Krill are small, shrimp-like crustaceans and zooplankton, typically measuring around two inches in length. They form dense swarms that can stretch for miles across all the world’s oceans, representing a vast and mobile energy source. Krill are a foundational element in the global ocean’s food structure and a primary food source for a wide array of marine life. Small fish consume krill, making them major consumers alongside the ocean’s largest creatures.
Why Krill Are Essential to the Ocean Food Web
Krill are considered a significant link connecting the microscopic base of the food web to the higher trophic levels. They primarily feed on phytoplankton, which are minuscule, single-celled plants that convert sunlight into energy. By consuming this primary production, krill transform energy from the smallest organisms into a larger, nutrient-rich form suitable for consumption by bigger predators.
The sheer volume of krill biomass underscores their ecological importance, with Antarctic krill alone estimated to have a total biomass of several hundred million tonnes. This massive population is organized into incredibly dense swarms, which can reach up to 100,000 individuals per cubic yard. These aggregations make krill an extremely attractive and energy-efficient food source for animals ranging from small fish to the largest whales.
Krill are especially valued for their high nutritional content, being rich in Omega-3 fatty acids and the powerful antioxidant astaxanthin. This concentration of nutrients provides the necessary energy to fuel the metabolism and reproduction cycles of countless marine species. The efficiency of krill in transferring energy from phytoplankton upwards establishes them as a central component of marine ecosystem health.
Specific Small Fish That Feed on Krill
Many species of small, schooling fish, often referred to as forage fish, rely heavily on krill to sustain their populations. Species like Capelin, Atlantic Herring, and various types of Sand Lance are known to include krill, specifically euphausiids, as a major part of their diet. Krill are also a regular food source for commercially valuable fish like juvenile Salmon, Rockfish, and Sardines.
The importance of krill often shifts as fish mature, a process known as ontogenetic diet shift. Smaller, juvenile Capelin, for example, tend to feed predominantly on smaller zooplankton like copepods. However, as these Capelin grow larger, their diet shifts significantly to incorporate the more energy-dense krill, which become the largest contributor by weight to their food intake.
These small fish employ different methods to consume the dense krill swarms, predominantly using ram feeding. This involves the fish swimming rapidly through the water with their mouths open to engulf the krill and the surrounding water. They then filter the water out through specialized gill structures, retaining the krill. This technique is highly effective when targeting the massive, slow-moving krill aggregations.
Other small predators, such as Lanternfish found in deeper waters, also actively hunt krill during their daily vertical migrations. These small fish are adapted to pursue krill as they move up the water column at night to feed on phytoplankton near the surface, highlighting how krill availability influences predator behavior across different depths.
Krill as a Primary Food Source for Large Marine Animals
Krill serve as a universal food source, meaning small fish must compete with the ocean’s giants for this energy-rich prey. The most well-known consumers are the large baleen whales, such as the Blue Whale and Humpback Whale. A single Blue Whale can consume up to four tons of krill daily during peak feeding seasons.
These immense mammals are highly adapted for consuming krill, using specialized baleen plates to filter massive quantities of the small crustaceans from the seawater. Their feeding migrations often follow the seasonal abundance and distribution of krill swarms in polar and subpolar waters.
Krill are also the primary prey for several species of seals and seabirds, particularly in the Southern Ocean. Crabeater Seals, despite their name, primarily eat krill, using specialized lobed teeth to sieve the crustaceans from the water. Penguins, including Adélie and Emperor species, and various seabirds like Petrels and Albatrosses, rely on krill for their own survival and to feed their young.