Atlantic and Pacific herring feed primarily on tiny crustaceans called copepods, along with other zooplankton such as krill, fish larvae, and the occasional phytoplankton bloom. This seemingly modest diet powers one of the most important energy pipelines in the ocean, because herring convert microscopic plankton into calorie-dense packages that seabirds, marine mammals, cod, salmon, and dozens of other predators depend on. What herring eat, when they eat it, and how much of it they find ripples outward through entire marine ecosystems and even affects the nutritional quality of the herring fillets on your plate.
What Herring Actually Eat
If you opened a herring’s stomach on any given day, you would almost certainly find copepods. These rice-grain-sized crustaceans dominate the diet across herring populations worldwide. In the Norwegian Sea, the copepod Calanus finmarchicus is the centerpiece of the herring diet, with fish preferring larger, energy-rich individuals that are close to spawning age.1ICES Journal of Marine Science. Diets of herring, mackerel, and blue whiting in the Norwegian Sea in relation to Calanus finmarchicus distribution and temperature conditions In the Baltic Sea, herring target species like Temora longicornis and Eurytemora affinis.2ICES Journal of Marine Science. Selecting for three copepods—feeding of sprat and herring in the Baltic Sea Beyond copepods, herring also eat euphausiids (krill), amphipods, fish eggs, small fish larvae, and various other plankton. They are not strict specialists; herring adapt to whatever zooplankton is abundant in their local waters. But copepods remain the backbone of their diet in most regions and seasons.
Two Ways to Catch a Meal
Herring have a trick that many fish lack: they can switch between two completely different feeding strategies depending on how dense the prey field is. When prey is sparse, herring pick off individual organisms one at a time, darting and biting like most predatory fish. But when prey concentrations climb above a certain threshold, herring switch to filter feeding, swimming forward with their mouths open and straining plankton through their gill rakers. Laboratory video recordings showed that filter feeding becomes more profitable than biting above a critical prey concentration because the capture rate scales directly with how many organisms are in the water, while biting hits a ceiling set by how fast the fish can snap its jaws.3Journal of Fish Biology. The relative profitability of particulate‐ and filter‐feeding in the herring, Clupea harengus L.
This flexibility matters for understanding herring ecology. In patchy ocean environments where zooplankton can be extremely dense in one spot and nearly absent a few hundred meters away, herring can exploit both conditions. They bite selectively when they encounter a few high-value copepods, then switch to bulk harvesting when they hit a swarm. The ability to toggle between strategies helps explain why herring can thrive across such a wide range of ocean conditions.
How Diet Changes as Herring Grow
A herring larva fresh out of the egg does not eat the same things as an adult. The shift is gradual and tied to mouth size and swimming ability. In a study of autumn-spawned herring larvae in Newfoundland, the earliest larvae fed on nauplii (the tiny juvenile stages) of copepods like Temora longicornis and Oithona similis. As the larvae grew, their diet shifted to Pseudocalanus species, and by the late-larval stage they had graduated to larger calanoid copepods such as Calanus.4Journal of Plankton Research. Feeding ecology of autumn-spawned Atlantic herring (Clupea harengus) larvae in Trinity Bay, Newfoundland: Is recruitment linked to main prey availability?
This progression has real consequences for whether a generation of herring survives. If the right size of copepod is not available at the right time, larval herring starve. The match between when larvae hatch and when their preferred prey peaks in abundance is one of the strongest predictors of how many young herring make it to adulthood. A bad match, even by a few weeks, can mean poor recruitment for that entire year class.
Seasonal Swings in Diet and Body Condition
Herring do not eat the same quality or quantity of food year-round. Plankton blooms follow seasonal rhythms driven by light, temperature, and nutrient availability, and herring body condition tracks these rhythms closely. In the Baltic Sea, the lipid content of herring drops substantially from autumn through spring, falling from roughly 7.4% in November to about 4.7% by March.5DTU Orbit. Seasonal lipid dynamics of herring and sprat in the Baltic Sea and possible implications for cod reproduction The polyunsaturated fatty acids in herring flesh, which originate from phytoplankton and pass up through copepods, peak in autumn when feeding conditions are best and decline through winter.6ICES Journal of Marine Science. Forage fish quality: seasonal lipid dynamics of herring (Clupea harengus L.) and sprat (Sprattus sprattus L.) in the Baltic Sea
These seasonal changes are not just a physiological curiosity. They determine when herring are most nutritious as prey for other animals and when they are most valuable as food for humans. A herring caught in September is a different nutritional product from one caught in March, and the difference traces directly back to what zooplankton was available in the months before capture.
The Food Web Funnel
Herring occupy a position in the ocean that ecologists sometimes call a “wasp waist.” They sit between a vast base of plankton below and a wide array of predators above, funneling energy from microscopic organisms into forms that larger animals can use. Research on the North Sea food web showed that planktivorous fish like herring play a central role in initiating complex cascading effects both up and down the food chain.7PubMed Central. Interaction between top-down and bottom-up control in marine food webs When herring populations are healthy and well-fed, predator populations tend to follow. When herring decline or shift their distribution, the effects spread across trophic levels in sometimes unpredictable ways.
Ecosystem modeling in the Northeast Pacific has reinforced how critical this role is. Researchers who built detailed food web models of British Columbia’s marine ecosystem found it necessary to separate herring into juvenile and adult groups and to distinguish herring from other forage fish like eulachon and sand lance, because herring’s trophic connections are so numerous and complex that lumping them together with other small fish misrepresented the dynamics.8PLOS ONE. Herring supports Northeast Pacific predators and fisheries: Insights from ecosystem modelling and management strategy evaluation Herring are not just one forage fish among many; they are often the primary conduit for energy flow.
When Herring Diet Fails, Seabirds Pay the Price
Some of the clearest evidence for why herring diet quality matters comes from seabird studies. On Machias Seal Island in the Bay of Fundy, researchers tracked the breeding success of terns over multiple years and found roughly 50% variation in the energy density (fat content) of the juvenile herring that seabirds were feeding to their chicks. Breeding success of both Arctic and common terns tracked the energy density of herring in their diet closely.9PubMed. Seabirds as indicators of changes in marine ecosystems: ecological monitoring on Machias Seal Island In years when herring were lean, fewer chicks survived.
A parallel study of black-legged kittiwakes in Alaska found that Pacific herring, sand lance, and capelin together made up about 80% of kittiwake diets, and all three were characterized by high energy density. Diet quality stayed consistently high during the study period. What actually drove variation in breeding success was how much food parents could deliver to nests, not what kind of fish it was.10Marine Ecology Progress Series. Assessing the nutritional stress hypothesis: relative influence of diet quantity and quality on seabird productivity Together, these studies show that both the quality and quantity of herring in predator diets matter, and which factor dominates depends on local conditions. A fat herring delivered too infrequently is no better than a lean herring delivered on schedule.
Sharing the Table with Sprat
Herring are not the only fish chasing copepods. Sprat, a smaller relative, overlaps heavily with herring in both habitat and diet, and the two species compete directly for food. In the Baltic Sea, both species consume predominantly the same small copepods, particularly Temora longicornis and Eurytemora affinis. Sprat may actually have an edge in this competition: they had fewer empty stomachs and appeared more successful at finding and consuming prey, especially when the zooplankton community was dominated by small-bodied species. The dietary overlap between the two species decreased when zooplankton diversity increased, suggesting that a richer plankton community reduces competition by giving each species more options.2ICES Journal of Marine Science. Selecting for three copepods—feeding of sprat and herring in the Baltic Sea
In the North Sea, a study of pelagic fish feeding in Belgian waters confirmed significant dietary overlap between herring and sprat but noted that herring behaved more opportunistically, with a more varied diet composition than sprat. Sprat clustered tightly in what they ate, while herring stomachs were more scattered across prey types.11ICES Journal of Marine Science. Selective feeding by pelagic fish in the Belgian part of the North Sea Herring’s dietary flexibility may be a survival advantage: when their preferred copepods become scarce, they can pivot to alternatives that sprat cannot or do not exploit as readily.
This competition has population-level consequences. An age-structured modeling study of Bothnian Sea herring found that zooplankton availability and interspecific competition from sprat were more important drivers of herring population dynamics than top-down pressures like fishing or predation by cod and grey seals, especially for younger age classes.12PubMed. Interacting trophic forcing and the population dynamics of herring In other words, what herring eat and who they share it with can matter more to their survival than what eats them.
Climate Change Is Rearranging the Menu
Warming ocean temperatures are already affecting what herring can find to eat. In the Norwegian Sea, unusually warm conditions in 2002 caused Calanus finmarchicus to develop earlier in the season than normal. The second generation of copepods appeared ahead of schedule and in smaller body sizes, leaving herring with poorer feeding conditions because the fish prefer larger, pre-spawning copepods. To find suitable prey, herring had to migrate farther north than usual.1ICES Journal of Marine Science. Diets of herring, mackerel, and blue whiting in the Norwegian Sea in relation to Calanus finmarchicus distribution and temperature conditions
This kind of mismatch, where warming shifts the timing or size structure of plankton communities, is projected to become more common. If herring consistently have to travel farther or settle for smaller, less energy-rich copepods, the downstream effects on their body condition, reproductive success, and value to predators could be substantial. The concern is not that copepods will disappear entirely but that the right copepods will not be in the right place at the right time.
Ocean acidification adds another layer of complexity. In a large-scale mesocosm experiment, herring larvae were raised as top predators in a pelagic food web exposed to projected end-of-century carbon dioxide levels (around 760 microatmospheres). Rather than the decrease in survival seen in some laboratory studies, the larvae actually survived about 19% better under elevated CO₂ conditions. The likely explanation: higher CO₂ stimulated primary production at the base of the food web, which cascaded up through the plankton community and gave larval herring more to eat.13Nature Ecology & Evolution. Food web changes under ocean acidification promote herring larvae survival The finding is a useful reminder that indirect effects through the food web can outweigh the direct physiological stress of a changing environment, at least under some conditions. Whether this benefit persists at more extreme acidification levels or across different ecosystems remains an open question.
From Plankton to Your Plate
The nutritional value of herring for human consumption is directly shaped by what the fish have been eating. Herring are among the richest dietary sources of omega-3 fatty acids, particularly EPA and DHA, but the levels vary substantially depending on when and where the fish were caught. Norwegian spring-spawning herring belly flaps sampled across multiple months had mean EPA levels around 2.0 grams per 100 grams of tissue and mean DHA levels around 2.3 grams per 100 grams, with September fish carrying the highest concentrations of both.14Heliyon. Belly flap from Norwegian spring-spawning herring (Clupea harengus L.): A potentially new product with high content of vitamin D, EPA and DHA
The connection between herring diet and human nutrition runs through fatty acid composition. An abrupt increase in EPA levels in North Sea herring flesh between February and May was not explained by the fish’s age, sex, size, or spawning status. Instead, researchers suggested it was related to seasonal changes in the fatty acid composition of the calanoid copepods the herring were feeding on. Herring caught in May with high EPA levels also had relatively low total lipid content (around 7%), making them especially suitable for human consumption because they delivered concentrated omega-3s without excessive fat.15Journal of the Science of Food and Agriculture. Fatty acid composition of herring (Clupea harengus L.): influence of time and place of catch on n‐3 PUFA content In practical terms, the quality of a herring fillet as a health food depends not just on the species but on the season and what plankton blooms preceded the catch.
Contaminants That Travel the Food Chain
The same food web pathway that delivers omega-3 fatty acids to herring flesh also delivers pollutants. Herring accumulate contaminants from their prey, and because they are eaten by so many other species (including humans), they can act as a conduit for those contaminants up the food chain. A study examining tissue distribution of mercury, cadmium, lead, and several per- and polyfluoroalkyl substances (PFAS) across Northern European fish species found that herring had notably high liver-to-muscle concentration ratios for PFOS, with a ratio roughly double that of eelpout. For other PFAS compounds, the difference between herring and some other species was even larger, up to a factor of four.16Science of the Total Environment. Fish tissue conversion factors for mercury, cadmium, lead and nine per- and polyfluoroalkyl substances for use within contaminant monitoring
These species-specific differences in contaminant distribution matter for both environmental monitoring and food safety. If you are measuring pollutant levels in herring to assess ocean health, which tissue you sample (liver versus muscle) dramatically affects the number you get. And because herring are consumed by everything from cod to seals to puffins, their contaminant load propagates widely through the ecosystem. Herring’s position as a dietary staple for so many species means they function as both a nutritional lifeline and a potential vector for bioaccumulated pollution.
Parasites Acquired Through Prey
Herring also pick up parasites from the organisms they eat. The relationship between herring and their parasites has been studied extensively across the fish’s entire geographic range, encompassing both protozoan and metazoan parasites. Many of these parasites use copepods or other zooplankton as intermediate hosts, so the act of feeding is the primary route of infection. The parasite community a herring carries can actually serve as a biological tag, telling researchers about where the fish has been feeding and which prey it has been consuming. Different herring populations carry different parasite assemblages depending on the local zooplankton community, and these differences have been used to trace migration routes and distinguish between stocks that are otherwise difficult to tell apart.
For the fishing and aquaculture industries, the most familiar herring parasite is probably Anisakis, a nematode whose larvae live in small crustaceans before being consumed by fish. Herring that feed heavily on euphausiids in certain areas tend to carry higher Anisakis loads. This is why proper freezing or cooking of herring is standard practice in food preparation: the parasite burden is a direct consequence of the herring’s diet, and it varies by region and feeding ground.
Why Fisheries Managers Watch the Plankton
All of this helps explain why modern herring fisheries management does not just count fish. Increasingly, managers pay attention to what herring are eating and how well-fed they are. The condition of herring at spawning, their lipid reserves, and the match between larval herring and their copepod prey all feed into stock assessments and harvest decisions. A population of herring that looks numerically stable but is chronically underfed produces fewer viable eggs, grows more slowly, and provides lower-quality forage for predators.
In the Bothnian Sea, the finding that zooplankton availability and competition with sprat were stronger drivers of herring dynamics than fishing pressure suggests that managing the herring fishery in isolation from plankton monitoring and sprat management would miss the most important factors.12PubMed. Interacting trophic forcing and the population dynamics of herring Ecosystem-based management, which accounts for these food web connections rather than treating each species as an independent stock, has become the direction most fisheries scientists advocate for herring. The fish themselves are inseparable from their diet, and managing them well requires understanding what is happening several links down the food chain.