K.O. & Ponwith, B.J. 1998. Comparative ecology of eleotrid fishes in Central American coastal streams. Environmental Biology of Fishes 53, 373–384.214 Wootton, R.J. 1990. Ecology of Teleost Fishes. Fish and Fisheries Series 1. Chapman & Hall, London.
215 Wuenschel, M.J. & Able, K.W. 2008. Swimming ability of eels (Anguilla rostrata, Conger oceanicus) at estuarine ingress: contrasting patterns of cross‐shelf transport? Marine Biology 154, 775–786.
216 Yáñez‐Arancibia, A., Lara‐Dominguez, A.L., Rojas‐Galaviz, J.L., et al. 1988. Seasonal biomass and diversity of estuarine fishes coupled with tropical habitat heterogeneity (southern Gulf of Mexico). Journal of Fish Biology 33(Supplement A), 191–20.
217 Yáñez‐Arancibia, A., Linares, F.A. & Day, J.W. 1980. Fish community structure and function in Terminos Lagoon, a tropical estuary in the southern Gulf of Mexico. In: Estuarine Perspectives (ed., Kennedy, V.S. ), pp. 465–482. Academic Press, New York.
Chapter 3 Reproduction, Ontogeny and Recruitment
Edward D. Houde, Kenneth W. Able, Nadine A. Strydom, Eric Wolanski, and Timo Arula
3.1 Introduction
Estuaries are universally regarded as important spawning and nursery environments. In part, this occurs because estuaries are diverse and transitional habitats between marine and freshwater environments (Able et al. 2022). Accordingly, diverse reproductive modes and patterns are represented amongst taxa that spawn in estuaries or in those taxa that spawn elsewhere but use the estuary during early‐life stages. Numerous reproductive modes may be represented in a single estuary (Elliott et al. 2007, Potter et al. 2013). Abiotic and biotic processes support the transport, retention and trophic requirements of eggs, larvae, juveniles and some adults of estuary‐dependent and estuary‐associated fishes, ensuring the success of reproduction and replenishment of the populations.
The replenishment process of estuarine fishes includes spawning and nursery functions that ensure life‐cycle closure. The production of progeny is often accompanied by complex ontogenies of young fishes as they develop and grow towards recruitment, and eventually replenishment of the adult population. Following Costa et al. (2002), we accept their definition of the replenishment of adult population as the culmination of the recruitment process. This narrow definition adheres to the fishery manager recognition of recruitment as size/age of fish that enter a fishable population. However, in our review, we address recruitment from a much broader perspective, a perspective held by many ecologists, that considers early‐life‐stage dynamics and ecological processes as recruitment processes. For example, we describe processes that result in recruitment of larval‐stage fishes from offshore to estuaries and recruitment of young fishes to specific estuarine habitats (Able et al. 2022). These processes generally refer to development, growth and survival, particularly in the first year of life. Our approach is to address reproduction and recruitment in an integrative way, recognising that, while processes in the earliest life stages may underpin reproductive success, no single life stage controls outcomes in terms of success and variability in recruitment (Houde 2008, Lowerre‐Barbieri et al. 2016, Somarakis et al. 2019).
For a century, most fish biologists have considered the eggs and earliest larval stages, which are highly vulnerable to environmental perturbations, nutritional deficiencies and predation losses as most important, if not critical, in determining the success of reproduction and recruitment (e.g. Hjort 1914, 1926, Cushing 1990, Houde 2008, 2016, Peck et al. 2012b). In general, while processes controlling early‐life survival are similar in estuaries, freshwaters or the open sea, environmental factors and variability that affect fish production almost certainly are more extreme in estuaries because they are relatively small in size, exposed to terrestrial perturbations and thus responsive to coastal weather and other environmental stresses (Teichert et al. 2017). For many estuary‐dependent species, transitional linkages amongst early‐life stages occur, for example between coastal ocean and estuary, estuary and freshwater systems or even open ocean to estuary to freshwater, e.g. eels from the families Anguillidae and Congridae using estuaries as conduits between aquatic ecosystems. Transitions often require remarkable migrations by adult spawners to spawning sites, followed by directional transport of eggs and larvae that depends on circulation features (from small to large spatial scales). This is coupled with selective behaviours by larvae and juveniles, appropriately cued to estuarine and coastal features and physico‐chemical drivers that ensure success in transport or retention and, ultimately, recruitment of young fishes to the adult populations (Boehlert & Mundy 1988, Churchill et al. 1999, Beck et al. 2001, Strydom 2003, Cowan et al. 2012, Wolanski & Elliott 2015, Teodosio et al. 2016, Wolanski 2017).
The hydrodynamics of estuaries generally result in a seaward flushing that may transport pelagic fish eggs and larvae from an estuary. However, there are important mechanisms that result in transport of larvae to the estuary from offshore and also for retention of eggs and larvae within the estuary, especially for resident‐spawning and anadromous fishes. Retention processes for pelagic larvae seldom persist for more than a few days or weeks in small estuaries, a period too brief for young fish to recruit using hydrodynamic processes alone, but these processes can be persistent in large estuaries with long residence times. Accordingly, behaviour of larvae is vital to facilitate recruitment in most estuaries. Estuarine and coastal gradients, especially in salinity, and other physico‐chemical features (Basset et al. 2013) such as fronts (Kruger & Strydom 2011), eddies, plumes (Grimes & Kingsford 1996, Strydom 2003) and Estuarine Turbidity Maxima (Dodson et al. 1989, North & Houde 2001, 2006), or processes such as tidal dynamics (Strydom & Wooldridge 2005, Pattrick & Strydom 2014) are used by larvae and juvenile fishes to facilitate recruitment (see Section 3.3.1). Physical gradients represent more than physical features. They translate into biological gradients in productivity and abundances of prey, and probably predators, which the young fishes must face.
Utilisation of estuaries and dependence on estuarine nurseries by young fishes is important for reproductive success and population continuity of estuary‐dependent species (Potter et al. 2013). Many species of fishes are obligate estuarine spawners and their young use estuarine nurseries that serve as suitable, sheltered refugia relative to less sheltered habitats in the ocean and shelf environments. However, some fishes labelled as estuary dependent are facultative users of estuaries for spawning and rearing of young (Able 2005) and would be better described as estuary‐associated species. Estuary‐dependent and ‐associated fishes experience multiple challenges to the reproductive process and during early life in transitions between the ocean and estuaries, as well as experience numerous, potential bottlenecks that may occur at hatching, metamorphosis and settlement, sexual maturation and spawning (Figure 3.1). Critical estuarine nursery habitats are deteriorating in much of the world's coastal areas, mainly from anthropogenic causes (Houde et al. 2014, Elliott et al. 2019), posing threats to the sustained success of reproduction and recruitment of estuary‐dependent and ‐associated fishes (see Section 3.5 and Cabral et al. (2022)).
Estuaries are often depicted as ‘safe’ nursery environments for larval and juvenile fishes. Relatively favourable temperatures and quiescent waters, coupled with high phytoplankton and zooplankton production, often ensure abundant food for fast‐growing larvae (Strydom 2015, Whitfield et al. 2022b). Predator numbers are reputed to be lower in estuaries than in the coastal marine environment (Blaber & Blaber 1980, Boesch & Turner 1984, Beck et al. 2001), although this conjecture requires further scrutiny (Sheaves 2001, Baker & Sheaves 2007, 2009a, 2009b). In a synthesis of data on fish early‐life dynamics, mortality rates of larval fishes from estuarine systems were found to be similar to those of larvae in coastal and ocean ecosystems (Houde & Zastrow 1993).
