estuary‐dependent fishes ranged from 0.05 to 0.52 d−1 (4.9 to 40.5% d−1) and expected mortality increased by approximately 0.01 d−1 for each 1 °C increase in temperature (M = 0.0277 + 0.0137T), a rate similar to that for weight‐specific growth rate (G) of estuarine fish larvae. In the Houde & Zastrow (1993) synthesis, rates of M and G of individual species of estuary‐dependent larval fishes were positively correlated, indicating a strong, although coarse, concordance between the rate of instantaneous mortality (M) and G that depends on an ecosystem's temperature.
When recruitment outcomes of marine and estuarine fishes are evaluated with respect to stage‐specific survival, the levels and variability of mortality rates during the early‐larval stage were, in many cases, found to be the most important determinant of recruitment level (e.g. Bannister et al. 1974, Secor & Houde 1995, Martino & Houde 2012, Van der Veer et al. 2015). Mortality rates of juvenile estuarine fishes are lower than that for eggs and larvae (Bergman et al. 1988, Scharf 2000, Martino & Houde 2012, Nash & Geffen 2012). A long juvenile stage (weeks to years, depending on species) can, however, generate high and variable cumulative mortality that may determine, and can adjust or stabilise, recruitment levels (Beverton & Iles 1992, Kimmerer et al. 2000, Houde 2008). In temperate and high‐latitude estuaries, overwinter mortality of pre‐recruit juveniles is particularly important as a process that regulates recruitment, in which age‐0+ juveniles in poor nutritional condition experience high and size‐selective mortality (Hurst & Conover 1998, Hare & Able 2007, Hurst 2007).
Mortality of marine organisms generally declines with size in a predictable, if variable, way (e.g. Peterson & Wroblewski 1984) where M = aW b with the exponent b relating decline in mortality (M) relative to mass (W) theoretically equal to −0.25. In an analysis on larvae that included three estuary‐dependent taxa (Houde 1997b), Anchoa mitchilli, Alosa sapidissima and Morone saxatilis, mortality rates did decline with respect to mass. But, rates of decline were faster than predicted (exponents −0.32 for A. mitchilli; −0.39 for A. sapidissima and −0.42 for M. saxatilis), indicating that the probability of dying declines rapidly with respect to ontogeny and growth. The exponent b in the relationship of M to W for larvae of these three species also varied inter‐annually. The ratio of instantaneous mortality rate (M) to weight‐specific growth rate (G) is the physiological mortality rate (Beyer 1989). The trend in this ratio for the three species in the example, with respect to ages or sizes at which M/G became <1 (i.e. the ‘critical size’, an indicator that a cohort or year class was gaining biomass) varied amongst the species (Houde 1997b). The levels of M/G, and its trend in the earliest days posthatch, identify cohorts that have the potential to contribute strongly to recruitment (Cowan et al. 2000).
Estimating mortality rates of early‐life stages is difficult, especially so in large, open marine systems (Houde 2002) where the earliest life stages of many estuary‐dependent and ‐associated fishes are found. Within estuaries, estimating mortality is more tractable, particularly so in small systems that can be quickly and repeatedly sampled, or for which estuarine retention times are known. Dispersion losses in the estuary may be less problematic in biasing abundance estimates of eggs and larvae than in continental shelf and open ocean systems. Dispersion losses may constitute a large fraction of mortality when eggs or larvae are advected away from nursery areas and may confound estimates of mortality (Helbig & Pepin 1998).
In well‐designed studies, some estuaries can be rapidly sampled for eggs and larvae within time periods much less than estuary residence times, facilitating estimation of egg and larval abundances. In estuaries, experimental approaches also are possible to evaluate growth and estimate mortality of larvae and small juveniles. For example, releases of hatchery‐produced, chemically marked larvae can be effective to conduct mark‐recapture research on early‐life stages in estuarine systems (Tsukamoto 1985, Tsukamoto et al. 1989, Secor et al. 1995, 2017).
Stage durations
Variability in growth rates translates into variability in larval‐stage durations, with longer durations generally occurring in colder estuarine ecosystems. Egg and larval‐stage durations are variable, perhaps more so in estuarine fishes than in species from ocean habitats, owing to effects of weather, especially temperature, precipitation and freshwater flow variabilities that can quickly alter an estuary's water quality and potential to support production of early‐life stages of fishes. The stage‐duration hypothesis (Cushing 1975, Anderson 1988) indicates that fast growth and short stage durations during early life are associated with higher recruitment potential. In many, but not all cases, shorter stage durations are linked to higher survival and, ultimately, higher recruitment (Leggett & Frank 2008, Houde 2016).
Duration of the larval stage differs amongst species and is dependent on growth rate. For example, in the moronid Morone saxatilis, early‐larval‐stage duration (at 8 mm TL) in Chesapeake Bay ranged from 13 to >40 days, an effect primarily attributable to temperature on growth rates (Rutherford & Houde 1995). In laboratory experiments on larval pleuronectiforms, 9–25 mm Paralichthys dentatus and Pseudopleuronectes americanus were highly vulnerable to predation by Crangon septumspinosa shrimp, suggesting that stage duration of newly settled individuals in that length range, and not only the daily mortality imposed by the predator, could have a substantial effect on cumulative mortality and recruitment (Witting & Able 1993).
Across taxa, stage durations (D) of estuarine fish larvae decline rapidly and non‐linearly with respect to increasing temperature (Houde & Zastrow 1993): D = 462.28T −0.85. For typical, estuary‐associated taxa, reported larval‐stage durations range from at least 19 to 100 days (Houde & Zastrow 1993). Temperature and prey availability are the two factors contributing most to variability in stage duration (Blaxter 1992, Houde & Zastrow 1993). Those factors may control recruitment levels through effects on the cumulative mortality during variable larval‐stage durations (Cushing 1975, Houde 1987, 1997a, Anderson 1988, Pepin & Myers 1991).
Mortality rates usually decline as larvae transition to juveniles, but juvenile stage durations typically are longer than larval‐stage durations (Houde 1997b, 2002). Juveniles of some exploited estuarine fishes may spend months or years in a pre‐recruit stage, providing ample time to incur variable cumulative mortalities that can significantly tune or regulate levels of recruitment.
3.3.4.2 Predation
Predation is known or presumed to be the principal source of mortality on early‐life stages of marine and estuarine fishes. As such, predation plays a major role in controlling and regulating recruitment (Bailey & Houde 1989). Predation mortality on the earliest life stages of fishes is generally attributable to planktonic predators, both invertebrates and fishes. Jellyfishes, carnivorous zooplankton and other fishes may be the major predators on eggs, larvae and juveniles (Bailey & Houde 1989, Houde 2016). Laboratory experiments conducted with jellyfish or fish predators, together with modelling research, have often quantified the predation process, estimating vulnerabilities of eggs and larvae, and sometimes predicting consequences for recruitment (Cowan & Houde 1992, 1993, Breitburg et al. 1994, Purcell et al. 1994, Shoji et al. 2005a). For example, in research conducted in large experimental enclosures, high predation mortality was inflicted on the engraulid Anchoa mitchilli eggs and the gobiid Gobiosoma bosc larvae by jellyfishes and juvenile fishes at abundance levels typically recorded in Chesapeake Bay (Cowan & Houde 1993, Houde et al. 1994), indicating that upwards of 20% d−1 of A. mitchilli eggs and larvae could be lost to mortality from jellyfish and juvenile fish predation.
However, in research conducted in the estuary or other marine environments, it has been difficult to quantify predation mortality or to partition it amongst predators. Quantification of predation mortality on early‐life stages of fishes is complex in trophically diverse, size‐structured communities with multiple predators and prey of mixed sizes and behaviours. Accordingly, while stomach