American gizzard shad
The Gizzard shad (Dorosoma cepedianum) is a member of the herring family of fish, and is native to large swaths of fresh and brackish waters of the United States of America. The adult has a deep body, with a silvery-green coloration above fading to plain silver below. The gizzard shad commonly resides in freshwater lakes, reservoirs, rivers, and streams but can reside in brackish waters, as it does on the Atlantic coast of the United States. Their range is across most of the continental United States, although they typically go no further north than New York and no further west than New Mexico. They are large parts of many of the ecosystems they inhabit, and can drive changes in phyto- and zooplankton, thereby indirectly affecting other planktivorous fishes. The gizzard shad has been widely used as a food source for game fish, with varied success in management and effectiveness.
The gizzard shad's dorsal fin starts behind the insertion of the pelvic fins, and the last ray is greatly lengthened in adults but not in the young. They have a long anal fin, with 25 to 36 long soft-rays on the fin. The mouth of the Gizzard shad has a short, wide upper jaw with a deep notch along the ventral margin, and a weak, relatively smaller lower jaw. The mouth itself is subterminal to inferior (on the lower portion of the head), and the adults possess no teeth. The gizzard shad also has 90 to 275 gill rakers along the lower limbs. Their ventral (pelvic) fins are in the thoracic position, or in the chest region of the fish. The gizzard shad can range from millimeter and milligram size when fry to a maximum recorded length of 477 millimetres (18.8 in) and maximum weight of 1.56 kilograms (3.4 lb). The average length is typically larger in northern waters, and ranges from 284 millimetres (11.2 in) mm at age three years to 399 millimetres (15.7 in) at age 10. They have a highly reduced lateral line system, a feature shared by other members of the herring family. To compensate for this reduction in their mechanical sensory system, they have developed other methods of sensing movement.
Range and habitat
They have historically gone from North Dakota in the north-west of the United States down to New Mexico in the south-west, over to Florida in the south-east and up to the 40°N latitude line (they have historically been seen no farther north than the lower New York Harbor). They were not seen in many of the Great Lakes until the late 1800s and early 1900s, although they are suspected to be endemic to Lake Erie, reaching it after the last ice age. Gizzard shad typically live in lakes and reservoirs, although they can live in rivers and streams, and brackish waters. They reside in the limnetic zone, and can comprise up to 80% of fish biomass in certain lake systems. They prefer shallow lakes with muddy bottoms and relatively high turbidity. This may be due in part to their breeding preferences, but it probably arises from the fact that they have lower survival rates in clear waters and waters with high vegetative cover.
Diet and feeding habits
The gizzard shad is planktivorous in its early life, feeding mainly on phytoplankton, before switching to a diet of zooplankton when it becomes older. Their consumptive demand is so heavy it can cause collapses in the zooplankton community, which has far-reaching effects through the ecosystem they are a part of. They can switch to diets containing detritus, but their growth rates are decreased and they only do so when density of conspecifics is high and the zooplankton population has been depleted. Daphnia and other crustaceans make up a large portion of some gizzard shad diets. Gizzard shad feed mainly during the day, with minimal activity at night.
The start of the spawning period is typically between mid-May and early June, and is triggered by rising water temperatures. The number of eggs per individual varies between populations, but typically it is 12,500 eggs for a two year old individual and peaks at 380,000 eggs in an age 4 individual. The eggs are lain in shallow water in clumps, with seemingly no pairing off occurring between individuals. They spawn during the evening and the early hours of the night, and the eggs adhere to underwater vegetation and do not receive any parental attention. Feeding begins three to four days after hatching, and most individuals are 3.25 mm long upon hatching. The gizzard shad has very high fecundity and a rapid growth rate, meaning it can become a large part of an ecosystem, in terms of abundance and biomass, very quickly. They are capable of hybridizing with the closely related threadfin shad.
Gizzard shad were introduced into many lake and river systems as a source of food for game fish, such as walleye, bass, and trout, because of their small size and relatively high abundance. It was thought they would be easy food for game fish and would help increase the numbers of fish available, as well as decrease the pressures on prey species (like the bluegill). However, due to their rapid growth they can quickly grow beyond the size available to many predators. In addition, they spawn in large numbers and can reach densities high enough to ensure that many of them survive past the first year, making them essentially invulnerable to predation. They are also harmful exploitative competitors to other species, leading to declines in other fish species populations. They can increase productivity in an ecosystem through redistribution of nutrients (bringing the nutrients up from detritus deposits and making it available to other predator species).
The gizzard shad is so named because it possesses a gizzard, a sack filled with rocks or sand, that aids the animal in the breakdown of consumed food. The first part of the scientific name, Dorosoma is a reflection of the fact that, when young, the fish has a lancelet-shaped body (doro meaning lanceolate and soma meaning body). The second part of the name, cepedianum, is a reference to amateur French ichthyologist La Cépède.
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