mortalities in egg, larval and juvenile stages. Hydrodynamics, migration histories and feeding success, in addition to poorly understood predation processes, mold survival outcomes. For many species, hydrodynamics set the stage while trophodynamics fuel growth and survival trajectories. Growth and mortality processes do not act independently (Cushing 1975, Houde 1987, Anderson 1988). For example, variable growth rates result in variable stage durations, a key factor controlling stage‐specific mortality and abundance at the end of the larval stage, or at recruitment (Houde 1997b, 2016).
Estuary‐dependent and ‐associated fishes display numerous life‐history modes, increasing the possibility of stage‐specific and habitat‐specific variability, or shifts, in growth and mortality rates. For taxa that spawn offshore, variability in the offshore environment, including circulation patterns and hydrographic features that are critical for transport of eggs and larvae, may govern the observed variability in growth and mortality rates (Heath 1992). Within the estuary, numerous environmental factors act on eggs and larvae of resident taxa, anadromous spawners and pre‐settlement immigrants from offshore.
The high and variable mortality rates and processes acting on early‐life stages are major drivers that generate variable recruitments (Houde 1987, 1997b). Much of the literature on early‐life mortality, while directed to ocean species, is equally applicable to estuarine fishes. The influence of Hjort (1914, 1926), who proposed massive and variable mortalities during the first‐feeding period (i.e. the critical period), became widespread, but Hjort's hypotheses were hardly evaluated for marine and estuarine fishes until the 1970s. Despite shortcomings (Leggett & Deblois 1994), the critical period hypothesis has appeal in explaining recruitment variability (Houde 2008). An equally compelling and related idea put forward by Cushing (1990) is that synchrony in larval production with that of their planktonic prey in a ‘match‐mismatch’ scenario could determine the fates of year classes. There is considerable evidence supporting this hypothesis for estuarine and marine fishes (Peck et al. 2012b, Houde 2016).
Earliest life stages of estuarine and marine fishes suffer high (typically >99.9%) mortality. But, high mortality rates alone may not be the decisive factor controlling abundance at recruitment. It is cumulative mortality and the stage‐specific variabilities prior to the stage defined as recruitment that primarily determine recruitment levels and variability (Sissenwine 1984, Houde 2008). The life stage at which recruitment is set may differ amongst years, or amongst cohorts produced in a year, depending on variability in mortality rates, growth rates and stage durations. In the estuary, factors governing survival can be highly variable and, additionally, mortality of early‐life stages in estuaries sometimes is episodic (Houde 1989b). Environmental challenges may be especially critical when associated with transitions from the ocean to the estuary, including metamorphosis and settlement. An example of this circumstance is seen in the paralichthyid Paralichthys dentatus, which ingresses into estuaries during the metamorphic, eye‐migration stage (Figure 3.4). A potential for bottlenecks in the path to recruitment exists if the factors influencing a stage either block or impede its transition (i.e. survival) to subsequent stages. The complex life histories of many estuary‐dependent species elevate the possibility of reproductive and recruitment bottlenecks and associated mortality, attributed to environmental variability, e.g. weather events, freshwater availability, dissolved oxygen, altered flows and habitat deficiency.
In early‐life stages of estuary‐dependent fishes, growth is rapid and mortality rates are high, but generally similar to rates of other marine fishes (Houde & Zastrow 1993). Usually, the environmental and trophic processes controlling mortality and growth of eggs and early‐stage larvae are mostly density independent and the rates are sensitive to availability of prey resources and intensity of predation (Cowan et al. 2000; Cowan & Shaw 2002). Later, processes acting on juveniles, while still mostly driven by density‐independent, environmental controls, may include a substantial component of density‐dependent regulation (Van der Veer 1986, Houde 1987, Myers & Cadigan 1993, Rose et al. 2001, Van der Veer et al. 2015). For example, in estuary‐dependent pleuronectiforms, such as Pleuronectes platessa, Platichthys flesus and Solea solea, there is a period at settlement when abundance of newly metamorphosed juveniles may be regulated by density‐dependent predation (Van der Veer 1986, Van der Veer et al. 1991, 2015, Nash & Geffen 2012). In other taxa, shifts towards density‐dependent regulation in young‐of‐the‐year juvenile stages were demonstrated, e.g. in the lateolabracid Lateolabrax japonicus (Shoji & Tanaka 2007a, 2008) and moronid Morone saxatilis (Kimmerer et al. 2000, Martino & Houde 2012).
3.3.4.1 Rates and variability
Temperature is most cited as a factor controlling rates of growth in larval fishes and contributing to variable growth (Strydom et al. 2014a, Houde 2016). Additionally, it is clear that prey availability also exercises control over growth (Peck et al. 2012b), although the evidence is not always clear‐cut (e.g. Leggett & Deblois 1994). For estuary‐associated taxa, reported mean, weight‐specific growth rates (G) of larvae vary at least 16‐fold (G = 0.022 to 0.365 d−1), with taxa from warm estuarine systems growing fastest (Houde & Zastrow 1993). In the across‐taxa synthesis, expected mean values of G for estuary‐associated taxa increased by approximately 0.01 d−1 (i.e. ~1%) per 1 °C increase in temperature (T), i.e. G = −0.0236 + 0.0098T. This across‐taxa relationship was similar to that for all marine fish larvae (Houde & Zastrow 1993).
Growth rates of individual taxa also vary widely and are responsive to temperature. For example, weight‐specific growth rates of the moronid Morone saxatilis larval cohorts in Chesapeake Bay varied from 0.15 to 0.35 d−1 (equates to 0.19 to 0.39 mm d−1) and were directly responsive to temperatures ranging from 14–24 °C (Rutherford & Houde 1995). The consequences of variability in growth rates are indicated in stage durations of M. saxatilis larvae. At 14 °C, larval M. saxatilis require 58 days to grow from 4 to 15 mm TL, but only 28 days at 24 °C. In the sillaginid Sillaginodes punctatus, early larval growth in offshore environments is positively related to temperature, as is subsequent level of recruitment in Port Phillip Bay, Australia (Jenkins & King 2006). Although a key variable for most taxa, temperature may have relatively small effects on larval growth rates of some species, for example in the pleuronectid Pseudopleuronectes americanus, in which other site‐specific factors determined variability in rates of growth in New Jersey (USA) estuaries (Sogard et al. 2001). In another example, post‐settlement Pleuronectes platessa juveniles in European estuarine and coastal nurseries exhibited inter‐annual variability in growth that depended on temperature, but within‐year variability in growth rates was nursery‐specific and apparently highly dependent on factors other than temperature (Ciotti et al. 2014). Given higher mortality rates and selective predation on small or slow‐growing young fish (Sogard 1997), variability in growth and stage duration during early life potentially can control levels of recruitment.
Mortality rates of early‐life stages often exceed 10% d−1 and larval‐stage cumulative mortality often exceeds 99% (Houde 2002). Predation usually is presumed to be the major direct cause of mortality to fish eggs and larvae (Bailey & Houde 1989), but environmental factors at lethal or stressful levels also contribute to mortality and are discussed in examples and case studies presented below. The highest mortality rates of estuary‐dependent species, as for most marine species, generally occur in the egg and early‐larval stages. These high and variable rates may act to coarsely set levels of recruitment in estuary‐dependent fishes, as in the pleuronectid Pleuronectes platessa (Van der Veer 1986, Nash & Geffen 2012). Size‐ or growth‐rate selective mortality, usually from predation, not only can obscure starvation as the source of mortality (by selective removal of small or slow‐growing individuals) but it also contributes to shifts in size distributions, age structure and apparent growth rates of survivors, complicating interpretation of dynamics in early life (Houde 2002, Houde & Bartsch 2009).
For
