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Alosa Sapidissima (Wilson 1811): American Shad

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Alosa Sapidissima (Wilson 1811): American Shad Alosa Sapidissima (Wilson 1811): American Shad (Also: common shad, Atlantic shad, white shad) KC Dill FISH 423 – Fall 2011 http://nas.er.usgs.gov/queries/factsheet.aspx?SpeciesID=49 numbering more than 60 on the first arch, before Diagnostic Information the bend (Moyle 1976). The Pacific sardine Taxonomy: Order – Clupeiformes (Sardinops sagax) is the fish in this region that most closely resembles A. sapidissima (Figure Family – Clupeidae 2), but it is much smaller (up to 41 cm in length, but usually no more than 30 cm). S. sagax does Genus – Alosa have dark spots similar to those of the American Species – sapidissima shad, but they have more variation in number and placement. In addition, it has a shallower body and less pronounced scutes than the shad. Finally, the Pacific sardine is a pelagic fish and American shad is an anadromous would not be located in river systems as is the member of the herring family. It is the largest American shad (Peterson et al 1999). clupeid and the only anadromous herring on the West Coast. As a clupeid, A. sapidissima is thin- bodied and has a small dorsal fin approximately midway down the back, with pelvic fins positioned abdominally. Herring have no adipose fin or fin spines. Clupeids also possess a deeply forked caudal fin and cycloid scales, which grow by adding concentric layers (Figure 1). The head lacks scales (Peterson et al 1999). As the only member of the genus Alosa present on the West Coast, American shad is relatively easy to identify. It is large, up to 76 cm, and has a deep belly with bony scutes, giving it a saw Figure 1. Diagram of cycloid-type scales tooth appearance. The coloring of live fish tends (reproduced from earthlife.net to be a steel blue on the back with silver sides http://nathistoc.bio.uci.edu/Fish/Sardinops%20sa and belly. A row of dark spots starting just h) behind the operculum is a useful identifying feature for this fish. The gill rakers tend to be long and numerous, typically al 2003). This usually results in spawning in the main stems of rivers, rather than the more distant and colder tributaries preferred by salmon. Males are generally the first to arrive and are three years old on average, while the females follow after and are typically around four years of age. Historically, American shad begin to run in small numbers in March or April, with a peak around June. Shad spawn soon after entering freshwater, with the females releasing between 30,000 and 300,000 eggs, depending on her age Figure 2. Pacific sardine pictured on top, American shad is on bottom. Note differences and size (Moyle 1976). Based on their location in body shape and depth. and the ambient temperature, American shad may be either semelparous or iteroparous. This occurs along a latitudinal cline, with semelparity Life History and Basic Ecology dominant in the more southern, warmer waters Life Cycle and Reproductive Strategies and iteroparity more typical of the northern, colder areas (Leggett and Carscadden 1978). The life history of the American shad is The timing of spawning also varies spatially, complex and spatially variable. Like our native apparently in response to temperature. As shown salmon, A. sapidissima is anadromous, with the in Figure 3, spawning time moves earlier in the exception of one isolated population in Millerton year with decreasing latitude and increasing Lake in California, which has been landlocked temperature (Limburg et al 2003). since 1955 (DJ Hasselman University of Once the eggs have been released, they tend to Washington personal communication). Most of remain suspended in the water column and are the research in this area has been focused on the gradually carried downstream by the currents. American shad’s native range, along the eastern The eggs hatch quickly, on the order of three to coast of North America. In its native area, the six days (Moyle 1976). Larvae then recruit to the shad exhibits a high rate of philopatry, with 97% shorelines to rear before transforming to young- of spawners returning to their natal stream of-the-year juveniles. In the Columbia River (Melvin et al 1986). Adults swim upriver and basin, this process appears to take approximately can spawn in a wide range of habitats, although two months, with larvae emerging in June and they appear to prefer areas with temperatures July (for 1995 and 1996), recruiting to the between 14°C and 24.5°C and a low water shoreline in July and August, and finally velocity, typically less than 0.7 m/s (Petersen et River systems. Adults spend three to six years in the marine environment before returning to their natal streams to spawn (Petersen et al 2003; Moyle 1976; DJ Hasselman personal communication). Feeding Habits American shad primarily feed on zooplankton, although the adults will consume crustaceans and small fish in some areas. Adults typically do not feed at all during spawning or Figure 3. Periods of spawning runs for the American when in freshwater (Facey and Van Den Avyle shad within its native range (Source: Limburg et al. 1986). Juveniles feed on smaller zooplankton 2003) and their diet in the Pacific Northwest consists mainly of Cyclopoid and Calanoid copepods and appearing as young juveniles in late July and various cladocerans, but they will feed September (Petersen et al 2003). Juveniles opportunistically on insect larvae and other exhibit diel migration, tending to stay at the small planktonic organisms (Petersen et al bottom of the river channel or near shore during 2003). Again, the Millerton Lake population the day and move through the water column at presents an exception, as there is anecdotal night (Gadomski and Barfoot 1998). Little is evidence they have become piscivorous, known about the migration patterns of American predating on the threadfin shad. The gill rakers shad once they leave the rivers. Juveniles will have become reduced in both length and number often overwinter in nearshore estuaries, but accordingly (DJ Hasselman personal others leave immediately for the open ocean. communication). Mature adults in the Atlantic may migrate to the Bay of Fundy to feed between spawning Environmental Optima and Tolerances seasons, although this pattern does not hold true American shad have the ability to adapt for the first year of life (Walther and Thorrold to a fairly wide range of environments, as 2010). Even less is known about migration evidenced by their anadromous lifestyle and patterns in the Pacific, but it appears the shad variable life histories and feeding strategies migrate north from spawning sites, as indicated based on temperature and location. However, by higher than usual by-catch of American shad temperature is a very important factor for shad on trawls north of the Columbia and Sacramento as it determines when and where they will migrate (Leggett and Whitney 1972). When only five hours (Facey and Van Den Avyle spawning, A. sapidissima prefers a water 1986). temperature between 14°C and 21°C and their An adequate supply of dissolved oxygen is also outer limits for temperature have been recorded important for American shad. A dissolved as low as 8°C and as high as 26°C (Facey and oxygen content greater than 5.0 mg/l is ideal for Van Den Avyle 1986). However, the highest spawning, with 4.0 mg/l being the minimum survival rates for eggs and larvae are achieved amount necessary for success (Chittenden 1973). between 15.5°C and 26.6°C (Leggett and Larvae exposed to concentrations lower than 2.9 Whitney 1972). Temperatures below 9°C tend to mg/l experience high mortality rates, with 100% be lethal to both eggs and larvae. Juvenile shad mortality below 1.0 mg/l (Facey and Van Den can tolerate colder waters and have been Avyle 1986). A minimum oxygen content of 2.5 recorded in waters as cold as 2°C, but they stop to 3.0 mg/l is necessary during juvenile feeding and begin experiencing negative effects migration, but they may be able to survive at around 6°C (Facey and Van Den Avyle 1986). lower levels for short periods of time When in the Atlantic Ocean, shad have been (Chittenden 1973). noted in temperatures ranging from 3°C to 15°C, The velocity of water movement is also very although most were found between 7°C and important during spawning and early life stages. 13°C (Neves and Depres 1979). No data has High velocities can affect mature adults been collected on American shad’s tolerances or swimming upstream by increasing the amount of preferences in the Pacific Ocean. energy expended to reach their spawning American shad must be able to acclimatize to a grounds. After spawning, the flow must be wide range of salinities over their lifetime. Eggs strong enough to keep the eggs suspended, but are laid in freshwater systems, but studies have not so strong that they are washed out into the determined some eggs can successfully hatch in sea or into other inhospitable environments mild salinities of 7.5 ppt to 15 ppt. None of the (Facey and Van Den Avyle 1986). The optimal eggs in the study were viable at salinities of 22.5 water velocity for American shad is around 0.7 ppt and above. Juvenile fish are capable of m/s (Petersen et al 2003). Turbidity level, depth tolerating both fresh and brackish waters, and of water, and type of substrate do not appear to frequently travel between estuaries and rivers affect shad much, and they are able to (Chittenden 1972). Adult shad must spend two successfully spawn in a wide range of conditions or three days in brackish water before venturing (Facey and Van Den Avyle 1986).
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