2001) and probable Ekman transport towards the coast (Nelson et al. 1977). Based on vertical distributions and wind conditions (Govoni & Pietrafesa 1994), some larvae are initially advected offshore, then entrained into the Gulf Stream flow to the north, before beginning a cross‐shelf drift towards estuaries (Hare & Govoni 2005). A southerly, alongshore drift of postflexion larvae has been observed and modelled that may largely explain the delivery of postflexion (>16 mm) larvae to Mid‐ and South Atlantic estuaries (Hare et al. 1999, Quinlan et al. 1999, Simpson et al. 2017). Larval behaviours to support cross‐shelf and along‐shelf transport are not clearly demonstrated although larvae reside primarily in near‐surface waters, generally above the pycnocline, and undertake vertical migrations to the surface at night to engulf air and inflate their swim bladders (Hoss & Phonlor 1984, Forward et al. 1994). Laboratory experiments revealed that salinity and temperature gradients and daily light cycles help to establish an endogenous rhythm of daily vertical migrations (Forward et al. 1996, 1999) that acts to ensure cross‐shelf, shoreward transport under most wind and weather conditions.
Mechanisms that operate at the mouths of estuaries to ensure entry of late‐stage larvae of Brevoortia tyrannus are only partly understood. Recently, Hale and Targett (2018) tentatively concluded that larvae use vertical migrations to initiate and facilitate up‐estuary transport at the mouth of Delaware Bay. However, Forward et al. (1999) found that B. tyrannus larvae made diel vertical migrations when offshore but detected no patterns at the mouth of Beaufort Inlet, North Carolina. In the laboratory, Forward et al. (1996) reported that larvae were more active and surface‐oriented at night (diel vertical migration), in accord with field observations (Govoni & Pietrafesa 1994), but without evidence of STST or tidal rhythms.
Lozano & Houde (2013) conducted repeated surveys for Brevoortia tyrannus larvae at the Chesapeake Bay mouth over three winters and found no clear patterns in variable vertical distributions that might indicate consistent adoption of STST or other tidally induced behaviours to aid up‐estuary transport by the >20 mm larvae. Abundances did not differ significantly across the 18 ‐km‐wide Chesapeake Bay mouth, suggesting that substantial ingress occurred under favourable winds on flooding tides over the entire mouth, although greatest inflow occurs on the north side (Valle‐Levinson et al. 2001). Hale & Targett (2018) suggested that there may be fluxes (repeated re‐entry after flushing by tides) of larvae into and out of the Delaware Bay mouth. Lozano & Houde (2013) also had evidence that larvae may experience flushing and re‐entry at the Chesapeake Bay mouth. In Chesapeake Bay, winds and flood‐tide forcing, and possibly swimming, appear to be sufficient to control and assure ingress. Once within Chesapeake Bay, up‐estuary migration can be rapid. Late‐stage larvae averaging 28 mm TL had migrated 300 km up‐estuary in 30 days (>10 cm s−1) (Lozano & Houde 2013).
Species of Brevoortia share several characteristics of their early‐life history. North American B. tyrannus and B. patronus spawn primarily in the coastal ocean where larvae spend the first one to two months of their lives before recruiting to estuaries. The South American B. aurea and B. pectinata have broadly similar reproductive behaviours, but most spawning and larval ontogeny may occur within the estuary and coastal lagoons (Acha & Macchi 2000, Lajud et al. 2016, Salvador & Muelbert 2019). Offshore temperatures control growth rates of B. patronus and B. tyrannus larvae (Warlen 1988, 1992). Size‐selective mortality of B. patronus larvae, probably from predation, occurs before larvae ingress to estuaries (Grimes & Isely 1996). A growth‐rate analysis of B. tyrannus larvae offshore from North Carolina concluded that storm winds and temperature fluxes (falling) were the primary variables affecting growth rates and potentially recruitment (Maillet & Checkley 1991). Growth rates steadily declined as larvae emigrated from offshore, warmer waters to cooler waters near the North Carolina coast (Warlen 1992) and to Chesapeake Bay (Lozano & Houde 2013). In Chesapeake Bay, temperature continued to be an important variable controlling growth of age‐0+ juvenile B. tyrannus. Growth rate is a predictor of age‐0+ recruitment success (Annis et al. 2011, Humphrey et al. 2014). Recruitment levels of juvenile, young‐of‐the‐year B. tyrannus and other Brevoortia species are governed by estuarine hydrological factors and measures of primary productivity, e.g. chlorophyll a (Deegan 1990, Govoni 1997, Houde et al. 2016, Salvador & Muelbert 2019).
3.6.5 Morone saxatilis (Moronidae)
The anadromous Morone saxatilis spawns near the salt–fresh interface in estuaries on the east and west coasts of North America, although the west coast populations were introduced. Daily cohorts are produced during a two‐month spawning period (Rutherford & Houde 1995). Year‐class strength is coarsely set when larvae are only 8 mm in length and is determined by mortality and growth rates operating during the egg and larval stages (Secor & Houde 1995, Rutherford et al. 1997). The highest mortalities occur in egg, yolk‐sac and early‐feeding larval stages when >99% cumulative mortality typically occurs in spawning tributaries of Chesapeake Bay (Rutherford & Houde 1995, Secor & Houde 1995) (Table 3.2). Weight‐specific growth and mortality rates of larvae are high and variable and are responsive to local environmental factors (Houde 1996), particularly those near the estuarine salt front and turbidity maximum zone (North & Houde 2001, 2006). Amongst the interacting factors that control mortality and growth of larvae are precipitation and freshwater flows, prey abundances, temperature, conductivity (salinity) and pH (Uphoff 1989, Secor & Houde 1995, Rutherford et al. 1997, Limburg & Pace 1999, Kimmerer et al. 2001, Secor et al. 2017). In estuaries with the most extensive spawning areas, for example the tidal Hudson River system (USA), biotic variables, such as prey abundance, may play a more important role than hydrography in determining survival and growth (Limburg & Pace 1999). However, in the large and dynamic St Lawrence River system (Canada), the estuarine turbidity maximum region is a key feature associated with successful feeding and production of young (Vanalderweireldt et al. 2019a, 2019b).
Table 3.2 Stage‐specific mortality rates and abundances at the onset of stage for Morone saxatilis in the Patuxent River sub‐estuary of Chesapeake Bay in 1991. YSL = yolk‐sac larvae; FFL = first‐feeding larvae; PFL = post‐finfold larvae; t = days in stage; N stg = abundance calculated at youngest age for each stage: Z t = daily mortality rate (d−1); Z stg = cumulative mortality per stage (from Secor & Houde 1995).
| Stage | N stg | t | Z t | Z stg | Stage (%) |
|---|---|---|---|---|---|
| Egg‐YSL | 6.46 × 108 | 2.2 | 0.09 | 0.20 | 18.1 |
| YSL‐FFL | 5.43 × 108 | 6 | 0.97 | 5.80 | 99.7 |
| FFL‐PFL | 1.64 × 106 | 20 | 0.15 | 2.95 | 94.8 |
| PFL | 0.09 × 106 | 25 | 0.07 | 1.81 | 83.6 |
Mortality rates and cumulative mortalities of Morone saxatilis larvae are dependent on temperature. Larval cohorts experiencing lowest mortality