in larvae of the closely related South African sciaenids Argyrosomus japonicus and A. inodorus differed in small ways, but resulted in species‐specific differences in feeding ability (Deary et al. 2015). Ontogenetic shifts in habitats occupied that represented areas with differing prey types and abundances were documented for postflexion larvae and juveniles of the pleuronectid Platichthys flesus in the Lima Estuary, Portugal (Amorim et al. 2016).
Figure 3.6 Illustration of ontogenetic diet shifts in early‐life‐history stages of sciaenid taxa from Chesapeake Bay, USA. Shifts are illustrated with respect to development of the feeding and sensory systems and with respect to transition from pelagic, pre‐settlement habitats to post‐settlement, early‐juvenile habitats. The larval lengths (SL) of the observed dietary shift for each foraging guild are noted over the arrows. Dominant prey types are represented but are not drawn to scale. The line drawings of the oral jaw structures are drawn to scale except for the generalist sciaenid larva illustration. Blue represents the water column and brown represents the benthos
(from Deary et al. 2017, their figure 7).
The coincidence between morphological and physiological transformation and settlement is evident in many temperate estuarine fishes, as well as estuarine species in the South Atlantic Bight of the USA (Hoss & Thayer 1993), the Gulf of Mexico (Yáñez‐Arancibia 1985), Spain (Arias & Drake 1990), South Africa (Day 1981, Beckley 1984, 1986) and Australia (West & King 1996). The typical ontogeny and associated behaviours, however, do not apply to all demersal species. For example, all Urophycis spp. (Phycidae) in the Middle Atlantic Bight of the Western Atlantic have a pelagic juvenile stage and they do not settle until they are older and larger (>23–80 mm) than most other species (Able & Fahay 1998). Others, such as the anadromous salmonids which spawn in freshwater, may use estuaries in their transition from freshwater to the ocean for days, weeks, months or years as parr or smolts, e.g. Oncorhynchus spp. and Salmo salar (Healey 1985, Thorpe 1994, Weitkamp et al. 2014, Levings 2016). Furthermore, the patterns of estuarine use by juvenile salmonids can vary within species and include variation amongst rivers and estuaries (Thorpe 1994, Levings 2016, Quinn 2018).
The culmination of the above developmental patterns influences the degree of estuary dependence in fishes, and thus potentially their survival. For example, in the western North Atlantic, estuary dependence is variable across a variety of scales for a large sample of estuarine fishes (Figure 3.7). Many of these species are obligate users, but the spectrum is broad and not all fishes ingressing into estuaries are dependent on these ecosystems for their growth and survival. In contrast to the above situation, most fish species ingressing into Australian and southern African estuaries use these systems ‘opportunistically’ (Potter et al. 1990). For those species that are dependent on estuaries as nursery areas, the supply of larvae, and sometimes juveniles, that enter estuaries can have a great influence on the numbers of individuals that survive to be reproducing adults.
Figure 3.7 Characterisation of degree of estuary dependence for representative estuarine fishes from the northeastern USA
(based on Able 2005).
3.2.2 Sources of variability in reproductive success and recruitment
Recruitment in estuarine fish is variable, with annual variability sometimes exceeding a factor of ten. In a review, recruitment success of anadromous and estuarine fishes varied on a scale similar to that for marine fishes (Rothschild & DiNardo 1987). Recruitment levels and patterns over time are often adopted by fishery managers as measures of reproductive success. It is probable that estuary‐dependent spawners and pre‐recruit fishes that utilise estuaries have evolved and are adapted to the variable habitats and unstable water‐quality conditions that are common in estuaries and which can threaten reproductive success. On the other hand, the high productivity of estuaries may act positively to support reproduction and recruitment. The nature of spawning migrations by adults of estuary‐dependent fishes and cues that trigger spawning are properties that have evolved to ensure eggs are spawned in appropriate habitat and under favourable environmental conditions (Secor 2015). The sources of variability that act on eggs and larvae, followed by processes acting during the lengthy life stages of juveniles and, finally, adult spawning processes combine to generate the observed variability in reproductive success.
3.2.2.1 Habitat and water quality
Temperature, precipitation and associated freshwater flow are factors frequently identified as exercising control over reproductive and recruitment success in estuarine fishes. Effects are notably prominent on reproductive success of anadromous fishes such as the moronid Morone saxatilis, mugilids, anguillids, alosine species and salmonids (Crecco & Savoy 1984, Blaber 1987, Martino & Houde 2010, 2012, Kettle et al. 2011, Quinn 2018). In a more specific example, the influence of freshwater flow resulting from precipitation is especially evident for ingress of the glass‐eel stage of the catadromous Anguilla rostrata (Sullivan et al. 2006) and Anguilla mossambica (Bruton et al. 1987). Reduced freshwater flow during droughts can diminish the size and extent of estuarine plumes onto the continental shelf and the resulting cues to estuarine ingress of larvae from offshore (Baptista et al. 2010). Low freshwater flows also can reduce the availability and extent of suitable nursery areas within small estuaries, thus reducing abundance of marine fishes that recruit, for example, to South African estuaries (Whitfield & Harrison 2003). Some droughts can induce fish kills in estuaries, likely due to synergistic effects of hypoxia and resulting decline in food availability (Wetz et al. 2011), or increasing hypersalinity as a result of evaporation (Whitfield et al. 2006). In a comparison of two Australian estuaries with differing freshwater discharge, the estuary with higher discharge, and higher dissolved oxygen levels, had higher survival of eggs and larvae of the sparid Acanthopagrus butcheri (Nicholson et al. 2008). Certainly, low temperatures during the winter in temperate estuaries can negatively influence recruitment of the sciaenid Micropogonias undulatus (Hare & Able 2007, Hare et al. 2010). Sudden decreases in water temperature (<14 °C) also cause mortalities in tropical estuaries (Kyle 1989, Cyrus & McLean 1996). In postflexion larvae of the Baltic clupeid Clupea harengus, increases in summer temperatures on nursery grounds to above the physiological optimum negatively affect survival (Arula et al. 2016). Diel variation in hypoxia (<2 mg L−1 dissolved oxygen) may negatively influence reproductive condition, e.g. gonadosomatic index and egg production in Fundulus grandis (Cheek 2011).
Long‐term patterns in recruitment and reproductive success, particularly declines, may be driven by deterioration of estuarine habitat and water quality from human causes (see Section 3.5). Many recruitment patterns of fishes in estuaries heavily utilised by humans actually are long‐term trends of reproductive failures and declines, and some may prove to be ‘one‐way trips’ as human influences on estuaries continue to escalate (e.g. alosines, the endangered osmerid Hypomesus transpacificus, anguillids).
3.2.2.2 Hydrography and physics
Hydrography, hydrodynamics and physical features of estuaries define the spawning environment and habitat that influence successful spawning and retention, dispersal or transport of eggs and larvae of estuarine‐associated fishes (Strydom