and weather patterns (Wood & Austin 2009). Similar recruitment levels are sometimes recorded for a series of years, with the autocorrelated levels responsive to years with similar weather patterns that contribute to either low or high success in reproduction and recruitment.
Some estuary mouths open and close on scales from centuries to years and seasons, and sporadically due to storms. These changes can influence local ingress of estuarine‐associated species and thus survival. This seasonal opening and closing of estuaries is most evident in temperate estuaries of Australia and South Africa. Typically, this occurs when a sand bar forms across the estuary mouth and prevents exchange of water and fishes until a bar‐breach event (Bell et al. 2001, Young & Potter 2002, Whitfield 2019). The response by recruiting fishes is influenced by their access to the estuary (Strydom 2003, James et al. 2007b), to changes in salinity within an estuary (Whitfield et al. 2006) and to bar breaches, allowing a cueing response in plume water for marine larvae, which was found to be independent of measurable salinity changes in the surf zone (Strydom 2003). Aggregation of fish larvae at the mouths of temporarily closed estuaries in South Africa may be dependent on olfactory cues that allow larvae to sense the estuary mouth in anticipation of favourable hydrological conditions that would support ingress (Tweddle & Froneman 2017). Because not all closed estuaries in South Africa are supported by aggregating fish larvae in the surf zone, the potential for ingress appears to be governed by cohorts of available larvae (Strydom 2003). Recruitment of postflexion stage larvae into such estuaries has even been recorded when the sand bar at the mouth is closed, but marine overwash occurs during storm events (Figure 3.8) (Bell et al. 2001, Cowley et al. 2001).
Figure 3.8 Diagrammatic representation of the life cycle and proposed recruitment mechanisms of the sparid Rhabdosargus holubi which spawns in the coastal ocean and recruits (ingresses) to the Kleinemonde Estuary (South Africa) under (a) open estuary mouth conditions and (b) closed estuary mouth conditions
(from Cowley et al. 2001, their figure 5).
On larger timescales, but also after episodic events, estuarine inlets can reform in response to hurricanes (Gobler et al. 2019), with a subsequent increase in fish diversity and biomass (Olin et al. 2019) once ingress into the estuary is restored. When inlet opening occurred as a result of human dredging in estuaries undergoing change due to human activities, the response varied from producing higher mean densities of newly settled, estuary‐dependent fishes (Reese et al. 2008, Hall et al. 2016) to a lack of apparent response (Milbrandt et al. 2012). The artificial release of a freshwater pulse from an upstream impoundment into a freshwater‐deprived South African estuary did not appear to stimulate the level of natural fish recruitment that was expected (Strydom & Whitfield 2000) when compared to the response to natural river flow into the surf zone (Strydom 2003).
Overwinter mortality, especially in the first winter of life, is becoming better documented and may have important implications for reproductive and recruitment success. This may be especially true for small, relatively immobile estuarine species subjected to low temperature and cumulative loss of energy reserves (Hurst et al. 2000, Hales & Able 2001, Hurst 2007) that contribute to elevated mortality. Winter mortality is certainly evident for species that enter temperate estuaries in the late summer or early fall, when temperatures are warmest, but followed soon thereafter by cold temperatures. This is the case for the tropical chaetodontid Chaetodon ocellatus, which typically occurs in southern New Jersey (USA) estuaries each year in late summer (Able & Fahay 2010) but suffers total mortality when fall–winter temperatures drop to 12 °C (McBride & Able 1998). Some species have greater tolerance, e.g. recruits of the serranid Centropristis striata may survive winter temperatures (Hales & Able 2001) (Figure 3.9).
Figure 3.9 Phenology of ingress of larvae of southern species in a representative north temperate estuary relative to representative warm and cold winters, and temperatures at which mortality occurs for the young‐of‐the‐year (YOY) juvenile stage of two selected species
(from Able & Fahay 2010).
There are many tropical and subtropical species that colonise temperate estuaries and typically suffer winter mortality, but such expatriates are difficult to detect and monitor. A well‐described example of the varying effect of overwinter weather is that for young‐of‐the‐year of the North American sciaenid Micropogonias undulatus, which shows increased survival and population outbursts during warmer winters (Hare & Able 2007, Hare et al. 2010). Similar temperature‐related phenomena also occur in subtropical estuaries, especially related to storm effects (Günter 1947, Mora & Ospina 2002). Recruitment and colonisation by the juveniles of tropical species into warm‐temperate South African estuaries as a result of global warming have also been recorded (James et al. 2008b).
In a New Jersey estuary, using a time series based on weekly sampling over 26 years, from 1990 to 2015, the larval fish assemblage, with 65 species well represented in the samples, changed significantly in response to warming water temperatures (Morson et al. 2019), indicating substantial shifts in their levels of reproduction and recruitment potential. Of these, the 5 species originating from spawning north of Cape Cod (Latitude 41.67 °N) all decreased, while 18 of 21 originating south of Cape Hatteras (Latitude 35.25 °N) increased in occurrence. As a result, total fish density and species diversity increased over the period of the study. A specific example of temperature effect is that for Pseudopleuronectes americanus in which there was no overt response to annual mean monthly temperatures, but in spring seasons when temperatures were warm (5–7 °C) there was lower abundance of larvae of this northern form (Able et al. 2014).
There are several short‐term (days) weather‐related phenomena in estuarine habitats that influence reproduction and recruitment of fishes. For example, upwelling events off the coast of New Jersey (USA) occur during the summer when alongshore southerly wind stress causes the transport of cold, nutrient‐rich bottom water onshore and displacement of warm, nutrient‐poor surface water offshore, resulting in cold upwelling conditions (Neuman 1996, Hicks & Miller 1980). These events occur from one to five times per year and last from 2–12 days. In most years, overall abundance of larval fishes at a long‐term study site within an adjacent estuary was greater than abundance observed there during upwelling events. In addition, there was a significant positive correlation between (i) the number of upwelling events in a given year and larval species diversity and (ii) the total number of upwelling days in a given year and species diversity during upwelling events (Able & Fahay 2010). Despite these general patterns, there was no overall agreement relating increased diversity to upwelling except that most fish groups (e.g. estuary/shelf spawners, estuary spawners) peaked at the same time. A study at the mouth of Chesapeake Bay detected upwelling effects on larval occurrences but found that estuarine plume dynamics were also important (Reiss & McConaugha 1999).
Freshwater discharge and flow‐related events are common in estuaries and are associated with either high or low flows that support estuarine dynamics and features favouring retention or transport of larvae within estuaries. Consequences of very high river flows to poor recruitment of young fishes into the Thukela Estuary (South Africa) were documented by Whitfield & Harrison (2003), with very low or zero flows corresponding to poor recruitment and moderate flows leading to good recruitment in other South African estuaries (Whitfield 1994). Anthropogenic changes to flow dynamics in estuaries also may result in flushing of estuary‐resident fish
