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). Factors contributing to structure and function of estuary habitats have been recently crystallised into an improvement of our understanding of the nursery concept. We have advanced from the earliest mention of estuarine nurseries (Günter 1967) to a specific approach based on production per unit area contributions to the adult population (Beck et al. 2001) and subsequent refinements (Dahlgren et al. 2006, Sheaves et al. 2006) to a broader view of seascape nurseries (Boström et al. 2011, Nagelkerken et al. 2015, Litvin et al. 2018). Still, there is a need to continuously re‐evaluate and interpret the nursery concept over entire life histories and across the estuarine mosaic, as developed further by Able et al. (2022).
Figure 3.1 All fishes, whether estuarine transients or residents, have complex life histories with multiple physiological and ecological bottlenecks (as during hatching, metamorphosis and settlement, sexual maturation, spawning), but those that transition from the estuary to the ocean undergo additional levels of complexity as indicated for estuary‐dependent fishes.
Recruitment success in fishes has been hypothesised to depend on hydrodynamic and production processes of enrichment, concentration and retention (e.g. Boehlert & Mundy 1988, Bakun 1996). Structural and physical mechanisms in estuaries that enrich, concentrate or retain early‐life stages of fishes are both numerous and diverse, relative to waters on the continental shelf or open sea.
3.1.1 Scope of the chapter
Our goal is to summarise and synthesise knowledge on reproduction and recruitment in estuary‐dependent and ‐associated fishes. We describe patterns in reproduction, including ontogeny and habitat utilisation by early‐life stages. We examine the scales and patterns in recruitment variability and the processes that control and regulate recruitment. Case studies that exemplify the patterns, modes and processes are presented. We review and discuss the following:
The state of knowledge of fishes that use estuaries for reproduction.
Egg, larval and juvenile stages of fishes that occur in or ingress to estuaries, including ecology, dynamics and behaviour, especially regarding the role of estuaries as nurseries.
Recruitment variability of estuary‐dependent and ‐associated fishes, including causes and trends.
The adult stage of estuary‐dependent and ‐associated fishes, focusing on contributions by adults to reproductive success and recruitment variability.
Recruitment forecasting; predicting reproductive success of estuarine fishes.
Threats to reproduction and recruitment of estuary‐dependent and ‐associated fishes.
3.2 Estuarine support of reproduction and recruitment
3.2.1 Replenishment: modes and patterns
The observed patterns and behaviours for reproduction are remarkably diverse in estuarine and estuary‐dependent bony fishes (Dando 1984, Blaber 2000, Elliott et al. 2007) as well as in sharks, skates and rays (Musick 2010). The modes of reproduction known in marine and freshwater fishes are represented in estuarine‐occurring species, but many reproduction‐related characteristics are broadly variable amongst biogeographic regions (Table 3.1). Successful reproduction depends on coordinated behaviours and dynamic processes acting on multiple life stages (Blaber 2000, Able & Fahay 2010, Whitfield 2019). Completion of the replenishment process by resident fishes, including recruitment to the adult stock, may be fully accomplished by many small species within the estuary (e.g. fundulids, atherinids, gobiids, blenniids, engraulids). For many elasmobranchs that use estuaries for reproduction, it is primarily for pupping their well‐developed young (Merson & Pratt 2007). In other taxa, adults may migrate from freshwater habitats to brackish estuarine waters to spawn and produce young (e.g. the moronid Morone americana).
Table 3.1 Representative reproductive characteristics of estuarine fishes by broad biogeographical region. Primary sources include Blaber (2000), Elliott & Hemingway (2002), Able & Fahay (2010) and Whitfield (2019).
| Tropical | Subtropical | Temperate/Boreal | Arctic | |
|---|---|---|---|---|
| Spawning stimuli | Watershed flow, salinity, monsoons, temperature | Temperature, dissolved oxygen | Temperature, photoperiod | Temperature, ice conditions |
| Spawning method | Serial spawning over a long season | Serial spawning over a long season | One or few batch spawning over a short season | One or few batch spawnings over a very short season |
| Spawning location | Often continuous between estuary and ocean | Ocean or estuary | Ocean, estuary, freshwater | Freshwater, estuaries and coastal ocean |
| Aspects of reproduction | Ocean: large number of pelagic eggs Estuary: fewer, larger eggs, parental care in some | Ocean: large number of pelagic eggs Estuary: fewer, larger eggs, parental care in some | Ocean: large number of pelagic eggs Estuary: fewer, larger eggs, parental care in some | Rivers: fewer, larger eggs Estuary and coastal ocean: larger number of benthic and pelagic eggs |
| Dominant families | Ariidae Carangidae Clupeidae Elopiformes Engraulidae Gobiidae Haemulidae Mugilidae Polynemidae | Ariidae Carangidae Clupeidae Elopiformes Engraulidae Gobiidae Haemulidae Mugilidae Polynemidae | Clupeidae Engraulidae Pleuronectidae Sciaenidae Sparidae Syngnathidae | Clupeidae Salmonidae Osmeridae Gadidae |
Adults of catadromous