retain early‐life stages of G. morhua within the deepwater region of the Bornholm Basin (Hinrichsen et al. 2001). Upon transition to the juvenile stage, most juveniles inhabit deeper sites close to the Bornholm Basin.
Reproductive and recruitment success of the eastern Baltic Gadus morhua has declined in response to changing climate conditions that have reduced salinity and dissolved oxygen on the spawning grounds. These conditions, combined with high fishing pressure on adults and probable high egg predation by the clupeid Sprattus sprattus, drove G. morhua recruitment to low levels in the early 1990s (Westin & Nissling 1991, Wieland & Jarre‐Teichmann 1997). Low recruitment persisted, despite improving hydrographic conditions for egg survival in the mid‐1990s, due to insufficient larval prey concentrations, i.e. low abundance of the copepod Pseudocalanus sp. (Köster et al. 2005).
Adults of Clupea harengus migrate from offshore in the Baltic Sea to the coast where they spawn demersal eggs (Parmanne et al. 1994, Arrhenius & Hansson 1996). Early‐stage larvae reside in the littoral zone (Urho & Hilden 1990) until 30 mm length, after which they migrate to the offshore pelagic zone. Year‐class strength of western Baltic C. harengus is determined early in life (Oeberst et al. 2009). Factors affecting survival of larvae >20 mm in length and juveniles are similar amongst years, suggesting that inter‐annual variability in mortality of the late‐stage larvae and juveniles is relatively small. Unusually high mean surface water temperature during summer may reduce abundance of C. harengus larvae (Arula et al. 2016). Similarly, in the western Baltic, Dodson et al. (2019) concluded that adult C. harengus may shift dates of spawning to partly mitigate effects of variable rates of spring warming that compromise successful hatching. In that study, the abundance of preflexion and flexion C. harengus larvae was defined by dome‐shaped responses to temperature, with maximum abundances observed in the 10.3–13.4 °C (preflexion) and 13.7–18.5 °C (flexion) ranges.
In recent decades, year classes of Clupea harengus have become more abundant, apparently responding favourably to prevailing weather conditions, including mild winters in the Baltic region, with above normal rainfall leading to increased river run‐off and reduced frequency of major, high‐salinity inflows from the North Sea (Matthäus & Schinke 1994, Ojaveer et al. 2011, Arula et al. 2016, ICES 2018). The favourable conditions for reproduction have resulted in a doubling of biomass in the Gulf of Riga in recent years (Arula et al. 2014, 2016).
Another abundant clupeid, Sprattus sprattus, spawns pelagic eggs and its larvae predominantly occur in surface waters of the Baltic Sea (Voss et al. 2003) where variable, wind‐driven circulation patterns result in dispersal of early‐life stages. A retention index based on hydrodynamic modelling (Baumann et al. 2004) for the Bornholm Basin, a major S. sprattus spawning area (Köster et al. 2001) is significantly related to abundance of age‐0 S. sprattus recruits in the central Baltic. Retention within the deep‐basin benefits recruitment, while dispersion to the southeastern Baltic leads to lower abundances and recruitment failure (Baumann et al. 2006, Voss et al. 2012). Recruitment success of young‐of‐the‐year S. sprattus in the Baltic Sea is positively related to spring‐summer temperatures for which statistical models have predictive power (MacKenzie et al. 2008). Bottom‐up trophic processes apparently play a more important role in controlling recruitment of Baltic S. sprattus than do top‐down predation processes, although numerous factors including feeding, predation and water circulation patterns during the larval and also the juvenile stages contribute to recruitment variability (Voss et al. 2012). Adult spawning biomass may be less important as a regulator of year‐class size (Figure 3.22).
Figure 3.22 Recruitment of Sprattus sprattus in the Baltic Sea showing the fates of the 2002 and 2003 year classes. Spawning stock biomass (left panel) was higher in 2002 but recruitment success was better in 2003, in part because of better survival during late‐larval to early‐juvenile stages
(from Voss et al. 2012, their figure 9).
3.6.7 Lateolabrax japonicus (Lateolabracidae)
Lateolabrax japonicus is abundant in East Asian coastal waters. In the marine Ariake Bay (Japan) spawning occurs in winter, where eggs hatch and early‐stage larvae develop and grow (Matsumiya et al. 1981, 1982, 1985). Postflexion larvae and small juveniles (≥15 mm SL) migrate to the tidal Chikugo River where their variable growth and mortality rates (to ~25 mm SL) determine recruitment outcomes (Shoji et al. 2006, Shoji & Tanaka 2007a, 2007b). Highly variable growth and survival rates of larvae to 15 mm SL in Ariake Bay (Shoji & Tanaka 2007a, 2008) are density independent and survival increases at higher temperatures (Shoji et al. 2006, Shoji & Tanaka 2007b), which are positively related to Chikugo River freshwater discharge.
After ingressing to the tidal Chikugo River, early‐stage juveniles (15–20 mm SL) of Lateolabrax japonicus find best feeding conditions near the salt front and estuarine turbidity maximum (Islam et al. 2006) where the copepod Sinocalanus sinensis is dominant prey. In this period, weight‐specific growth (G), mortality (M) and an index of recruitment potential (G/M) all become density dependent (Shoji & Tanaka 2007a, 2008) while continuing to be strongly related to temperature (Figure 3.23) (Shoji & Tanaka 2007b). In 11 years of observations, 24‐fold variability in annual abundances of 15 mm larvae declined to 9.4‐fold annual variability at 20 mm SL (Shoji & Tanaka 2008), which suggests that density‐dependent mortality and regulation of recruited abundance occur in the 15–20 mm, earliest juvenile stage. Limited abundance of the dominant prey S. sinensis is the probable factor driving density dependence.
The most important controls and regulators of recruitment variability in Lateolabrax japonicus are (i) density‐independent processes controlling growth and mortality of pre‐immigration larvae in Ariake Bay; (ii) positive effects of temperature on growth and survival of pre‐ingress larvae in Ariake Bay and post‐ingress larvae in the Chikugo River; (iii) positive effects of moderate freshwater discharges in the Chikugo River on survival and growth and (iv) density‐dependent growth and mortality of 15–25 mm SL juveniles during Chikugo River residence.
3.6.8 Fundulus heteroclitus (Fundulidae)
Estuarine shorelines, fringes and adjacent marshes provide habitat for numerous, small, abundant and primarily demersal resident fishes (Able et al. 2022). These fishes reproduce in the estuary and their young recruit to local habitats, often on the fringing shorelines. An example is Fundulus heteroclitus, a small fundulid found along the Atlantic coast of the USA and Canada (Able & Fahay 2010). Adults have limited home ranges and spawn intertidally. Although relatively little is reported on growth and mortality of its eggs and larvae on regional scales, or of recruitment variability, there is considerable knowledge of its dynamics at smaller scales. For example, Kneib (1993) reported fivefold variability in weight‐specific growth rates (0.027–0.143 d−1) and major variability in mortality rates (0.001–0.118 d−1) in enclosure experiments in a Georgia (USA) estuary, indicating that substantial variability in cohort‐specific recruitment levels would be expected. In this case, variability in recruitment was primarily attributed to duration of tidal flooding; longer flood‐tide durations positively influenced growth of larvae and resulted in reduced mortality rates, apparently related to tidal delivery of food. The importance of larval and small juvenile stages was demonstrated in a Delaware (USA) marsh, where production by young‐of‐the‐year individuals contributed 71.6% to the total annual production
