ingress to the Bay.
In the northern Gulf of Mexico, larvae of several sciaenid species experience slow onshore (northerly) transport, followed by rapid lateral transport to the west–northwest in a westward‐flowing, coastal boundary current that delivers larvae to estuaries in Texas, far west of the offshore spawning areas sampled by Cowan & Shaw (1988). Transit times for sciaenid larvae to reach estuaries from offshore sites range from 50 to 90 days (Cowan & Shaw 1988, Shaw et al. 1988). A numerical model (Brown et al. 2004) indicated that larvae of Sciaenops ocellatus were transported to a Texas estuary in a coastal boundary current, as hypothesised by Cowan & Shaw (1988). Sciaenid larvae may also have a swimming capability sufficient to gain ingress to estuaries from nearshore coastal waters. For example, postflexion larvae of S. ocellatus can swim at velocities, and with endurance, sufficient to direct their transport (Faria et al. 2009).
There is extensive knowledge on ages and stages of ingress by sciaenid larvae on the Atlantic and Gulf coasts of the USA, e.g. Sciaenops ocellatus, Leiostomus xanthurus, Micropogonias undulatus. At ingress, larvae appear to use STST or at least remain at moderate to deep depths in the water column, to facilitate ingress in near‐bottom inflows at estuary mouths (Holt et al. 1989, Norcross 1991, Govoni & Pietrafesa 1994, Forward et al. 1999, Reiss & McConaugha 1999, Schieler et al. 2014, Hale & Targett 2018). A high proportion of larvae of M. undulatus at the mouth of Chesapeake Bay remains below the surface mixed layer in the classic two‐layer flow near the mouth (Norcross 1991), favouring ingress of larvae. As noted by Norcross (1991), not all larvae need to remain consistently in the bottom layer to assure successful ingress and recruitment. Additionally, Hare et al. (2005a) reported tidally driven vertical migrations by M. undulatus at the Chesapeake Bay mouth and calculated a substantial up‐estuary larval flux.
Sciaenops ocellatus provides a good example in which mortality rates and back‐calculated growth rates were analysed to evaluate early‐life processes affecting recruitment. A seasonal pattern of growth and mortality rates of larvae indicated highest growth and lowest mortality during fall months under falling temperatures in the Aransas Estuary (Texas, USA) (Rooker et al. 1997, 1999). Research on larvae and newly settled juveniles indicated that mean rates of weight‐specific growth (about 5% d−1) and mortality (about 13% d−1) were similar in two years. In a two‐decade analysis of age‐0+ juveniles of S. ocellatus in Texas estuaries, Scharf (2000) reported substantial spatial variability in growth and mortality rates amongst estuaries. Results supported the probability that juvenile mortality and growth processes play a significant role in regulating recruitment and year‐class sizes of S. ocellatus. Abundances of juveniles amongst Texas estuaries were positively correlated, indicating that offshore abundances of eggs and larvae in each year, and hydrodynamic processes that disperse larvae to estuaries, set the stage for subsequent estuarine processes in which local environmental and habitat variables governed growth and mortality of settled juveniles. Additionally, there was weak evidence for density‐dependent mortality in S. ocellatus juveniles that could regulate recruitment (Scharf 2000).
3.6.3 Anchoa mitchilli (Engraulidae)
The engraulid Anchoa mitchilli is believed to be the most abundant fish in coastal waters on the east coast of North America and the Gulf of Mexico (Able & Fahay 2010). The spawning behaviour and early‐life‐stage distributions indicate retention and production in estuaries, but A. mitchilli also occurs on the inner continental shelf (Able 2005, Able & Fahay 2010). Eggs and larvae occur ubiquitously in estuaries and coastal embayments but, in large estuaries such as Chesapeake Bay, are most abundant in seaward segments at salinities >10 (Rilling & Houde 1999, Auth 2003). Anchoa mitchilli matures at less than one year of age and <50 mm TL (Zastrow et al. 1991). Few recruited individuals (four to five months of age) survive to age 1 (Newberger & Houde 1995, Lapolla 2001). Accordingly, key recruitment processes are concentrated in early‐life stages. Rates of growth, mortality and biomass production in the larval stage are high and variable (Jung & Houde 2004b). Annual recruitments (age‐0, 30 mm TL) in Chesapeake Bay varied ninefold over a six‐year study period (Jung & Houde 2004a).
Preflexion larvae (<10 mm) utilise tidal dynamics or adopt other behaviours to migrate up‐estuary in tributaries of Chesapeake Bay and the Hudson Estuary (Loos & Perry 1991, Schultz et al. 2000, 2003, 2005a). Analysis in the Hudson River indicated active migration by larvae, but STST was not the probable behaviour (Schultz et al. 2005b). Instead, a combination of diel and tidally mediated behaviours were utilised to maintain location and facilitate migration. In contrast, Loos & Perry (1991) presented evidence that postflexion larvae did utilise STST to migrate up‐estuary in a tidal tributary of Chesapeake Bay. Applying otolith chemistry, Kimura et al. (2000) detected up‐estuary migration by >20 mm A. mitchilli larvae in Chesapeake Bay. That migration probably was facilitated by active swimming and accounted for a >4 km d−1 up‐estuary movement.
Mortality rates of Anchoa mitchilli eggs in Chesapeake Bay are high, averaging 0.066 hr−1 (Dorsey et al. 1996). At that rate, more than 73% of a daily cohort perished before hatching that occurs at 20 hour post‐fertilisation and 27 °C. At 48 hour post‐fertilisation, the average cumulative mortality of eggs and yolk‐sac larvae of a daily cohort was 92.5% (Dorsey et al. 1996). Such high loss rates of eggs may seem surprising. But survival of A. mitchilli at hatching (averaging 27%) is higher than estimated for the pleuronectid Pleuronectes platessa (19%), based on temperature and hatching success reported by Harding et al. (1978). Egg and yolk‐sac larvae mortality rates of A. mitchilli in Great South Bay, New York (Castro & Cowen 1991), and in Biscayne Bay, Florida (Leak & Houde 1987), were similar to those in Chesapeake Bay.
Growth and mortality of Anchoa mitchilli larvae are temporally and spatially variable and positively related to temperature and prey levels. In a five‐year study in Chesapeake Bay, mean growth rates ranged from 0.68 to 0.81 mm d−1 (Auth 2003), which were similar to those reported from other areas (Leak & Houde 1987, Castro & Cowen 1991, Rilling & Houde 1999, Jordan et al. 2000). Lapolla (2001) and Castro & Cowen (1991) noted that A. mitchilli larvae and small juveniles grew faster in high‐latitude bays and estuaries within the distributional range of this species, a possible expression of latitudinal compensation. Mortality rates of A. mitchilli larvae differed spatially within Chesapeake Bay and were higher in June (M = 0.41 d−1) than in July (M = 0.23 d−1), attributable to probable higher predation by jellyfishes in June (Rilling & Houde 1999). Averaged mortality rates of larvae in Biscayne Bay, Florida, were similar to Chesapeake Bay (Leak & Houde 1987), but rates in Great South Bay, New York (M > 0.50 d−1) (Castro & Cowen 1991), were higher. In Chesapeake Bay, mortality rates declined as larvae grew, without indication that mortality was density dependent (Rilling & Houde 1999).
3.6.4 Brevoortia tyrannus and Brevoortia spp. (Clupeidae)
The abundant clupeid Brevoortia tyrannus is distributed broadly in shelf waters along the US Atlantic coast. Spawning occurs primarily in nearshore coastal waters (Able & Fahay 1998, Checkley et al. 1988, MDSG 2009) during fall and winter months. Some spawning and larval production occur in the lower portions of estuaries and may be increasing in recent decades during the spring‐summer periods (Ortner et al. 1999, Simpson et al. 2017). Larvae are dispersed in shelf waters and advected alongshore and towards estuaries (Werner et al. 1999, Epifanio & Garvine 2001). Large estuarine systems such as Chesapeake Bay, Delaware Bay and the Carolina sounds, at least historically, received the largest shares of ingressing larvae. Recent research found that larvae ingressing into a New Jersey estuary had two primary sources that influenced time of arrival. Historically, larvae arriving in the fall/early winter were from local spawning, while those arriving in late winter were from coastal North Carolina (Warlen et al. 2002, Light & Able 2003). The pattern of ingress to New Jersey estuaries has changed since the late 1990s and ingress now occurs primarily in summer and fall (Able & Fahay 2010).
The dispersal of Brevoortia tyrannus larvae is mostly