Читать книгу Fish and Fisheries in Estuaries - Группа авторов - Страница 86

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).