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 overfishing is a prime example (Goodyear 1985, Richards & Rago 1999) as is that of the percid Sander lucioperca in the shallow embayments of the Baltic Sea (Müller‐Karulis et al. 2013, Mustamäki et al. 2014). In South Africa, immature stages of the sciaenid Argyrosomus japonicus and the sparid Lithognathus lithoganthus, amongst others, are exploited as juveniles in estuaries, coupled with exploitation of adults in surf zones (Griffiths 1997, Lamberth & Turpie 2003). The heavy exploitation has led to major stock collapses in these species and current spawner stock biomass per recruit ratios at <5% of pristine levels. Adult stock abundance also is vulnerable to poor water quality and habitat deterioration. In North America, failed recruitments of alosines, anguillids, moronids and salmonids provide unfortunate examples of recruitment declines that are at least partly attributable to depleted adult abundances.
3.4.1.1 Stock structure, contingents and cohorts
Spawning populations of species using estuaries, particularly fishes that spawn within estuaries or their connected freshwaters, often have a diverse genetic makeup that is expressed in their particular spawning habits and behaviours (e.g. Secor 1999, Bottom et al. 2005a, 2005b, Secor and Kerr 2009, Levings 2016). Adult stock structure and variability are notably evident in anadromous fishes in which spawning adults return to their natal rivers and estuaries (Secor 2015). In the case of Pacific salmon species (Oncorhynchus spp.), there is diverse dependence on estuaries by fry, juveniles and smolts amongst the several species and races that reflect evolved spawning behaviours and estuarine dependence by fry and smolt stages prior to ocean migration (e.g. Simenstad et al. 1982, Thorpe 1994, Levings 2016). In the case of salmonids, the loss of spawning contingents, and overall losses of diversity in genotypic and phenotypic variability in pre‐smolt juveniles, combined with losses of estuarine nursery habitat, is a particular concern for recruitment success of O. tshawytscha in the Columbia Estuary (USA) (Bottom et al. 2005a, 2005b) and of salmonids in general in many Pacific coast estuaries in North America (Levings 2016, Quinn 2018).
Spawning contingents that display different life‐history patterns and variable dependence on estuaries and marine habitats are recognised for taxa other than salmonids, e.g. the moronid Morone saxatilis in Chesapeake Bay (Secor 1999, Secor & Piccoli 2007), Hudson River (Secor et al. 2001) and St Lawrence River (Morissette et al. 2016) populations. Similarly, research on the sciaenid Argyrosomus japonicus in South Africa indicates that this species has distinct marine and estuarine subpopulations, with the estuarine subpopulation showing critical declines in recruitment and abundance due to overfishing (Childs et al. 2015) with concomitant genetic diversity limitations and incidence of hybridisation with a related sciaenid species (Mirimin et al. 2014). In acipenserids, spawning seasons are highly variable (Bemis & Kynard 1997), and there is evidence of spring‐ and fall‐spawning races, for example, in Acipenser oxyrinchus living in the same estuaries and tidal rivers on the Atlantic Coast of North America and, similarly, dual‐spawning groups of acipenserid species in Europe and Asia (Balazik & Musick 2015). In another example of contingent behaviour, sub‐adults of some individuals of the amphidromous plecoglossid Plecoglossus altivelis ryukyuensis may repeatedly move between freshwater and estuarine habitats during their juvenile stage (Murase & Iguchi 2019).
In the Baltic Sea, the spring‐spawning clupeid Clupea harengus is represented by subpopulations or spawning stocks that are assessed and managed in fisheries as separate units (ICES 2018). Four C. harengus stocks are recognised for management: Baltic proper, Gulf of Riga, Gulf of Bothnia and Bay of Bothnia. Major differences between Gulf/Bay and Baltic proper stocks are related to their salinity tolerances and adaptations. A fifth spawning stock occurs in the Western Baltic, which is largely comprised of adults that feed and overwinter in the Danish Straits or outside the Baltic Sea (Dodson et al. 2019). Baltic C. harengus stocks have spring‐spawning and autumn‐spawning components that exhibit stock‐specific reproductive/recruitment behaviours, although the contribution by the autumn spawners is minor (Ojaveer 1988).
3.4.1.2 Maternal effects
Variability in age structure, growth and condition of adults contributes to variability in reproductive output and potentially to success of recruitment of estuary‐dependent fishes. Quantity and quality of eggs and sperm may depend on the condition, size and age of adults. As such, the quality and potential of eggs and larvae to survive depend on maternal (or paternal) investments. In a review of marine fishes, maternal effects were documented for many taxa (Marshall 2016). However, Ottersen et al. (2013) analysed recruitment success of 38 stocks of marine fishes, including some estuary‐dependent or ‐associated stocks (e.g. the Baltic Sea clupeid Clupea harengus membras, Baltic Sea gadid Gadus morhua and European pleuronectid Pleuronectes platessa) and found little evidence of a significant effect of maternal age on recruitment success in those stocks although the authors acknowledged that their data were sparse and difficult to interpret. In a meta‐analysis of effects of maternal age and size on marine fish stocks and implications for recruitment, Venturelli et al. (2009) reported that populations represented by older, larger individuals have a higher maximum reproductive rate than populations of equivalent size represented by younger, smaller individuals, concluding that expanded age structure is important for sustainability in many exploited fish stocks. The relationship between female size and egg number for Pacific salmons can vary with latitude (Fleming and Gross 1990). In the salmonid Salmo trutta, the anadromous traits displayed in some females provide an adaptive advantage and greater fitness during early ontogeny (Goodwin et al. 2016).
Based
