migrants, fall; c = yearling smolts; d = early spring migrants (recently hatched young or repeat migrants); e = kelts or returning veterans
(modified from Levings 2016).
In some instances, the location and timing of spawning vary amongst cohorts. Pomatomus saltatrix demonstrates cohort‐based differences of estuarine use and recruitment along the east coast of the USA (Wuenschel et al. 2012). Production of a spring‐spawned cohort occurred in both South Atlantic Bight and Middle Atlantic Bight habitats (south and north of Cape Hatteras, respectively), while a summer‐spawned cohort was limited to the Mid‐Atlantic Bight and south of Cape Hatteras. Estuarine habitat use by P. saltatrix varies between cohorts and time of year. Based on these characteristics, modelling indicates that the Mid‐Atlantic and South Atlantic bights provide similar numbers of recruiting young produced by the spring‐spawned cohort, but only the Mid‐Atlantic Bight produces recruits from the summer‐spawned cohort.
There is evidence of spatial partitioning of spawning by resident taxa and by spawning migrants within estuarine systems, e.g. selection of different salinity zones by some adults of Morone americana that ensures variability in hatching and nursery areas (Kerr & Secor 2012). Some species clearly have adapted to human alteration of estuary habitat and now spawn in areas that remain accessible after dams have blocked access to historical spawning sites, e.g. alosine species (see Limburg & Waldman 2009, Able et al. 2020). Although modes of reproduction are diverse, relatively few species reproduce within estuaries, possibly because of extreme and sometimes rapid changes in temperature and salinity, as well as microbial‐rich conditions that can prevail (Dando 1984). In addition, other water quality conditions such as hypoxia can be stressful to young fishes (Teichert et al. 2017).
There are numerous estuary‐associated species that spawn both in estuaries and the adjacent ocean (Able 2005). The diversity of spawning modes and early‐life ecology is also expressed in the fecundity, egg and larval size, morphology, embryonic duration and their pelagic, demersal or semi‐demersal habitats, all characteristics that influence dynamics associated with the recruitment processes of fishes (Forsgren et al. 2002, Cole 2010, Fogarty & O'Brien 2016, Houde 2016).
Many estuary‐dependent species have adults that spawn in the ocean and their larvae grow and survive there, either distantly or near the spawning sites. This is clearly apparent in catadromous species such as the eels Anguilla rostrata (in North America), A. anguilla (in Europe), A. japonica (in Japan) and Conger oceanicus (in North America) and C. conger (in Europe) (Able & Fahay 2010, Kettle et al. 2011, Tsukamoto et al. 2011). Their leptocephalus larvae transform to ‘glass eels’ before entering temperate estuaries in the western North Atlantic, such as those entering Little Egg Inlet in New Jersey (USA). These can result from spawning that occurred 100 s to >1000 km distant, e.g. spawning that occurred in the Sargasso Sea for Atlantic anguillids, southern Florida for elopomorphs or Georges Bank to the north for some gadids (Figure 3.3). Other larval taxa entering New Jersey estuaries originate from north and south of Cape Hatteras or elsewhere in the Mid‐Atlantic Bight.
Four species of anguillid eels utilise tropical, subtropical and warm‐temperate river catchments in southern Africa as nursery areas (Bruton et al. 1987). Like other anguillids, these species are obligate catadromous migrants that require estuaries for both the immigration of the ‘glass‐eel’ life stage and the emigration of adults to spawn in the marine environment. The four species Anguilla bicolor, A. mossambica, A. nebulosi and A. marmorata spawn in the open sea east of Madagascar and larvae are transported to the west by major ocean currents (Robinet et al. 2003, Pous et al. 2010). Evidence suggests that the most common of the eel species on the subcontinent, Anguilla mossambica, migrates from South Africa to marine waters off Madagascar where spawning occurs (Jubb 1961), a distance of at least 3000 km. Fertilised eggs and larvae are then swept southwards by the Agulhas Current, with the larval leptocephalus stages exiting the current and moving onshore along the coasts of Mozambique, east and south east South Africa to enter perennially flowing rivers via estuaries entering the Western Indian Ocean (McEwan & Hecht 1984). Additionally, spawning areas of six species of southwest Pacific anguillids (Anguilla bicolor pacifica, A. marmorata, A. australis, A. reinhardtii, A. megastoma and A. obscura) that ingress to estuaries and freshwaters of islands and continental land masses were delineated from small leptocephali larvae collected broadly within latitudes embraced by the South Equatorial Current (Kuroki et al. 2020), highlighting the extensive spawning migrations by adults and sometimes overlapping spawning periods of these species.
The elopomorphs Megalops atlanticus and Elops spp., which also have leptocephalus larval stages, spawn offshore in the coastal seas of tropical and subtropical regions and, like the anguillids, spawn pelagic eggs in the open sea. Leptocephali of these elopiforms enter estuaries where they metamorphose and grow (Crabtree et al. 1997, McBride et al. 2001). In the case of M. atlanticus, the prolonged juvenile stage may encompass 7–10 years of estuarine habitation before maturation in Florida and Louisiana, USA (Kurth et al. 2019). A high percentage of individuals of another elopomorph, Albula vulpes, also may ingress to estuaries as advanced leptocephali and utilise estuaries as nurseries in Florida and Cuba (Santos et al. 2019).
Figure 3.3 Sources of larvae transported from several locations in the western North Atlantic to Little Egg Inlet in southern New Jersey, USA.
Migrations of other early‐life stages from ocean spawning sites to estuarine nurseries can extend to hundreds of kilometres or more, and migrations by adults to estuarine spawning habitats by anadromous fishes (e.g. some salmonids, the moronid Morone saxatilis) may approach or exceed 1000 km. However, the early‐life stages of these fishes, once within the estuary or estuarine tributaries, are often confined to small areas (1–10 km) with hydrographic features that promote retention and ensure growth, survival and successful recruitment. In the long‐lived acipenserids (sturgeons), adults may migrate hundreds of kilometres along the coast or within large estuaries, before ascending freshwater rivers where they spawn demersal eggs. Young‐of‐the‐year and juvenile acipenserids may remain in the rivers or migrate to estuaries and shallow coastal regions, sometimes undertaking extensive coastal migrations (100 s to 1000 s of km) during their years in a protracted juvenile stage (Van Eenennaam et al. 1996, Bemis & Kynard 1997, Birstein et al. 1997, Secor et al. 2002).
Patterns in spawning locations by estuary‐dependent fishes also vary. Some estuarine taxa spawn selectively in the vicinity of the salt front (e.g. moronids) while others may spawn widely in the estuary, or select spawning habitats on its fringing shoreline (Able & Fahay 2010, Whitfield 2019). The clupeid Brevoortia tyrannus typically spawns pelagic eggs in the coastal ocean, but substantial spawning can occur within larger estuaries (Able & Fahay 2010) such as Chesapeake Bay (Hildebrand & Schroeder 1928, Murdy et al. 1997, MDSG 2009). The engraulid Anchoa mitchilli also spawns both on the continental shelf and in estuaries along the Atlantic coast of North America (Able & Fahay 2010), with most spawning probably occurring within estuaries. In the Eastern Atlantic, the engraulid Engraulis encrasicolus spawns in continental shelf waters but also in coastal estuaries of Europe and the Mediterranean region (Morais et al. 2012). The tropical alosine Ethmalosa fimbriata spawns both in estuaries and at sea off West Africa, with spawning concentrated during cooler months in the estuaries and in warmer months at sea (Faye et al. 2014). Indo‐Pacific alosines in the genus Tenualosa are economically important fishes that spawn in coastal, estuarine and freshwaters (Blaber et al. 2003a, 2003b, Wahab et al. 2019) from the Arabian Gulf to the Philippines. Some, for example T. toli, are anadromous while others, e.g. T. ilisha, are estuarine spawners. Variability in spawning habits and modes are present in the Tenualosa species (Blaber et al. 2003a). Some species, for example T. macrura and T. toli, are