an important role in controlling offshore, winter spawning and early‐life dynamics of Pleuronectes platessa (Bannister et al. 1974, Harding et al. 1978, Bergman et al. 1988). Offshore survival of eggs and larvae in the Southern Bight of the North Sea, before they reach estuarine nurseries, is inversely related to sea temperatures (Bannister et al. 1974, Harding et al. 1978). The inter‐annual variability in offshore larval mortality rates was sufficient to generate the observed differences in abundance of recruited year classes, suggesting that early‐life, offshore dynamics plays a more important role in governing recruitment strength than subsequent juvenile dynamics in estuaries and embayments. Mortality for Irish Sea and North Sea P. platessa showed a steady decline in daily mortality from ≥0.10 d−1 for eggs to about 0.01 d−1 for young‐of‐the‐year juveniles on estuarine and coastal nursery grounds (Nash & Geffen 2012). Stage‐specific mortality rates in the Wadden Sea nursery indicated highest cumulative mortality (98.9%) in the larval stage (Bergman et al. 1988). However, >90% of eggs (offshore) and also of settlers (estuary) perished, indicating considerable scope for regulation of abundance during those stages.
Growth offshore of larval Pleuronectes platessa is strongly and positively related to temperature (Hovenkamp & Witte 1991, Comerford et al. 2013). Consequently, duration of the larval stage can vary by more than 20 days due to the temperature effect. Upon settlement in coastal and estuarine systems, growth rate may decline. There is substantial variability in growth of settled P. platessa amongst nurseries, with weak dependence on temperature, probable dependence on prey resources and inconsistent evidence for density‐dependent growth (Ciotti et al. 2014). Variability in growth rates of settled individuals amongst nurseries may be less dependent on temperature than other habitat factors (Ciotti et al. 2014). However, in the Wadden Sea, growth varied relatively little amongst years and, in settled juveniles, did depend mostly on temperature (Bergman et al. 1988).
Within the estuarine Wadden Sea, the density‐dependent component of mortality in newly settled juveniles of Pleuronectes platessa is significant (Beverton & Iles 1992). Van der Veer (1986) and others (e.g. Bergman et al. 1988, Van der Veer et al. 2015) detected density‐dependent mortality, generated by decapod crustacean (i.e. Crangon crangon) predation during a brief 60‐day period from April to June. This mortality probably acts to regulate and reduce variability (fine‐tuning) in recruitment of P. platessa in the Wadden Sea (Nash & Geffen 2015). Predation by Crangon septumspinosa on juveniles in New Jersey (USA) estuaries may operate to regulate recruitment of other pleuronectiforms, e.g. Paralichthys dentatus and Pseudopleuronectes americanus (Witting & Able 1993, 1995).
A recent review noted that food quantity and quality and temperature stand out as variables that control growth during the offshore larval and estuarine juvenile stages of pleuronectiforms (Nash & Geffen 2015). For example, Tanaka et al. (1989) demonstrated that larvae of Paralichthys olivaceus showed a strong and direct relationship of growth to experimental temperature, with rates increasing from 0.2 to 0.6 mm d−1 in the temperature range 12–23 °C. In the pleuronectid Rhombosolea tapirina from Australia's Swan Bay, there was a positive effect of temperature on growth of winter cohorts of recently settled juveniles, but a negative effect on spring cohorts that may experience stressful high temperatures (May & Jenkins 1992). As reported for Pleuronectes platessa, declines in growth rates in some newly settled pleuronectiforms are associated with metamorphosis (Nash & Geffen 2015) and with shifts in habitat from offshore pelagic to demersal estuarine systems. Latitudinal variability in growth of estuarine juveniles occurred for European stocks of P. platessa, Platichthys flesus and Solea solea, with faster growth in the northernmost estuaries within their respective ranges (Freitas et al. 2012). In general, variability in growth was greater for pleuronectiform juveniles in low‐latitude nurseries than in high‐latitude nurseries.
Growth variability of pre‐settlement larvae and settled juveniles of flatfishes may be tuned more to local environmental conditions at particular estuarine sites than to broader measures of climate or regional habitat variables, as reported for Pseudopleuronectes americanus in New Jersey (USA) estuaries (Sogard & Able 1992, Sogard et al. 2001). Settled juveniles of P. americanus grew at variable rates amongst sites that were consistent from year to year, with no apparent density dependence in growth. This finding is similar to that for other pleuronectiform species (Nash & Geffen 2015). Sogard et al. (2001) found a positive correlation between growth rate and temperature for P. americanus at time of settlement but not for settled juveniles, a result contrasting with that reported for this species in the Mystic River Estuary (Connecticut, USA) where temperature was positively correlated with growth of juveniles (Pearcy 1962).
Larvae of the pleuronectid Pseudopleuronectes yokohamae from Hakodate Bay, a shallow Japanese embayment, had growth patterns positively related to temperature (Joh et al. 2013). However, in Tokyo Bay, larvae of P. yokohamae had low survival in years when temperatures were >10 °C, and young‐of‐the‐year recruitment was best in years when larvae experienced temperatures <10 °C (Lee et al. 2017). Settled juveniles of the pleuronectid Platichthys bicoloratus had substantially higher growth rates at estuarine sites compared to settlers on exposed coastline in Sendai Bay (Japan) (Malloy et al. 1996, Yamashita et al. 2003). Recruitment to the fishery for P. bicoloratus whose juveniles had resided in estuarine nurseries was relatively high compared to juveniles that had resided in coastal, non‐estuarine habitats (Yamashita et al. 2000), despite substantial mortality of estuarine juveniles attributed to predation by the decapod Crangon affinis (Yamashita et al. 1996b).
For Platichthys flesus larvae and settled juveniles in the Wadden Sea, Van der Veer et al. (1991) reported an early‐life pattern of mortality similar to that for P. platessa. Mortality rates of settled age‐0 juveniles of P. flesus during the summer were highly variable, and there was probable density dependence attributable to invertebrate predation (by Crangon crangon and Carcinus maenas) during the early settling period. Mortality rates of larval and juvenile Pseudopleuronectes americanus in the Mystic River Estuary (USA) differed between these life stages (Pearcy 1962). Estimated loss rates of early‐stage larvae were high, averaging about 20% d−1, compared with 4% d−1 for postlarvae and <1% d−1 for age‐0+ juveniles. The total cumulative mortality of P. americanus during larval and age‐0+ juvenile stages exceeded 99% and was mostly generated during the early‐larval stage (Pearcy 1962).
3.6.2 Sciaenidae
Fishes in the family Sciaenidae are common estuarine species of tropical and temperate regions. Many share a common reproductive strategy defined by offshore spawning, larval dispersal to estuaries and recruitment of postflexion larvae or juveniles to estuarine nurseries (Able & Fahay 2010). Sciaenid larvae provide some of the best examples of depth‐regulated and tidally mediated behaviours that support ingress into estuaries and retention of pre‐settlement stages. Research on ichthyoplankton in western Atlantic and Gulf of Mexico coastal waters of the USA has provided a reasonably good understanding of dispersal processes. Newly hatched larvae are broadly distributed in shallow coastal waters, indicating widespread spawning (Cowan & Shaw 1988, Shaw et al. 1988, Norcross 1991, Reiss & McConaugha 1999, Schaffler et al. 2009). For example, in the shallow shelf waters of the Gulf of Mexico, spawning by sciaenid fishes occurs from a few kilometres to >100 km offshore (Cowan & Shaw 1988).
Shelf processes that disperse sciaenid larvae indicate that residual onshore currents, favourable winds and, for some species, vertical migratory behaviour or depth selection by larvae are involved in the transport dynamics from offshore to the coast (Cowan & Shaw 1988, Hare et al. 1999, Hare & Govoni 2005). Norcross (1991) found that most Micropogonias undulatus larvae collected offshore (5–25 km) from Chesapeake Bay were at mid‐depth or near bottom in a stratified water column of ~20 m depth but were homogeneously distributed when waters were not stratified. The author believed that larvae were well positioned to enter the Bay through wind‐driven or passive entry in bottom flow. Schaffler et al. (2009) reported that M. undulatus larvae generally were located deep in the water column in offshore, nearshore and estuary‐mouth collections