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 analyses of predators that contain eggs, larvae and juveniles of estuary‐dependent fishes are common, reliable estimates of mortality rates from predation on pre‐recruit, estuary‐dependent fishes are only infrequently accomplished (Purcell et al. 1994, Van der Veer et al. 1997). Moreover, while predation may be the dominant cause of mortality on early‐life stages of fishes, the losses to predation in some cases may be attributable to nutritional deficiencies or slow growth that increase vulnerability of pre‐recruit fish to predators.
Historically, estuaries were considered a refuge for young fishes to avoid predation because of high turbidity, complex shorelines, structural habitats that may exclude predators and vegetation or other cover that offers shelter and protection (Levings 2016). For estuarine‐associated species with offshore eggs and larvae, they face a gauntlet of predators as they develop and transition to an estuarine environment (Baker & Sheaves 2009b). However, there are trade‐offs in estuaries, in which predation mortality, even if substantial, is compensated by fast growth and high production that assure high recruitment.
Predation mortality is size‐specific; fish and invertebrate predators consume fish larvae that, on average, are approximately 10% of their body lengths (Paradis et al. 1996). As larvae grow and develop, their swimming ability and predator‐detection capabilities improve, generally leading to lower rates of predation mortality. Mortality from predation on the pleuronectid Pleuronectes platessa eggs is size‐specific, with smaller eggs suffering higher mortality (Rijnsdorp & Jaworski 1990). Predation on fish larvae can be both size‐specific and growth‐rate dependent (Cowan et al. 1997, Takasuka et al. 2007, Houde & Bartsch 2009). For settled, estuary‐dependent pleuronectiforms, predation mortality appeared to be mostly dependent on predator abundances rather than on size‐selective predator behaviour in controlling recruitment levels (Van der Veer et al. 1997), but this conclusion might differ for other fishes. For example, predation on the newly settled sparid Lagodon rhomboides in Galveston Bay (USA) was strongly size‐selective, with survival of recruiting postlarvae skewed towards larger individuals (Levin et al. 1997).
In the Baltic Sea, demersal eggs of the clupeid Clupea harengus are vulnerable as prey to resident predators. In a predator exclusion experiment, a high percentage of C. harengus eggs (42% of all eggs between spawning and hatch) were consumed, primarily by the resident, gasterosteid fish Gasterosteus aculeatus (Kotterba et al. 2017b). In contrast, no significant predation on larvae of C. harengus was detected within the Baltic's Greifswald Bay, indicating that predation on larvae in that habitat may be negligible (Kotterba et al. 2017a). Eggs of C. harengus are appealing prey for the invasive gobiid Neogobius melanostomus in the western Baltic Sea. Investigations indicated that spatio‐temporal overlap between N. melanostomus and spawning by C. harengus, and also the type of demersal habitat and its structure, controlled the level of predation. Only minor predation on C. harengus eggs occurred on sandy, vegetated beds, but predation was higher on beds with a structured, stony bottom (Wiegleb et al. 2019).
Upon entering estuaries, fish larvae and juveniles may encounter a diverse and abundant assemblage of predators, potentially more numerous than in the coastal ocean. The potential for high predation losses within the estuary is notable for juvenile fishes that are prey to piscivorous fishes, birds and mammals (see Able & Fahay 2010 for review from northeastern USA). Mortality from predation on juvenile, pre‐recruit fishes in estuaries can limit levels of recruitment. For example, predation on salmonid fry and smolts in freshwater and tidal nurseries, and during downstream smolt migrations through estuarine systems, may exercise substantial control on recruitment (Daniels et al. 2018), especially on estuarine‐resident Oncorhynchus tshawytscha juveniles that are highly susceptible to predation by piscivorous birds (Bottom et al. 2005b, Evans et al. 2019). While predation is likely an important source of mortality to Pacific salmonids, interpretation of predation impacts often is difficult to distinguish from the multitude of other factors causing pre‐recruit mortality (Grossman 2016). This is the apparent case in the San Francisco Estuary (California), where predation mortality on juvenile O. tshawytscha and on recruits of the endangered osmerid Hypomesus transpacificus, notably by introduced invasive fishes, may contribute to observed declines in recruitment (Nobriga et al. 2013, Grossman 2016, Moyle et al. 2016). Other studies have demonstrated how effects of food limitation on growth of the sciaenid Leiostomus xanthurus resulted in increased mortality due to predation (Craig et al. 2006, 2007).
3.3.4.3 Environmental factors
Fishes that are estuary‐dependent in the egg, larval and juvenile stages often encounter major changes in their environment as they undertake ontogenetic migrations. Early‐life stages of estuary‐dependent and ‐associated fishes are more likely to experience a broader range of environmental variability in shallow estuaries than marine fishes on continental shelves or in the open sea. Accordingly, early‐life stages must be adaptive or tolerant, for example, to shifting environmental conditions. Amongst the most important environmental factors are (i) temperature, (ii) precipitation and associated freshwater flow, (iii) salinity, (iv) measures of estuarine productivity, including prey resources, and (v) water quality, e.g. dissolved oxygen and pH. Within the estuary, environmental factors may act chronically or as episodic events (Houde 1989b) to control survival and growth of eggs and larvae or, in the longer term, to modulate production, particularly of juveniles in the weeks or months of extended, young‐of‐the‐year stages. Environmental factors that are threats to reproduction and recruitment, beyond causing usual variability, are addressed in Section 3.5. As one example, hypoxia is sometimes common in estuaries and may be a chronic stressor or an episodic threat to production and recruitment; it can also hinder or prevent migrations by adult spawners or early‐life stages through estuarine hypoxic zones (Breitburg 2002, Breitburg et al. 2018, Able et al. 2022).
Temperature is often the most important variable affecting growth
