2017). Residual currents, winds and tides and fronts and eddies are key processes and features that support reproduction and recruitment processes. Two major life‐history strategies, both dependent on currents and hydrography, are recognised: (i) estuarine spawners that select spawning sites and features to assure retention or dispersal of eggs and larvae within the estuary and (ii) offshore spawners that spawn in areas favourable for dispersal and directional transport of early‐life stages towards estuarine nurseries. Descriptions of hydrodynamic processes and physical features supporting recruitment, including examples for estuarine fishes, are described in Section 3.3.1. Physical processes not only assure connectivity amongst habitats in the recruitment processes of estuarine‐dependent fishes (e.g. Pineda et al. 2007) but also support marine and estuarine productivity that are closely linked to trophodynamics in the early life of fishes (e.g. Cushing 1990).
3.2.2.3 Foods of early‐life stages
Food availability for first‐feeding larvae historically has been recognised as critical for growth and survival of marine fish larvae (Hjort 1914, Cushing 1990, Whitfield et al. 2022b). Here we describe foods of larvae, deferring discussion of the feeding process and nutrition to Section 3.3.3.1. As in most marine fish larvae, foods of estuarine‐associated larvae are dominated by zooplankton, mainly copepods (Houde & Alpern‐Lovdal 1984, Llopiz 2013, Strydom et al. 2014b, Bornman et al. 2019) but may shift to include benthic invertebrates or larger pelagic prey as larvae grow and transition to the juvenile stage (e.g. Houde & Alpern‐Lovdal 1984, Islam et al. 2006, Campfield & Houde 2011, Arula et al. 2012a, Bils et al. 2017). Those species that are detritivorous as juveniles and adults also have larvae that are zooplanktivorous, often showing a gradual shift in diet from zooplankton, through small vertically migrating invertebrates, for example mysids, to small benthic organisms during the very early‐juvenile stages (Blaber & Whitfield 1977, Elliott et al. 2002). The feeding behaviour, including consumption levels, of larvae undergoing metamorphosis and ingress into estuaries may change during this transition (Deary et al. 2017). For example, in the paralichthyid Paralichthys dentatus, the evidence of feeding was reduced during eye migration (Grover 1998).
A review of young‐of‐the‐year fish diets for 47 estuarine species in the northeastern USA indicated seven categories of invertebrates were important prey (copepods, amphipods, mysids, decapod shrimp, polychaetes, crabs and insects/arachnids) (Able & Fahay 2010). Copepods were important prey for 33 species, amphipods for 27 species, mysids for 26 species, decapod shrimp for 21 species, polychaetes for 17 species, decapod crabs for 13 species and insects/arachnids for 11 species. Other prey categories frequently eaten were isopods, bivalves, gastropods, ostracods, invertebrate eggs, nematodes, cnidarians, algae and cladocerans. The occurrence of insects and arachnids in diets of juvenile estuarine fishes points to the potential importance of allochthonous foods for some species. The importance of microplankton and protists in larval fish diets, including estuarine fish larvae, has become clearer with the advent of improved diagnostic methods (e.g. Zingel et al. 2019).
The importance of copepods in feeding by estuarine fish larvae is illustrated for the clupeid Clupea harengus in the Baltic Sea where the copepod Eurytemora affinis dominated the diet of larvae, with an annual occurrence of 58–92% (Arula et al. 2012a). The significance of copepods in the diet of larval C. harengus also is reported in other studies, although the copepod species eaten differed amongst regions. For example, E. affinis dominated in the Baltic Sea (Schnack 1974); Acartia spp. in the Blackwater Estuary, England (Fox et al. 1999); Pseudocalanus sp., Acartia sp., Temora sp., Oithona similis and Centropages sp. in the North Sea (Fossum & Johannessen 1979, Checkley 1982); and Calanus finmarchicus in the North Atlantic (Fiksen et al. 2002). In South African estuaries, larvae of the estuarine‐dependent clupeid Gilchristella aestuaria fed extensively on eggs of the dominant copepod P. hessi (Whitfield & Harrison 1996) and larvae of five species in families Clupeidae, Haemulidae, Monodactylidae, Mugilidae and Sparidae fed predominantly on copepods (Whitfield 1985). Rotifers can also be an important prey item for first‐feeding larvae of C. harengus in the Baltic Sea (Margonski et al. 2006). In the Baltic clupeid Sprattus sprattus, the copepod Acartia sp. is important in the larval diet (Voss et al. 2003, Dickmann et al. 2007) and seasonal comparison of prey fields suggested that cladocerans become increasingly important during the summer months.
3.2.2.4 Predators
The role of predation in estuarine food webs has been frequently researched (Crowder et al. 1997, Juanes et al. 2002, Krause et al. 2002, Craig et al. 2007), but we still lack a unifying understanding of major predators and their roles in controlling or regulating fish recruitment. The frequency of predation on larval and juvenile stages of fishes is evident from a summary of adult and juvenile fish stomach contents in temperate estuaries of the US east coast. In this summary, larval and juvenile fishes were important prey in 34% of the species analysed, and they occurred in the diet of 72% of the analysed species (Able & Fahay 2010). There are limits to evaluating predation from a visual approach in gut analysis on predators, especially for small, easily digested fish eggs, larvae and small juveniles. For example, the evidence of piscivory by Fundulus heteroclitus was limited by fast digestion times of small conspecifics, with this prey unidentifiable in 1 hour and completely evacuated in 6–7 hours (Able et al. 2007). Furthermore, significant spatial variation in diets, as along estuarine salinity gradients (Able et al. 2017), occurs for some large predators and can confound understanding of this source of mortality.
Small fishes and jellyfish taxa are amongst the most important predators on early‐life stages of fishes (Purcell & Arai 2001). In Chesapeake Bay, the ctenophore Mnemiopsis leidyi, the cnidarian Chrysaora chesapeakei (previously C. quinquecirrha) and planktivorous fishes were demonstrated to be important predators on larvae of Anchoa mitchilli and Gobiosoma bosc (Cowan & Houde 1992, 1993, Breitburg et al. 1994, Purcell et al. 1994). In the Baltic Sea, demersal eggs of Clupea harengus are vulnerable to resident fish predators, e.g. Gasterosteus aculeatus and Neogobius melanostomus (Kotterba et al. 2017b).
Environmental factors may modify ability of predators to consume fish eggs and larvae. For example, under low dissolved oxygen conditions, jellyfishes increased their predation capability on estuarine fish larvae, while pelagic fishes had reduced capability (Breitburg et al. 1994, Shoji et al. 2005a). Piscivores, mostly fishes but also birds and mammals, are an important source of mortality to juveniles of estuarine forage fishes (Baker & Sheaves 2009b, Able & Fahay 2010). In another example of predation on juveniles of estuarine fishes, predation by large Morone saxatilis occurs on smolts of Salmo salar in Canadian systems (Daniels et al. 2018). Moreover, large piscivores can consume substantial quantities of young‐of‐the‐year individuals that are pre‐recruits of species exploited in managed estuarine fisheries, as reported for Chesapeake Bay (Ihde et al. 2015).
3.2.2.5 Weather, climate and estuarine change
Extreme weather events that already occur naturally and climate variability or change (Gillanders et al. 2022) that may exacerbate such weather events can act to elicit reproductive or recruitment responses either directly or indirectly (Vinagre et al. 2009, Feyrer et al. 2015, Houde 2016, Jeffries et al. 2016; Elliott et al. 2019). The events may cause temperature shifts, flooding, freezes and droughts and result in higher salinity in the estuary. Such events may produce episodic low dissolved oxygen, habitat destruction and poor water quality. Weather (event‐scale) as a factor may be more important in estuaries than in the ocean where effects of weather are diluted and buffered by the large surface area and volume. Event‐scale weather may be especially important with respect to wind‐induced advection and larval ingress into estuaries (Hare et al. 2005a, Schieler et al. 2014, Bruno & Acha 2015).
Long‐term changes such as inter‐annual variability in ingress patterns