The small, oily fish commonly known as the sardine is often misunderstood in terms of its diet. Sardines are not herbivores; instead, they are classified as omnivores with a specialized feeding strategy. They occupy a low trophic level, feeding on microscopic organisms that drift in the water column. Their role in the marine ecosystem depends on their ability to consume both plant and animal matter through filter feeding.
The Primary Sardine Diet
Sardines consume plankton, the diverse organisms suspended in the water, which includes both plant-like and animal-like forms. The plant portion of their diet is phytoplankton, consisting of single-celled algae such as diatoms and dinoflagellates. They also consume zooplankton, which are tiny animals like copepods, crustacean eggs, and various larval fish. Studies show that zooplankton, particularly copepods, often make up the bulk of their food intake in terms of energy and volume. The availability of these food sources dictates what they ultimately consume, highlighting their opportunistic nature.
The Mechanics of Filter Feeding
Sardines are ram-feeding filter feeders, a method where the fish swims forward with its mouth open to continuously strain particles from the water. This continuous swimming processes the large volumes of water required to meet their energy needs. The physical structures responsible for capturing microscopic food are the gill rakers, which are bony, comb-like projections extending from the gill arches.
These gill rakers act as a fine sieve, trapping food particles as water flows past them and out over the gills toward the esophagus. Sardines can effectively retain prey as small as a few micrometers, such as tiny phytoplankton cells. For larger individual prey, the fish can switch to a more selective feeding mode known as particulate feeding.
Sardines in the Marine Food Web
As plankton consumers, sardines play a key role in transferring energy from the base of the food web to higher trophic levels. They convert the energy stored in microscopic plankton into a food source for a wide array of predators. Their trophic level is estimated to be around 2.8, indicating their position between primary producers and higher-level carnivores. Sardines are categorized as a forage fish, serving as a primary food source for seabirds, marine mammals, and larger predatory fish such as tuna and sharks. Their tendency to aggregate in dense schools makes them a vulnerable target, influencing the population dynamics of the species that rely on them.