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 and survival of estuarine fish larvae (e.g. Rutherford & Houde 1995, Morongiello et al. 2014, Houde 2016). For example, in the Baltic Sea, temperatures above the physiological optimum were found to reduce survival of larval Clupea harengus (Arula et al. 2016). Further, decreasing larval growth rates and productivity of larvae occurred at temperatures >17 °C, a level that has occurred frequently in the past decade (Moyano et al. 2020). A thermal threshold index for the Baltic Sea, based on the number of days above optimal 16 °C during the spawning season (March–June) has increased in recent decades. Growth rates and postlarval abundance of the estuary‐dependent sillaginid Sillaginodes punctata are strongly and positively related to temperature (correlation >0.80) for larvae that ingress from offshore Australian waters to Port Phillip Bay (Figure 3.14). Recruitment outcomes for S. punctata are determined by temperature and offshore transport that is most favourable at relatively high temperatures (Jenkins & King 2006). In North American estuaries, low temperatures during winter threaten recruitment success of some fishes, e.g. the sciaenid Micropogonias undulatus (Hare & Able 2007, Hare et al. 2010) and the scophthalmid Scophthalmus aquosus (Neuman & Able 2003).
Figure 3.14 Growth rates (mean otolith increment widths, open circles) of larval stages of the sillaginid Sillaginodes punctata sampled after ingress to Port Phillip Bay, Australia, and postlarval abundances (closed circles) in 1998–2003 showing the strong correspondence between growth rate and abundance. Spawning by S. punctata occurs in the coastal ocean and larval growth rates are strongly related to temperature
(from Jenkins & King 2006, their figure 3).
Reviewing literature on factors affecting recruitments of estuary‐associated fishes, Martinho et al. (2012) reported that temperature and river flow were amongst the best predictors of recruitment potential of estuarine fishes. Levels of river flows usually were positively related to recruitment success. As examples, the moronid Morone saxatilis in Chesapeake Bay (Martino & Houde 2012) and the percichthyid