If you’ve spent any time around lakes or oceans, you’ve probably seen tiny specks floating in the water. While they may seem insignificant, these tiny organisms known as zooplankton play a crucial role in aquatic food chains. But do fish eat zooplankton?

The short answer is yes, many species of fish do in fact eat zooplankton as part of their diet.

In this comprehensive article, we’ll take an in-depth look at the relationship between fish and zooplankton. We’ll cover what exactly zooplankton are, the different types of zooplankton, why they are an important food source for fish, which species feed on zooplankton, and how changes in zooplankton populations can impact fish numbers.

What Are Zooplankton?

Definition and Characteristics

Zooplankton are tiny aquatic organisms that float and drift in the world’s oceans, seas, and bodies of fresh water. The name zooplankton is derived from the Greek words “zoon”, meaning animal, and “planktos”, meaning drifter or wanderer.

They are a crucial component of the aquatic food chain, providing a vital link between the primary producers such as phytoplankton and larger aquatic animals.

There are two main groups of zooplankton: holoplankton and meroplankton. Holoplankton spend their entire lifecycle drifting in the plankton. Meroplankton are planktonic only during certain larval stages, but adults are non-planktonic.

Zooplankton range in size from microscopic organisms to larger species up to 2-3 cm in length. They drift and flow at the mercy of tides and currents. With weak swimming abilities, they cannot travel against a current.Zooplankton are primarily heterotrophic, meaning they consumer other organisms for energy and nutrition rather than produce their own food through photosynthesis.

They feed on a variety of organic matter suspended in the water including phytoplankton, bacteria, protozoans, detritus, and even other zooplankton. Their diverse diets allow zooplankton to inhabit oceans from surface to deep waters, near coasts and the open sea.

Major Types of Zooplankton

There are a multitude of different zooplankton species occupying the world’s aquatic ecosystems. Here are some of the major groups:

  • Copepods – Small crustaceans around 1 mm in size. Considered the most abundant multicellular animals on Earth.
  • Krill – Shrimp-like crustaceans ranging from 1-6 cm long. A keystone species of the Antarctic food web.
  • Jellyfish – Gelatinous zooplankton existing in both adult medusa and polyp stages.
  • Salps – Barrel-shaped, transparent marine tunicates that move by contracting their bodies.
  • Euphausiids – Mid-sized crustaceans commonly called krill. An important link between phytoplankton and baleen whales.
  • Chaetognaths – Predatory worm-like animals with grasping hooks for capturing prey.
  • Pteropods – Planktonic marine snails also known as “sea butterflies”.
  • Foraminiferans – Unicellular zooplankton protected by calcium carbonate tests.

Zooplankton such as copepods and krill occur in such vast numbers that scientists estimate their total global biomass to be in the billions of tons. Some species undergo mass migrations and form dense swarms or patches easily visible by satellites.

Despite their small size as individuals, zooplankton collectively represent a major store of protein, lipid, and mineral nutrients in the seas. They help sustain ecosystems from shallow coastal zones to the deep sea through their roles as grazers, predators, and prey.

Why Are Zooplankton Important for Fish?

Highly Nutritious Food Source

Zooplankton are tiny aquatic organisms that serve as a crucial food source for many fish species. Ranging in size from microscopic to a few millimeters long, these plankton are packed with proteins, lipids, vitamins, minerals and other nutrients that fish need to survive and thrive (1).

Species like herring, mackerel and menhaden are filter feeders that eat zooplankton directly by straining them from the water. Other larger predator fish like tuna, salmon and cod primarily eat smaller fish species that have fed on zooplankton (2).

Without the nutritional foundation that zooplankton provide to aquatic food chains, most commercial fisheries would collapse.

Fundamental Part of Aquatic Food Chains

At the base of open ocean and freshwater habitats, phytoplankton (microscopic plants) harness the Sun’s energy through photosynthesis. Zooplankton then eat the phytoplankton, transferring that energy up the food chain. Small fish consume the zooplankton, then larger fish prey on the smaller fish (3).

This energy transfer powers the growth and survival of all organisms higher up the food chain. About 90% of the energy in any ecosystem is contained within the plant life at the base (4). Zooplankton eat this energy rich plant life, providing an abundant package of concentrated nutrition for fish.

Removing zooplankton from aquatic food chains would have catastrophic consequences for global fisheries production. Fish growth and reproduction would decline precipitously without this foundation. Simply put, zooplankton are the linchpin of open ocean habitats and essential for healthy fish populations worldwide.

Fish Species That Consume Zooplankton

Small Planktivorous Fish

Zooplankton are an important food source for many species of small fish, such as anchovies, herring, sardines, and sprat. These small planktivorous fish make up a significant portion of marine fisheries, as they are later eaten by larger carnivorous fish, seabirds, and marine mammals.

These fish possess gill rakers – comb-like structures in the mouth that enable them to filter out tiny prey like copepods, krill, and other small crustaceans from the water as it passes over their gills.

Many species will swim together in large schools as they feed on dense concentrations of zooplankton that are advected by ocean currents. Some specialized small pelagic fish even migrate vast distances to track zooplankton blooms.

Juveniles of Larger Predatory Fish

Zooplankton are an important food source for many juvenile fish that later develop into top predators, including cod, halibut, tuna, and salmon. As larvae and early juveniles, these larger fish have small mouths unsuited for hunting larger prey.

Planktonic prey provide them with a crucial source of nutrition during this early life stage.

For example, research suggests that the prolonged decline of certain zooplankton species in the Bering Sea has contributed to poor survival of juvenile salmon there. Climate-driven changes in the abundance or timing of key zooplankton such as Neocalanus plumchrus could impact growth and survival of young salmon entering the ocean.

Filter Feeding Fish

Multiple species of large filter feeding fish also regularly consume zooplankton as adults. This group includes whale sharks, basking sharks, mobulid rays, and certain large bony fish like menhaden and Asia’s Bryde’s whales.

These fish swim with mouths wide open to filter large quantities of water across specialized gill rakers that trap prey.

While they are capable of targeting dense pockets of krill or other sizable zooplankton, much of their diet can consist of less conspicuous copepods, ostracods, and larvae aggregated from processing huge volumes of water.

Climate-driven shifts in the abundance or distribution of zooplankton hotspots could strongly impact the foraging of these filter feeders.

Impacts on Fish When Zooplankton Decline

Lower Survival Rates in Larval Fish

When zooplankton populations decline, it can have devastating effects on the survival rates of larval fish. Larval fish rely heavily on zooplankton like copepods as their main food source. If copepod abundance is low, larval fish may not find enough to eat and will experience starvation.

Studies have shown larval fish survival rates can be reduced by 50-90% when zooplankton prey is scarce, leading to recruitment failure and population declines.

For example, research in the Barents Sea found that during years when the zooplankton Calanus finmarchicus was less abundant, the survival rate of larval cod dropped to only 10-30%. This highlights just how crucial zooplankton are for the early life stages of commercially important fish stocks.

Reduced Growth Rates

Even if larval and juvenile fish manage to find enough zooplankton to survive, reduced zooplankton availability can still impact them by slowing growth rates. When prey is limited, more energy must be spent foraging and less energy can be directed toward somatic growth.

Slower growth makes small/young fish more vulnerable to starvation and predation for a longer time until they reach a size refuge.

For example, a study in the Baltic Sea found that in years with lower zooplankton biomass, larval sprat had significantly reduced length and body mass. The researchers estimated this slower growth could increase the larval stage duration by 4-9 days, substantially increasing the mortality risk.

Slow growth during critical early life stages can have lasting impacts on recruitment success.

Population and Recruitment Declines

When the survival and growth of larval fish is impaired due to zooplankton limitation, it can have severe ramifications for the overall fish stock by reducing recruitment. Fewer larvae reaching adulthood means fewer fish joining the reproductive population.

Even a single year with high larval mortality can destabilize the population for years to come.

There are many examples of zooplankton declines contributing to fish population collapses. The Atlantic cod population off Newfoundland crashed after years of low zooplankton prey for cod larvae. Some researchers cite zooplankton scarcity as a factor in the 80% decline in Lake Huron’s alewife population over just a few years.

Studies suggest zooplankton declines driven by climate change or other ecosystem shifts could reduce global fish recruitment by more than 20% this century.

Conclusion

In conclusion, zooplankton play a vital role as a food source for many species of fish across aquatic ecosystems. While tiny, these organisms help sustain food chains by providing nutrition for small fish, developing juveniles, and filter feeders.

Declines in zooplankton can reverberate up the food chain, impacting fish growth, survival, and reproduction. Understanding these complex interactions between zooplankton and fish is key for effectively managing fisheries and protecting marine resources.

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