The Atlantic croaker, Micropogonias undulatus, is a common and ecologically significant fish found in the western Atlantic Ocean. As a member of the drum family (Sciaenidae), this species plays a crucial role in estuarine and marine ecosystems. This article delves into the dietary habits of the Atlantic croaker, exploring its food sources, feeding behaviors, and how diet variability impacts its condition and the broader ecosystem.
Identification and Distribution
The Atlantic croaker is a medium-sized fish, typically less than 14 inches long, with a slightly elongated and moderately compressed body. Its coloration is silvery with a pinkish cast, with grayish back and upper sides adorned with black spots forming irregular, oblique lines above the lateral line. The dorsal fin features small black dots and a black edge, while other fins range from pale to yellowish. Distinguishing characteristics include three to five pairs of small barbels or "whiskers" on their chins to help them feel for food on the sea floor; a lateral line that extends to the tip of its caudal (tail) fin; inferior mouth (located to the bottom of the head facing the ground), and brown vertical stripes on its sides. Adults are silver with a pinkish cast, while young are silvery and iridescent.
This species inhabits the northern and eastern parts of the Gulf of Mexico, along the Atlantic coast of the United States from south Florida to Massachusetts, in the Greater Antilles, and along the South American Atlantic coast from Surinam to Argentina. In Florida, they are seldom found south of Tampa Bay or the Indian River Lagoon.
Life Cycle and Habitat
Atlantic croakers typically live up to 8 years, maturing at the end of their first or second year when they are 6 to 10 inches long. Spawning occurs over the nearshore continental shelf during the late fall and winter. Post-larval and juvenile Atlantic croakers utilize estuarine nursery areas, where they feed on benthic plankton and invertebrates.
Atlantic croakers are euryhaline, meaning they can tolerate a wide range of salinities, from 0 ppt to full seawater at 35 ppt, and even hypersaline waters exceeding 35 ppt. They are also eurythermal, thriving in waters with bottom temperatures ranging from 9°C to 32°C (48°F to 90°F), with the highest abundance observed in waters over 24°C (75°F). Beginning in August, tiny young enter the Chesapeake Bay and travel to low-salinity and freshwater creeks. They move to deeper parts of tidal rivers for the winter. Juveniles leave the bay with the adults the following autumn.
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Dietary Preferences
The Atlantic croaker's diet varies depending on its life stage, habitat, and the availability of prey.
Larval Stage
Beginning at day three, post-hatch, rotifers (Brachionus sp.) are a common food source for marine fish larvae due to their smaller size (90-250 microns) although they must be cultured as a live feed. Houde and Ramsey (1971) successfully reared croaker larvae under static conditions by maintaining them for approximately one week in a dense culture of Chlorella phytoplankton followed by a slow water exchange rate with copepod nauplii and other wild zooplankters introduced at that time. Best results are obtained when larvae are reared in a static culture of Isochrysis galbana (phytoplankton) or similar algae with high levels of docosahexanoic acid (DHA). Algae should be added to culture tanks by 3 days after hatching when larvae are ready to feed. Rotifers should be fed on days 3-12 post-hatch. Microparticulate diets (250 micron particle size) can be added starting on day 6 at a rate of 0.001g/L/day. A small amount of Artemia nauplii (0.25-0.5/ml) should be added to the rearing tanks 10-12 days after hatching along with the rotifers to help wean larvae off rotifers. Artemia nauplii alone should be added to the tanks at a concentration of 0.5-1/ml on days 13 through metamorphosis (approximately day 25). On day 13, increase the particle size for dry feed to 400 microns. Each day, increase the amount of dry feed by 0.001g/L or as needed.
Juvenile Stage
Post-larval and juvenile Atlantic croaker occupy estuarine nursery areas where they feed on benthic plankton and invertebrates, such as grass shrimp and worms.
Adult Stage
Adult Atlantic croakers are bottom feeders with a diverse diet that includes:
- Polychaete worms
- Crustaceans (e.g., shrimp, crabs, mysid shrimp, Atlantic rock crab)
- Small fish (e.g., bay anchovy, Atlantic menhaden)
- Squid
- Detritus (dead and decomposing plant and animal matter)
A study in Chesapeake Bay revealed that fish, particularly bay anchovies (Anchoa mitchilli), can constitute a significant portion (at least 20% by weight) of the croaker's diet during the summer. This suggests a crepuscular feeding habit, where croakers may ascend in the water column to feed on pelagic prey during twilight hours.
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Methods of Studying Diet
Scientists use various methods to study the diet of Atlantic croaker, including:
- Stomach Content Analysis: This involves examining the contents of the fish's stomach to identify and quantify the prey items. As of 2021, 3,267 individual stomachs have been analyzed. Out of these, 1,276 stomachs have had identifiable prey items present. The top 5 prey items analyzed are: mysid shrimp, bay anchovy, Atlantic rock crab, Atlantic menhaden, and squid.
- Stable Isotope Analysis: This technique analyzes the ratios of stable isotopes (e.g., carbon and nitrogen) in the fish's tissues to infer its trophic level and primary food sources.
- Bioenergetic Modeling: This approach uses mathematical models to estimate the energy intake, expenditure, and growth of the fish based on its diet and environmental conditions.
Impact of Diet on Condition and Ecosystem
The diet of Atlantic croaker significantly influences its overall condition, growth, and reproductive success. Studies have shown that croakers feeding on anchovies exhibit better condition compared to those primarily consuming polychaetes. This is reflected in various condition measures, including RNA:DNA ratios, Fulton's condition factor, relative weight, energy density, hepatosomatic index, and gonadosomatic index.
Furthermore, the Atlantic croaker plays a vital role in the food web, both as a predator and as prey. Its consumption of various invertebrates and small fish helps regulate their populations, while it serves as a food source for larger predators such as striped bass, shark, spotted seatrout, and other croakers.
Bioenergetic models suggest that Atlantic croaker can be an important competitor with other fish species, such as weakfish and striped bass, for food resources, particularly during periods of high population abundance.
Aquaculture and Feeding Regimes
Aquaculture methods for Atlantic croaker are only partially known. However, research has provided insights into optimal feeding regimes for larvae and juveniles.
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- Larvae: Rotifers, copepod nauplii, and microparticulate diets are commonly used to feed croaker larvae in hatchery settings. Static cultures of phytoplankton, such as Isochrysis galbana, can also enhance larval growth and survival.
- Juveniles: Juvenile croakers respond well to high-protein (45%) formulated diets, exhibiting rapid growth and survival in cage systems. Experiments have shown that diets with moderate lipid levels (around 8%) promote better growth compared to higher lipid diets.
Threats and Conservation
The Atlantic croaker population faces several threats, including overfishing, habitat degradation, and pollution. Changes in prey availability due to environmental factors, such as hypoxia (low oxygen levels), can also impact their diet and overall health.
While the Atlantic croaker is not currently listed as an endangered or threatened species, it is essential to monitor its population and implement sustainable fishing practices to ensure its long-term survival. Protecting and restoring estuarine habitats, reducing pollution, and managing fisheries responsibly are crucial steps in conserving this valuable species.