Fish are gill-bearing, aquatic, vertebrate animals, representing the largest group of vertebrates with over 33,000 known species. They inhabit nearly all water bodies on Earth, thriving in diverse environments from mountain streams to the abyssal depths of the ocean. The sheer biological diversity and abundance of fish underscore their fundamental importance to the stability and function of global aquatic ecosystems. They are active agents that drive ecological processes, maintain environmental health, and connect different parts of the aquatic world.
Regulating Aquatic Food Webs
Fish operate at every level of the aquatic food web, exerting both top-down and bottom-up control that structures entire communities. Predatory fish, such as sharks, tuna, and large bass, regulate ecosystems through a trophic cascade. By preying on smaller fish, these top consumers prevent the overpopulation of their prey species. This allows zooplankton to flourish, which reduces the biomass of phytoplankton at the base of the food web.
The removal of large, piscivorous fish can disrupt this balance, leading to an increase in smaller fish and a decrease in zooplankton. This ultimately results in an overgrowth of phytoplankton that can cloud the water column. Conversely, smaller, planktivorous fish, like sardines and anchovies, exert a strong bottom-up influence. These forage fish consume vast quantities of plankton, preventing localized algal blooms and acting as a conduit for energy transfer.
This consumption moves energy, initially captured by primary producers, up the food chain to higher trophic levels. Fish are the primary mechanism for transferring stored energy from these lower levels to organisms like marine mammals, sea birds, and humans.
Driving Biogeochemical Cycles
Fish play a substantial role in the movement and transformation of essential elements within a body of water. They actively contribute to nutrient recycling through the excretion of waste products, primarily biologically available nitrogen and phosphorus. This continuous release of nutrients acts as a natural fertilizer, supporting the growth of phytoplankton in the surface water column.
Studies have shown that fish excretion can meet a significant proportion of an ecosystem’s demand for dissolved inorganic nitrogen, particularly in tropical streams. The concentration of these nutrients can create localized “hotspots” of fertility that stimulate primary production. This process is important in nutrient-limited environments where external inputs are low.
Fish also facilitate vertical nutrient movement, linking the bottom sediments with the upper water column through their feeding behavior, known as benthic-pelagic coupling. A detritivorous fish, such as the gizzard shad, feeds on detritus in the nutrient-rich bottom sediment. It then excretes these nutrients into the water column as it swims higher up, effectively translocating phosphorus and nitrogen from the lake floor to the surface waters.
Maintaining Habitat Structure
The feeding and physical actions of fish constantly modify and maintain the structural integrity of their aquatic environments. Herbivorous fish are particularly important in maintaining the physical structure of coral reefs by controlling algal growth. Species like parrotfish scrape algae from the reef substrate, preventing fast-growing macroalgae from suffocating slow-growing corals. This grazing activity keeps the hard surfaces clear, creating settlement space for new coral polyps and promoting reef recovery.
Fish also influence the dynamics of soft-bottom habitats through their interactions with sediment. Bottom-dwelling fish, like flounders and certain catfish, engage in bioturbation, the physical disturbance of the substrate through burrowing or foraging. This activity stirs up the top layer of sediment, which helps aerate the deeper layers and release trapped organic matter back into the water.
In freshwater systems, some fish species act as important agents of plant propagation, a dispersal mechanism called ichthyochory. Frugivorous fish, such as the pacu in the Amazon basin, consume fruits and seeds that fall into the river from riparian trees. The fish digest the fruit pulp but pass the seeds intact through their digestive tracts, depositing them significant distances away from the parent plant. This aids in the dispersal of vegetation along riverbanks and floodplains.
Linking Disparate Ecosystems through Migration and Dispersal
The migratory patterns of certain fish species connect geographically distinct ecosystems, transporting energy and nutrients across ecological boundaries. Anadromous fish, such as Pacific salmon, grow to maturity in the nutrient-rich ocean and then return to spawn in nutrient-poor freshwater streams and rivers. Upon their return, these fish carry vast quantities of marine-derived nutrients, including nitrogen and carbon, deep into inland watersheds.
When these spawning fish die, their carcasses release these oceanic nutrients into the freshwater and surrounding terrestrial ecosystems. This input provides a subsidy that supports riparian vegetation, invertebrates, birds, and mammals, such as bears, which further disperse the nutrients onto the landscape. This transfer is particularly noticeable in nutrient-poor headwater streams that depend on annual salmon runs for a substantial boost in fertility.
Other species facilitate connectivity between coastal and estuarine zones, which act as nurseries for many marine organisms. The movement of fish between saltwater and freshwater habitats ensures a constant flow of energy and organic matter. These continuous movements link the productivity of the open ocean with sheltered zones, ensuring the health and resilience of both interconnected environments.