Why are fish-eating clupeiform species usually less abundant than plankton-feeding species?

Fish-eating clupeiform species are generally less abundant than plankton-feeding species primarily due to the principles of energy transfer within an ecosystem.21

In any food chain, energy is lost as it moves from one trophic level to the next. On average, only about 10 percent of the energy available at one level is converted into biomass at the level above it. Because plankton-feeding clupeiforms occupy a lower trophic level—feeding directly on primary producers or small organisms—they have access to a much larger pool of available energy.12

In contrast, predatory (fish-eating) clupeiforms occupy a higher trophic level. By the time energy reaches this stage, the total available energy is significantly reduced, which inherently limits the potential size and abundance of these predator populations. Consequently, predatory clupeids are relatively scarce and typically hold less commercial value than their plankton-feeding counterparts.21