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Atlantic menhaden produce pelagic eggs about 1.5 mm in diameter which hatch within 2.5-2.9 days at an average temperature of 15.5°C. Embryonic development is completed in <36 hr at 20-25°C, but takes about 200 hr at 10°C. Egg mortalities observed in the laboratory were >90% at 10°C, and 48-92% at 15, 20 and 25°C.
Larvae which hatch offshore are transported shoreward and enter estuaries in the south Atlantic region after 1-3 months at sea at a size of 14-34 mm FL. Larval immigration into estuaries occurs during May-October in the north Atlantic region, October-June in the mid- Atlantic, and December-May in the south Atlantic.
Metamorphosis to the juvenile stage occurs at about 38 mm total length (TL) during late April-May in North Carolina estuaries and later in the year farther north. Most larvae entered the White Oak estuary (North Carolina) in March and moved upstream to a fresh water-low salinity zone where they transformed into "pre-juveniles" in late March-April and then into juveniles in late April-May. Other studies also show young menhaden are more abundant in shallow, low salinity (< 5%) estuarine zones. Metamorphosis to the "pre- juvenile" stage occurs at lengths >30 mm TL and to the juvenile stage beyond 38 mm TL. Metamorphosis is rarely successful outside of the low-salinity estuarine zone, although Atlantic menhaden have been successfully reared from eggs to juveniles in high salinity water.
The morphological changes that occur at metamorphosis are associated with a change in feeding behavior. Larvae feed on individual zooplankters, whereas juveniles rely more heavily on filter feeding. This shift in feeding behavior is associated with a loss of teeth and an increase in the number and complexity of the gill rakers through which sea water is filtered as it passes through the gills Older larvae (25-32 mm) feed on large copepods, but only rarely on small zooplanktonic organisms. Fish larger than 40 mm FL feed primarily on phytoplankton, but zooplankton has also been reported as an equally important food source in juvenile Atlantic menhaden. Juveniles are capable of filtering particles as small as 7-9 microns and, thus, directly utilize the abundant small photosynthetic organisms that are not consumed by most other species of fish. Detritus derived from saltmarsh cordgrass (Spartino altemiflora) has also been reported as a primary food source for juveniles in North Carolina saltmarshes. Based on calculations incorporating feeding rates and population estimates from eight east coast estuaries, other scientific studies concluded that juveniles must consume more food during estuarine residency than is available from a strictly phytoplankton-based food chain.
Young-of-the-year menhaden congregate in dense schools as they leave shallow, estuarine waters for the ocean, principally during August to November (earliest in the north Atlantic region) at lengths of 75-110 mm TL. Many of these juveniles migrate south along the North Carolina coast as far as Florida in late fall and early winter and then redistribute northward by size as age-1 fish during the following spring and summer. Larvae which enter the estuaries late in the season may remain there for an additional year and emigrate to the ocean at age 1.
Age-1 menhaden migrate north and south along the coast over a greater distance than young-of-the-year juveniles. Abundance and distribution of juvenile Atlantic menhaden is monitored by the marine resource agencies of most Atlantic coast states under a variety of estuarine surveys using trawls and seines. According to a survey conducted by the Atlantic Menhaden Advisory Committee in February 1990, juvenile menhaden have been taken from Massachusetts to Georgia (there is no survey on the Atlantic coast of Florida). Juvenile menhaden were observed in Gulf of Maine estuaries during 1998 and 1999.
Juveniles collected at 2-3 day intervals have shown growth rates of nearly 1 mm/day. Water temperatures >33°C caused death in young-of-the-year and age-1 Atlantic menhaden, although the time until death depended, in part, on acclimation factors. Sudden exposure to lethal temperatures, for example, caused greater mortality. Juvenile Atlantic menhaden can adjust rapidly to abrupt changes (increase or decrease) in salinity from 3.5 to 35% and vice-versa. Juveniles raised in low salinity water (5-10%) were more active, ate more, had higher metabolic rates, and grew faster than juveniles raised in high salinity water (28-34%).
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