et al. 2012). Alternatively, anthropogenically altered and excessive freshwater flow from interbasin water transfers can adversely affect nursery function (Strydom et al. 2002).
3.5.4 Power plants
Power plants for electricity generation frequently have been built on estuaries or their tributaries (e.g. Kennish & Lutz 1984, Cattrijsse et al. 2002) and may threaten production and recruitment of estuarine fishes (McLusky & Elliott 2004), including anadromous salmonids and alosines (Taverny 1990, Costa et al. 2002). For decades it has been recognised that these plants present multiple threats to reproduction and recruitment of resident and anadromous fishes through (i) habitat destruction/modification during power plant construction; (ii) releases of effluent cooling water with temperatures far above ambient that may exceed tolerances of fish eggs and larvae; (iii) entrainment and impingement of early‐life stages in waters drawn from the estuary to cool condensers; (iv) discharge of chlorinated by‐products where antifouling methods are used and (v) effects on behaviour of spawning adults and recruiting juveniles. Precautions and regulations usually limit the extent of damage from power plant operations (Barnthouse 2013), but they are, in some instances, a substantial source of mortality to young fishes (NRDC 2014), for example through impingement of dominant species, including the osmerid Osmerus eperlanus and clupeids Clupea harengus and Sprattus sprattus (Greenwood 2008).
3.5.5 Estuary contaminants: water quality degradation
Contaminants and toxins are frequent components of estuarine waters, especially where the surrounding watershed is extensively developed or industrialised (Lawrence & Hemingway 2003, Day et al. 2013, Weis 2014). Still, we often have poor understanding of contaminant effects on estuarine fish reproduction and recruitment. Potential contaminants are diverse and can include those known for decades, e.g. metals, petroleum hydrocarbons, pesticides, industrial chemicals, nutrients and sewage (Weis 2014). Other contaminants are only recently recognised, for example endocrine disruptors that can influence sex determination (Rochman et al. 2014) and microplastics (Oliveira et al. 2013, Rochman et al. 2013, Critchell et al. 2019) that may impact reproduction or recruitment. Research currently underway in South African surf zones shows extensive consumption of microplastics by an estuary‐associated mugilid Chelon richardsonii. Its larval stage feeding in surf zones is at risk for high consumption of microplastics entrained in rip channels (McGregor & Strydom 2020).
One exception to our limited knowledge of contaminant effects on estuarine‐occurring fishes is that for the fundulid Fundulus heteroclitus, which has been well studied in laboratory research. Exposure to mercury causes reduced fertilisation success (Khan & Weis 1987a, 1987b), embryonic deformities (Weis & Weis 1977a, 1977b) and reduced larval swimming and feeding ability (Zhou et al. 2001). Mercury also causes reduced predator avoidance by the larvae (Zhou & Weis 1998) and diminished prey capture ability by juveniles (Smith & Weis 1997). The accumulated evidence indicates that contaminated nursery sites contribute to reduced recruitment of this common species because of higher mortality in early‐life stages, slower growth and reduced condition and longevity (Weis et al. 2001). In an urbanised estuarine nursery area in South Africa, it was shown that select species of juvenile fishes, including mugilids, targeted by fishers for consumption, were already contaminated with cadmium and other metals. The long‐term effects of heavy metal contamination on reproductive biology of estuary‐occurring species are relatively poorly understood worldwide and need to become a focus in the Anthropocene.
Acidification of estuaries and coastal waters is an emerging threat to reproduction and recruitment of estuary‐associated fishes (Wallace et al. 2014). Acidification and its impacts on reproduction of anadromous fishes on the east coast of North America and in tributaries of the Baltic were identified as a substantial threat (Hall 1987, Hendrey 1987, Urho et al. 1990).
3.5.6 Eutrophication
Eutrophication via inputs of nutrients presents an increasingly common threat to reproduction of coastal and estuary‐associated fishes (FAO 1995, NRC 2000, Elliott and de Jonge 2002). While effects on fish production and yields to fisheries are not always clearly demonstrated, increasing nutrient loadings from agricultural run‐off, industrial wastewater and urban sewage (Hondorp et al. 2010) lead to eutrophication. Excessive nutrient loadings are closely tied to low dissolved oxygen levels (hypoxia) that threaten reproduction and recruitment of fishes (Holt 2002, Breitburg et al. 2018). A common consequence of eutrophication of many estuaries is the development or expansion of hypoxia (Wannamaker & Rice 2000, Buzzelli et al. 2002, Gray et al. 2002, Baird et al. 2004, Long & Seitz 2009). While hypoxia in estuaries has likely been present for millennia (Cooper & Brush 1991), the frequency, duration and spatial extent are increasing in recent decades, with interactive and sometimes complex impacts on estuarine fishes and their reproductive and recruitment potential.
In some estuaries, hypoxia adversely affects food habits of juvenile fishes (Pihl et al. 1992), growth (McNatt & Rice 2004, Stierhoff et al. 2006) and may elevate predation on fish larvae (Breitburg 1992, Breitburg et al. 1994, Shoji et al. 2005a) as well as cause direct mortality (Secor & Gunderson 1998, Shimps et al. 2005). A large portion of bottom waters in Chesapeake Bay is hypoxic (<2 mg L−1 dissolved oxygen) during summer months, rendering its deep waters inhospitable to organisms (Kemp et al. 2005) and strongly affecting abundances and depth distributions of larvae of the gobiid Gobiosoma bosc and engraulid Anchoa mitchilli, which avoided hypoxic bottom waters (Keister et al. 2000). Amongst the many experimental studies demonstrating impairment of growth and survival of young fishes under low dissolved oxygen conditions, impairment of growth is observed in estuary‐associated juvenile fishes. For example, in the pleuronectid Pseudopleuronectes americanus and the paralichthyid Paralichthys dentatus, growth rates declined by 25–50% under hypoxic conditions (Stierhoff et al. 2006).
An increasingly common consequence of excessive nutrient loadings and eutrophication in estuaries and coastal waters worldwide (Codd 1998) are harmful algal blooms (HABs). HABs are toxin‐producing micro‐phytoplankton that can kill fish, shellfish and other estuarine organisms. For example, high biomass blooms in eutrophic estuaries in South Africa cause oxygen supersaturation during bloom peaks followed by anoxia during decay, which impedes growth and nutritional condition of the larvae of the estuarine clupeid Gilchristella aestuaria, an important planktivore in these estuaries (Smit et al. 2021). Many species of dinoflagellates and cyanobacteria and some diatoms are implicated in HABs (Glibert 2016). While massive fish kills are frequently reported, effects on reproductive success and recruitment of estuarine fishes are less often documented. Persistent ‘red tides’ caused by blooms of the toxic dinoflagellate Karenia brevis in Tampa Bay (Florida) have disrupted spawning and reproductive success in the estuarine sciaenids Cynoscion arenarius, C. nebulosus and Bairdiella chrysoura (Florida Fish and Wildlife Commission 2013, Walters et al. 2013). In an example from the Baltic Sea, potentially toxic cyanobacteria (such as Nodularia spumigena, Microcystis spp. and Anabaena spp.) often cause HABs (Balode and Purina 1996). Nodularia spumigena may release its toxins during the spawning and hatching periods of the spring‐spawning and autumn‐spawning clupeid Clupea harengus (Ojaveer 1988). Microcystis aeruginosa and N. spumigena exert a harmful influence on the embryonic development and hatching of Baltic C. harengus attributable to abnormal development and high embryo mortality (Ojaveer et al. 2003).
3.5.7 Climate change
Effects of ongoing climate change on reproduction of estuary‐dependent and ‐associated fishes will be substantial for some species (see Sections 3.2.2, 3.4.2 and Gillanders et al. 2022). Rising temperatures,
