(Morongiello et al. 2014) experience better recruitment under conditions of elevated precipitation and river discharge. However, effects of freshwater flow may be negative for other species (e.g. Ramos et al. 2006). Very high flows in small South African estuaries result in temporarily reduced abundances of recruiting marine (Whitfield & Harrison 2003) and estuarine‐resident species (Strydom et al. 2002). In the lateolabracid Lateolabrax japonicus, exceptionally high levels of river discharge reduce its recruitment levels in the Ariake Sea‐Chikugo River (Japan) estuary (Shoji et al. 2006). Other factors may interact with flow and precipitation. For example, larval recruitment of the pleuronectid Platichthys flesus to the Lima Estuary (Portugal) was strongly negatively related to coastal sea‐surface temperature, positively related to coastal chl‐a and weakly but positively related to precipitation and freshwater flow (Amorim et al. 2016).
In salmonids, temperature, dissolved oxygen and precipitation are three factors frequently identified as exercising control over early‐life survival and recruitment. Salmonids provide exceptional examples due to their unique early‐life history. For these anadromous fishes, most mortality occurs during the period when the eggs and/or fry are buried in their gravel nests (redds) in freshwaters above estuaries (Quinn 2018). Several responses are clear. First, the rate of embryonic and fry development is clearly temperature dependent (Quinn 2018). For example, the number of days to hatch and the days to emergence from the gravel nest depend on temperature and vary with species. In some instances, extreme high or low temperatures can be lethal. Second, dissolved oxygen can influence survival, with lower levels a frequent source of mortality. This can occur when groundwater has lower levels of dissolved oxygen than surface waters (Quinn 2018). Furthermore, if interstitial spaces in the redd are filled with fine silts, the flow of oxygenated water is reduced (Peterson & Quinn 1996), which is particularly important because the time from egg to fry emergence in salmonids can range from weeks to months, during which time eggs and embryos can be susceptible to stress from low dissolved oxygen. Moreover, the oxygen demand increases during embryonic development (Quinn 2018). Thirdly, scouring of nests by high water currents, often closely tied to levels of precipitation, may displace embryos, exposing them to predators and other sources of mortality (Quinn 2018).
Air temperature during late winter is the primary environmental indicator of recruitment variability in spring‐spawning Clupea harengus in the Baltic Sea. Winter‐spring temperatures and other climate variables exercise control during the period of highest larval mortality, mainly by controlling production of planktonic prey (Ojaveer et al. 2011). The dependence of recruitment on spawning stock biomass (SSB) varies amongst years with differing temperature conditions. In years of cold winters, environmental conditions are the dominant factor controlling year‐class abundance of C. harengus, and spawning stock biomass is not important. In milder winters, the importance of spawning stock biomass increases and is significantly related to recruitment success.
In another example, enhancement of anguillid eel recruitment under high‐flow conditions is especially evident for the glass‐eel stage of the catadromous Anguilla rostrata (Sullivan et al. 2006). Reduced freshwater discharge during droughts can diminish estuarine plumes and associated cues onto the continental shelf that may facilitate estuarine recruitment of fish larvae from offshore (Baptista et al. 2010). Some droughts can induce fish kills in estuarine nurseries, likely due to synergistic effects of hypoxia and resulting in diminished availability of food (Wetz et al. 2011). Effects of prolonged drought and its negative consequences for the nursery function and larval fish assemblages in the Murray‐Darling Estuary (Australia) have been documented (Bucater et al. 2013). Recruitments of pelagic fishes in the San Francisco Estuary (USA) are negatively impacted by years of prolonged drought (Sommers et al. 2007). Similarly, cessation of river flow into certain South African estuaries due to freshwater abstraction and the resultant loss of cues to the marine environment has been suggested as the major reason for reduced ingress and recruitment of estuary‐associated, marine postlarvae into such estuaries (Whitfield 1994).
Event‐scale weather can generate variability in recruitment success and may have more impact in estuaries (Stevens et al. 2006, Biggs et al. 2018, Massie et al. 2019) than in the ocean where effects of such events may be dampened, diluted and buffered by the large volume. In the moronid Morone saxatilis, major storms with high precipitation and excessive river discharges, often associated with temperature drops, may result in episodic, down‐estuary losses of eggs or larvae and temperature‐related mortalities in the Hudson River and Chesapeake Bay (Dey 1981, Rutherford et al. 1997). Sudden drops in water temperature (<14 °C) also cause mortalities of young fishes in tropical estuaries (Kyle 1989, Cyrus & McLean 1996). Storm‐related impacts do not always have negative outcomes. For example, there was no indication of increased mortality of the sciaenid Micropogonias undulatus larvae attributable to Hurricane Isabel in Chesapeake Bay; in fact, levels of ingress by M. undulatus larvae from offshore increased in the aftermath of this major storm (Houde et al. 2005, Montane & Austin 2005). Similarly, larval fish recruitment of certain species into the St Lucia estuarine system (South Africa) appeared to be enhanced following Cyclone Domoina (Forbes & Cyrus 1992). The effects of Hurricane Sandy on juvenile fishes in Barnegat Bay (USA) were not discernable despite annual sampling before and after the storm (Valenti et al. 2020).
3.4 Adults and recruitment
Establishing a relationship between adult spawners and numbers of recruits has been an objective of fishery scientists and managers for decades (Cushing 1981, Rothschild 1986, 2000, Subbey et al. 2014), primarily to support fishery management and to document the status and trends in stock trajectories. Gaining an understanding of recruitment variability, its causes and degree of dependence on adult stock abundance is important to evaluate a stock's production potential and, for exploited species, to manage its fisheries, including those for estuarine species. These efforts often are confounded by high inter‐annual variability in reproductive success that is attributed to unpredictable and changing environmental factors (Myers 1998) that mask relationships of recruitment to parent stock.
While recruitment is often (usually) poorly related to adult stock, abundance and age structure of the spawning stock, especially at low stock abundance, can be a key factor controlling levels of recruitment (Myers 2001, Cury et al. 2014, Subbey et al. 2014, Houde 2016). Documenting a clear linkage between adult stock (numbers, biomass or fecundity) and recruit numbers has been elusive for most marine and estuarine species (Subbey et al. 2014, Szuwalski et al. 2015, Lowerre‐Barbieri et al. 2016, Somarakis et al. 2019, Sharma et al. 2019). In many cases, only at the lowest levels of adult stock abundance can recruitment be demonstrated to depend on spawner abundance or biomass (Hilborn & Walters 1992, Myers 2001). Furthermore, in many circumstances it is actually the abundance of adult stock that varies in its dependence on recruitment success (Szuwalski et al. 2015).
Intensive research has been directed at explaining dependence of recruitment on adult stock in the decades following development of stock‐recruitment models (Ricker 1954, Beverton & Holt 1957, Rothschild 1986, Myers 2001, Subbey et al. 2014) and continuing to the present day (Szuwalski et al. 2015, Sharma et al. 2019). Substantial knowledge has accumulated on scales and causes of recruitment variability, the role of environmental factors, role of adult stock and age structure and the effects of fishing. Understanding the relationship of recruitment to adult stock is not unique to fishes in estuarine ecosystems, but explaining and understanding causes of variability may be exacerbated in estuaries where a multitude of environmental and anthropogenic factors, in addition to fishing, act to modify stock‐recruitment relationships.
3.4.1 Adult stock
Adult abundance, population age structure, nutritional condition and access to spawning areas exercise controls over egg production and quality and initial early‐life stage abundances. Adults of many fishes that spawn in or utilise estuaries are highly vulnerable to fishing (e.g. alosines, salmonids, moronids, sparids, percids), leading to depleted adult abundances and to ‘recruitment overfishing’, a circumstance in which too few adults remain in the population to assure adequate replenishment of the stock. The depletion of the moronid Morone saxatilis on the east coast of North America from