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, changes in estuarine hydrography, rising sea level, increasing instances of hypoxia and acidification and associated shifts in spawning phenology are likely to be major effects (Ojaveer & Kalejs 2005, Nye et al. 2009, 2014, Gillanders et al. 2011) as climate change continues to affect shallow estuarine systems (James et al. 2013, Wallace et al. 2014, Elliott et al. 2019). Related, naturally occurring weather events also can pose hazards to reproduction of estuary‐associated fishes (see Section 3.2.2.5). A recent review of climate‐induced effects of acidification on settlement and metamorphosis of fish early‐life stages indicated that, based on a relatively few estuary‐associated species, these key life‐history processes could be compromised, e.g. in the catadromous latid Lates calcarifer (Espinel‐Velasco et al. 2018) although effects might not be realised for many decades. Incidence of harmful algal blooms (HABs), noted above, that may disrupt and potentially reduce reproductive success of estuarine fishes is likely to increase in response to changing climate (Glibert 2016).
The influence of changing climate is evident in a 26‐year time series of larval ingress for fishes in a Mid‐Atlantic (USA) estuary, where the number of higher‐latitude species is now reduced and the number of lower‐latitude species has increased (Morson et al. 2019). There are documented northward shifts in distributions and fishery landings of adults of estuary‐associated species, for example the paralichthyid Paralichthys dentatus, on the US east coast (NEFSC 2016, 2019). Although shifts in distribution of the adult stock do not confirm changes in reproduction and/or recruitment, or that populations are migrating in response to climate change, the weight of evidence indicates that climate is a driving force that may threaten reproductive success and cause major shifts in spawning areas and seasons in some species.
3.5.8 Catastrophic events
Human‐caused catastrophes, although relatively uncommon, may reduce reproductive success of estuarine fish populations. The increasingly heavy use of coastlines and estuaries by humans has elevated the risk from hazards, both man‐made and natural (Elliott et al. 2014). Hazards that lead to catastrophes often are acute events, e.g. toxic spills that could kill early‐life stages of fishes or, alternatively, catastrophes may evolve from chronic stressors that have long‐term impacts on habitat and severely reduce reproductive capacity. Amongst the best documented events are those caused by spills of oil in the coastal ocean and estuaries. One of the largest known catastrophes of human origin, the Deepwater Horizon oil spill in the Gulf of Mexico, did not result in any clear effects of the oil on marsh fish population abundance (Able et al. 2015). Nor was it possible to confidently understand negative effects on individuals versus populations, indicating critical knowledge gaps (Fodrie et al. 2014). However, there were clear changes in gene expression and associated gill immunochemistry (Whitehead et al. 2012) that could be indicative of stress on future reproduction. A similar difficulty in interpreting catastrophic effects occurred when trying to evaluate the response of estuarine fishes to Hurricane Sandy in Barnegat Bay (USA) in the Mid‐Atlantic (Valenti et al. 2020).
In southern Africa, the loss of Lake St Lucia as a fish nursery area, particularly for estuary‐associated marine fish species, primarily due to freshwater deprivation and the subsequent evaporation of more than 90% of the surface area of this 35 000 ha estuarine lake ranks as a major catastrophe for coastal fishes (Cyrus et al. 2010). The prolonged closure of the St Lucia Estuary mouth due to human‐induced,