (Sheaves 2009, Duffy‐Anderson et al. 2015, Van der Veer et al. 2015, Levings 2016, Teodosio et al. 2016).
We emphasise processes that embrace a broad definition of recruitment, including those that (i) control supply of eggs and larvae to estuaries, (ii) support immigration of early‐life stages and settlement of juvenile fishes in estuaries that serve as nurseries and (iii) guide or control recruitment to the juvenile stage. Feeding and nutrition of larval stages are reviewed, including aspects related to ontogenetic shifts in feeding and the sufficiency of prey levels to support larval growth. Vital rates (i.e. growth and mortality) in early‐life stages are discussed, particularly with respect to factors controlling or regulating the rates.
Recruitment, defined as the abundance of life stages added to a population, represents the outcome of a suite of dynamic processes and behavioural traits within a life stage, or integrated across life stages, and which ensure replenishment of stocks (Lowerre‐Barbieri et al. 2016). Historically, most of the dynamics associated with causes of variability in recruitment of marine fishes have been attributed to processes acting on the earliest life stages, i.e. eggs and early‐stage larvae (Hjort 1914, Leggett & Deblois 1994, Houde 2008). Recruitment in exploited species, defined as the size/age at which individuals in a fished stock enter the fishery (e.g. Costa et al. 2002), may not occur for months or years posthtach. As noted earlier, we take a broader view of recruitment and emphasise processes associated with the early‐life stages and the ontogenetic transitions associated with recruitment to the juvenile stage. For demersal fishes, metamorphosis and settlement represent key transition processes that signal the ending of the postflexion larval stage and beginning of the juvenile stage that is often taken to represent the onset of the ‘recruitment’ phase.
Estuaries are both complex and productive habitats, presenting challenges but also offering possibilities for elevated growth and production during the recruitment process. Under favourable conditions, the probability of successful recruitment may be enhanced in estuarine environments. Success in recruitment of marine fishes is dependent on connectivity amongst habitats, often in well‐defined, evolved pathways that assure linkages amongst life stages (Harden‐Jones 1968, Cushing 1975, Pineda et al. 2007, Sheaves 2009, Secor 2015, Duffy‐Anderson et al. 2015, Amorim et al. 2016, Teodosio et al. 2016), and often between offshore and estuary habitats. Hydrodynamic processes are important in guiding connectivity, creating pathways, delivering early‐life stages to estuaries and retaining early‐life stages in estuaries (Wolanski 2017). During ontogenetic and growth transitions, behavioural attributes that emphasise sensory capabilities (Boehlert & Mundy 1988, Forward et al. 1999, Teodosio et al. 2016) become increasingly important to assure connectivity amongst habitats and life stages.
3.3.1 Dispersal, transport and retention
A large proportion of estuary‐dependent and ‐associated bony fishes have pelagic larvae and many taxa also have pelagic eggs. Mechanisms and processes controlling transport of eggs or larvae to estuaries, or retaining them upon delivery, have been appreciated, if not fully understood, for decades. Norcross & Shaw (1984) and Boehlert & Mundy (1988) defined the problem, reviewed knowledge and noted important processes and mechanisms. They cited offshore circulation processes that disperse larvae and eggs and, once within an estuary, circulation, flows and larval behaviours. These processes are often described as mechanisms that ensure connectivity amongst habitats occupied by early‐life stages of estuary‐associated fishes (e.g. Duffy‐Anderson et al. 2015, Amorim et al. 2016, Teodosio et al. 2016). The processes and, most of all, the realisation that larval behaviour in response to a suite of sensory cues plays a major and decisive role in ensuring transport and retention are framing research designed to understand processes controlling recruitment to estuaries (Teodosio et al. 2016).
Here, we emphasise three broad, topic areas:
1 Processes supporting ingress of larvae to estuaries from offshore spawning.
2 Movements and retention of larvae within estuaries.
3 The nursery function of estuaries.
During the time that eggs or larvae are drifting in the plankton, their distribution and dispersal are dependent on the surrounding residual water currents and, if the eggs and larvae behave as passive particles, they may disperse long distances from spawning grounds. In marine fishes, for example the gadid Gadus morhua, which, in some stocks, may enter European estuaries as juveniles (Elliott et al. 1990), the finding that dispersal processes are important for transporting early‐life stages to suitable juvenile nurseries has been recognised for more than a century (Hjort 1914, 1926) and mechanisms controlling dispersal, including adult selection of spawning sites, have been addressed frequently (e.g. Harden‐Jones 1968, Werner et al. 1997, 1999, Secor 2015).
For many estuary‐associated species, physical processes that ensure connectivity between the ocean and estuaries, or that ensure retention in estuarine habitat, are even more critical to recruitment success than for species that spawn, develop and recruit offshore (Beckley 1985, Whitfield 1989b, Epifanio & Garvine 2001, Sheaves 2009, Duffy‐Anderson et al. 2015, Van der Veer et al. 2015, Levings 2016). As knowledge has accumulated, the assumption that larval fishes behave as passive
