Sardines are small, oily forage fish belonging to the Clupeidae family, which includes herrings and shads. They are known for swimming in vast, dense schools in the open ocean. Sardines are filter feeders, meaning their diet consists almost entirely of plankton—microscopic plants and animals drifting in the water column. This feeding strategy places them at a low trophic level, making them an important link in the marine food web.
The Plankton-Based Diet
The sardine diet is composed of two main categories of plankton: phytoplankton and zooplankton, making them omnivorous. Phytoplankton are microscopic, plant-like organisms, such as diatoms and algae. Zooplankton are tiny, animal-like organisms, which include small crustaceans like copepods and larval stages of other marine animals. While sardines consume both, their preference often leans toward the more energy-rich zooplankton. Studies frequently show a high percentage of zooplankton, especially when prey is abundant.
How Sardines Filter Prey
Sardines capture these microscopic organisms through continuous-flow filter feeding, or ram ventilation. The mechanism centers on the gill rakers, which are bony or cartilaginous projections found on the gill arches. These rakers form a comb-like structure that acts as a sieve to strain food particles from the water. The sardine swims forward with its mouth open, allowing a continuous flow of water to pass into the mouth and over the gill arches. The water passes through the fine-meshed gill rakers, which trap the plankton particles while the water exits through the gill slits.
Dietary Shifts Across the Life Cycle
A sardine’s diet changes predictably as it grows from a larva to a mature adult, a process known as an ontogenetic shift. Larval and young juvenile sardines initially rely on smaller organisms, primarily feeding on phytoplankton. As the fish grows, its feeding apparatus, including the gill rakers, develops the necessary structure and spacing to capture larger prey. Mature adult sardines shift their feeding to focus on larger, energy-dense zooplankton, such as copepods. Environmental factors like seasonal changes and upwelling events that concentrate plankton also heavily influence the specific species of plankton available for consumption.
The Sardine’s Role in the Food Web
The sardine’s plankton-based diet positions it as a forage fish in the marine food web. By consuming phytoplankton and zooplankton, sardines transfer the energy fixed at the base of the food web to higher trophic levels. This energy transfer supports a wide array of marine predators, including larger fish like tuna and mackerel, sharks, seals, dolphins, and seabirds. The health and abundance of sardine populations are directly linked to the availability of their plankton food source. Changes in ocean conditions, such as warming waters, can reduce the size and availability of plankton, forcing sardines to rely on less efficient filter feeding. When sardine populations fluctuate due to these environmental pressures, the entire ecosystem feels the impact, affecting the survival and reproductive success of the many predators that rely on them.