Anchovies are small, silvery, schooling fish belonging to the family Engraulidae, primarily found in marine and brackish waters worldwide. As small pelagic forage fish, they occupy a low position in the oceanic food web, making their feeding habits a central function in the health of the entire ecosystem. Their diet is intimately linked to the microscopic life in the water column, acting as a direct link between the ocean’s primary producers and its larger predators.
Primary Food Sources
The diet of anchovies consists predominantly of plankton, which are the tiny organisms suspended in the water column. This planktonic food source is divided into two major groups, phytoplankton and zooplankton. The adult anchovy diet is largely omnivorous, consuming a ratio of both. Phytoplankton are microscopic, plant-like organisms, such as diatoms and dinoflagellates, which form the base of the marine food web by converting sunlight into energy through photosynthesis. Adult anchovies will consume these plant cells, particularly when they are abundant during seasonal blooms.
Zooplankton, however, represent the most significant portion of the adult anchovy’s energy intake. This group consists of microscopic animals, including copepods, euphausiids (krill), and the larval stages of larger marine invertebrates and fish. Copepods are frequently cited as the main energy source for many anchovy species across different oceans. Juvenile anchovies and larvae tend to focus their consumption on the smallest zooplankton available, such as copepod eggs and nauplii.
The Filter Feeding Mechanism
Anchovies employ a specialized method of feeding called ram filter feeding to consume their microscopic diet. This mechanism involves the fish swimming forward with its mouth held wide open, allowing large volumes of water to continuously flow into the buccal cavity. The forward motion of the anchovy forces the water, along with the suspended plankton, through the filtering apparatus.
The filtering structure is composed of modified bony projections called gill rakers, which line the inner surface of the gill arches. These rakers function like a sieve, trapping the planktonic organisms as the water passes over them and exits through the gill openings. The captured food particles are then transported toward the esophagus, where they accumulate before being swallowed. While filter feeding is generally used for small, suspended prey, anchovies can also switch to actively biting and capturing larger individual zooplankton when prey density is low.
Influences on Dietary Intake
The exact composition of an anchovy’s diet is not static and changes based on several biological and environmental factors. A primary influence is the fish’s life stage, known as an ontogenetic shift in diet. Anchovy larvae, being small and less developed, must consume tiny prey, often focusing on the eggs and early larval stages of copepods.
As the fish matures, its feeding apparatus allows it to target and retain larger zooplankton, such as adult copepods and small crustaceans. Environmental conditions also introduce significant variability in their feeding habits, particularly the location and season. Anchovies in different geographical areas show distinct differences in their prey selection. Seasonal changes, such as the timing of spring and autumn plankton blooms, dictate periods of intense feeding, resulting in higher nutritional content for the fish.
Ecological Significance
The dietary habits of the anchovy position it as a foundational component of the marine food web, serving as a crucial energetic link. By consuming vast quantities of microscopic plankton, anchovies effectively transfer the energy and nutrients from the base of the food chain to higher trophic levels. This role is why they are classified as a forage fish.
Anchovies are a primary food source for a wide array of marine predators, including commercially important larger fish species, seabirds like cormorants and pelicans, and marine mammals such as whales and sea lions. Fluctuations in anchovy populations, which are tied directly to the availability of their planktonic diet, can therefore have widespread consequences for these predator species. When anchovy populations decline or shift their distribution due to changing food availability, it can negatively impact the foraging success and reproductive output of the animals that rely on them for sustenance.