Forage fish
Forage fish are small, schooling species of pelagic fish that primarily consume plankton, including zooplankton and phytoplankton, and function as a foundational prey resource for larger predatory fish, seabirds, marine mammals, and humans within marine ecosystems.[1][2] These fish, characterized by high reproductive rates and rapid population fluctuations driven by environmental conditions such as ocean temperature and nutrient availability, include prominent examples like herring (Clupea harengus), sardines (Sardinops sagax), anchovies (Engraulis encrasicolus), and menhaden (Brevoortia tyrannus).[3][4] Forage fish occupy a critical trophic position, efficiently transferring energy from primary producers to higher levels in the food web, which supports the productivity of commercial fisheries targeting predators and sustains biodiversity in coastal and open-ocean environments.[5] Their abundance, often reaching billions in biomass during peak periods, enables them to buffer ecosystem dynamics against variability, though natural boom-bust cycles—exacerbated by factors like climate oscillations—pose challenges for sustainable harvesting.[6] Fisheries for these species provide direct economic value through products like fish meal and oil, while indirectly bolstering yields of species such as tuna and salmon, yet management debates persist over balancing extraction with ecological roles, given evidence of resilience in well-regulated stocks.[7][8]
Definition and Characteristics
Biological Traits
Forage fish encompass small to medium-sized pelagic species, typically ranging from a few centimeters to approximately 40 cm in maximum length, such as Pacific sardines (Sardinops sagax) which can reach up to 40 cm.[9] These fish often exhibit streamlined bodies adapted for sustained swimming in open water, with silvery scales that provide camouflage through reflection of light, reducing visibility to predators.[10] Life history traits of forage fish include short lifespans, generally spanning 2 to 7 years, accompanied by high natural mortality rates exceeding 0.3 per year, which contributes to their boom-and-bust population dynamics.[9] They demonstrate rapid growth, attaining sexual maturity within 1 to 2 years, enabling quick population recovery under favorable conditions.[9] For species like herring (Clupea harengus), maximum age can extend to 15 years in some populations, though most individuals do not survive beyond 3-5 years due to predation and environmental factors.[11] Reproductive strategies emphasize high fecundity to offset elevated mortality, with females producing up to 100,000 or more eggs annually through multiple spawning events, often triggered by environmental cues such as temperature and food availability.[9] Eggs are typically pelagic or demersal depending on the species—pelagic in sardines and anchovies (Engraulis spp.), facilitating wide dispersal, while herring often deposit adhesive eggs on substrates.[12] This iteroparous or serial spawning pattern supports resilience, with batch fecundity varying by size and condition, as observed in anchovies and sardines where larger females allocate more energy to reproduction.[13] A defining behavioral trait is their propensity to form large, dense schools, which enhances predator evasion via the confusion effect and dilution of individual risk, while also optimizing plankton filtration during feeding.[9] Physiologically, these fish possess efficient gill rakers for straining zooplankton and high metabolic rates supporting fast growth and energy transfer in food webs, though they remain highly sensitive to environmental perturbations like temperature shifts that affect oxygen uptake and development.[9]Taxonomic Diversity
Forage fish comprise a functionally defined group of small, schooling, planktivorous species that serve as primary prey for larger marine predators, spanning multiple taxonomic orders and families rather than forming a monophyletic clade. The predominant taxa belong to the order Clupeiformes, which includes over 400 species characterized by their pelagic habits, filter-feeding on zooplankton, and high biomass in coastal and shelf ecosystems.[14] Key families within Clupeiformes are Clupeidae (herrings, sardines, shads, and menhadens, with approximately 200 species) and Engraulidae (anchovies, around 140 species), which dominate global forage fish assemblages due to their abundance and ecological centrality.[15] These groups exhibit morphological adaptations such as silvery scales for camouflage and adipose fins in some for schooling efficiency.[16] Beyond Clupeiformes, forage fish diversity extends to other orders, reflecting regional variations in prey availability. Osmeriformes, particularly the family Osmeridae (smelts like capelin, Mallotus villosus, and eulachon, Thaleichthys pacificus), contribute significantly in northern latitudes, with species numbering around 10-12 globally but forming dense schools during spawning migrations.[17] Perciformes includes Ammodytidae (sand lances, such as Pacific sand lance, Ammodytes hexapterus), noted for burrowing behaviors and high lipid content, encompassing about 30 species.[17] Myctophiformes (lanternfishes, family Myctophidae, over 240 species) dominate mesopelagic forage roles, bioluminescent and vertically migrating to support deep-sea food webs. These non-clupeiform groups, while less commercially targeted, amplify taxonomic breadth, with regional fishery management plans like those from NOAA incorporating over 50 species across 10+ families in areas such as the Gulf of Alaska.[18]| Order | Key Family | Example Species | Approximate Global Species Count | Ecological Notes |
|---|---|---|---|---|
| Clupeiformes | Clupeidae | Atlantic herring (Clupea harengus) | ~200 | Coastal schooling, zooplankton filterers |
| Clupeiformes | Engraulidae | European anchovy (Engraulis encrasicolus) | ~140 | Nearshore swarms, high fecundity |
| Osmeriformes | Osmeridae | Capelin (Mallotus villosus) | ~10 | Arctic spawning aggregations |
| Perciformes | Ammodytidae | Pacific sand lance (Ammodytes hexapterus) | ~30 | Sediment-burrowing, lipid-rich |
| Myctophiformes | Myctophidae | Northern lanternfish (Myctophum punctatum) | ~240 | Mesopelagic, diel vertical migration |
Ecological Dynamics
Role in Marine Food Webs
Forage fish occupy an intermediate trophic position in marine food webs, primarily consuming zooplankton and serving as the principal prey for larger piscivores, seabirds, and marine mammals, thereby channeling energy from primary producers to apex predators.[19][20] Small pelagic forage species exhibit a median trophic level of 3.10, acting as a key linkage between planktonic production and higher consumers across diverse ecosystems.[21] These fish account for approximately 43% of global fish production, drawing support from about 8% of primary production, with elevated efficiencies in upwelling systems where they utilize 10% of primary production.[21] Their biomass facilitates substantial energy transfer to predators, contributing to 22% of seabird production and 15% of marine mammal production worldwide; in upwelling regions, these figures rise to 33% for seabirds and 41% for mammals.[21] This role underscores their status as a critical nexus, often structuring "wasp-waist" ecosystems where forage fish abundance bottlenecks energy flow to upper trophic levels.[21] Fluctuations in forage fish populations, exacerbated by fishing pressures reaching 50–200% above average in some stocks, can induce collapses affecting dependent predators, with 27 of 55 monitored stocks falling below 25% of average biomass and median minima 44% lower than expected without exploitation.[19] Such dynamics highlight their ecological primacy, as they underpin not only predator populations but also associated fisheries yielding around 17 million tons annually, representing 65% of global forage fish catch since 2000.[19] In inner shelf habitats, forage fish integrate energy to sustain apex species of conservation concern, emphasizing the need for data on their contributions amid climate and anthropogenic stressors.[20]Diet and Foraging Strategies
Forage fish, including species such as sardines (Sardina pilchardus), anchovies (Engraulis encrasicolus), and herrings (Clupea spp.), primarily consume zooplankton, with copepods forming the dominant prey item across many populations.[22] Euphausiids, bivalve larvae, decapod larvae, and fish eggs or larvae supplement this diet, while some individuals ingest phytoplankton, classifying many as omnivores with narrow trophic niches focused on locally abundant plankton.[22] Diet composition exhibits spatial and seasonal variation tied to zooplankton distribution, such as higher copepod reliance in nutrient-rich northern regions.[22] Environmental conditions further influence prey selection; in warm ocean regimes like the 2015–2016 eastern Pacific "warm blob," anchovies, sardines, and herrings shifted toward gelatinous zooplankton (e.g., small jellyfish), comprising a larger dietary proportion than in cool years (2010–2011), when energy-dense copepods and euphausiids prevailed.[23] This opportunistic adjustment to less nutritious prey correlates with diminished fish growth rates and poorer body condition.[23] Ontogenetic shifts characterize feeding progression, with larvae targeting finer particles like phytoplankton and early-stage zooplankton, while juveniles and adults prioritize larger, mobile prey such as copepods to support rapid growth.[24] Foraging strategies emphasize suspension feeding via cross-flow filtration, where water passes parallel to gill raker arrays that retain particles smaller than mesh spacing through hydrodynamic forces, rather than simple sieving.[25] Clupeids alternate this with particulate feeding—targeted suction or biting—on discrete prey in mixed assemblages, optimizing intake rates when filtering proves inefficient.[26] Schooling amplifies efficiency, as groups integrate sensory cues from conspecifics to detect plankton patches, yielding near-optimal energy gains and equitable foraging success among members.[27] These behaviors enable exploitation of ephemeral, high-density resources, underpinning their ecological role as plankton-to-predator conduits.[27]Predation Pressures and Adaptations
Forage fish face intense predation from piscivorous fishes such as tunas, billfishes, and cod; seabirds including gannets and shearwaters; and marine mammals like dolphins, seals, and whales, with these predators often consuming forage fish comprising over 20% of their diet in 28 fish, 10 seabird, and 7 mammal populations studied.[28] This predation drives high natural mortality rates, frequently exceeding 1.0 year⁻¹ and dominating over fishing mortality, as evidenced in herring where average predation mortality surpasses standard total natural mortality by more than fivefold.[29][30] Predation intensity varies with forage fish abundance, becoming disproportionately significant when biomass falls below 15-18% of maximum recorded levels, amplifying pressure from seabirds and other consumers.[31] Key behavioral adaptations include tight schooling formations, which reduce per capita predation risk via the dilution effect—spreading attack probability across group members—the confusion effect that disorients predators through synchronized, unpredictable maneuvers, and enhanced collective vigilance enabling faster threat detection than solitary individuals.[32] These dynamics particularly thwart pursuit predators like sharks and tunas by complicating target selection and attack success in dense, coordinated groups.[33] Life-history traits further buffer predation pressures through r-selection strategies: rapid somatic growth allowing maturity in 1-2 years, semelparity or iteroparity with high fecundity (e.g., herring females yielding 20,000-200,000 eggs per batch), and frequent spawning to compensate for elevated mortality across life stages.[33] Such traits sustain populations despite heavy losses, as demonstrated in models where predation shapes boom-bust cycles but high reproductive output enables recovery.[34]Population Fluctuations and Migrations
Forage fish populations are characterized by large-amplitude fluctuations, often exhibiting boom-bust cycles on decadal timescales, driven primarily by variability in larval survival and recruitment success influenced by oceanographic conditions such as sea surface temperature, upwelling intensity, and large-scale climate modes like the Pacific Decadal Oscillation.[35] These shifts frequently manifest as alternating dominance between species, such as sardines and anchovies, where cool-water regimes favor anchovy recruitment while warmer conditions benefit sardines, as observed in the California Current where Pacific sardine (Sardinops sagax) biomass peaked in the 1930s before collapsing by the 1950s amid cooling and intensified fishing.[36] Similarly, historical records spanning centuries document global synchronies and anti-phase oscillations, with Japanese sardine outbreaks correlating to high anchovy abundance off California over 300-year periods, underscoring environmental forcing over localized factors.[37] Fishing pressure exacerbates these natural fluctuations, increasing the frequency and severity of collapses beyond what environmental variability alone would produce; empirical models indicate that sustained harvests during declining productivity phases can precipitate stock crashes, as evidenced in analyses of multiple forage species where overexploitation amplified downturns compared to unfished scenarios.[38] However, recoveries can occur rapidly post-collapse, with paleoecological data from anchovy and sardine scales off California showing returns to fishable biomass within 1-2 decades under favorable conditions, highlighting the resilience tied to high fecundity and opportunistic life histories.[39] Recent assessments disentangling climate and harvest effects across 92% of studied populations attribute stronger dynamical impacts to fishing than warming trends during the late 20th to early 21st centuries, though baseline environmental regime shifts remain the primary initiator.[40] Migrations in forage fish are predominantly seasonal and schooling-mediated, enabling efficient tracking of plankton blooms and access to spawning habitats over vast distances, often spanning continental shelves. In the Northeast Atlantic, Atlantic herring (Clupea harengus) undertake clockwise feeding migrations starting southward in winter, progressing westward, northward, and eastward to northern grounds before returning to overwintering areas, a pattern sustained by prey availability and water mass movements.[41] Pacific species like northern anchovy exhibit spatiotemporal shifts in distribution tied to larval abundance indices, with historical data from U.S. and Mexican surveys revealing fluctuations in spawning locations responsive to upwelling variability.[42] In upwelling systems such as the Canary Current, round sardinella (Sardinella aurita) perform shelf-wide migrations influenced by local hydrodynamics, while climate-driven alterations in thermal optima are prompting poleward distributional shifts in small pelagics globally, as modeled for European waters.[43][44] These movements underpin their ecological connectivity between coastal and open-ocean realms, though empirical tagging studies underscore the complexity of routes, with albacore tuna and associated pelagics showing variable coastal-offshore linkages.[45]Freshwater Counterparts
Species Composition
Freshwater forage fish encompass a diverse array of small, primarily planktivorous species that serve as prey for larger predatory fish, birds, and mammals in lakes, rivers, and reservoirs. Unlike their marine counterparts, which are dominated by clupeids like herring and anchovies, freshwater assemblages feature prominent cyprinids (minnows and shiners), clupeids adapted to inland waters, osmerids (smelt), and smaller centrarchids (sunfish). These species often school in large numbers, facilitating efficient energy transfer up the food web, and their composition varies by region, water temperature, and habitat type, with temperate North American systems hosting the most studied examples. In southern and central U.S. reservoirs and farm ponds, clupeids such as the gizzard shad (Dorosoma cepedianum) and threadfin shad (D. petenense) dominate, filtering zooplankton and algae while supporting sportfish like bass and catfish; gizzard shad, native to the Mississippi River basin, can reach biomasses exceeding 100 kg/ha in eutrophic waters, though their filtration may exacerbate algal blooms under high densities. Cyprinids like the fathead minnow (Pimephales promelas) and golden shiner (Notemigonus crysoleucas) are ubiquitous, with fathead minnows spawning up to nine times annually and tolerating low oxygen levels, making them resilient forage in variable conditions; golden shiners, reaching 30 cm, provide larger prey items in northern lakes.[46] In northern temperate lakes, such as those in the Great Lakes region, introduced species like the alewife (Alosa pseudoharengus) and rainbow smelt (Osmerus mordax) form key components, with alewives comprising up to 90% of pelagic biomass in some systems post-1950s invasions, sustaining predators like salmon but disrupting native zooplankton dynamics. Smaller sunfishes, notably bluegill (Lepomis macrochirus), act as forage in warmer shallows, growing to 10-25 cm and reproducing prolifically, though overabundance can limit predator growth due to spiny defenses and competition. Regional endemics, including suckers (Catostomidae) and darters (Percidae), supplement in streams and shallower habitats, but planktivores like emerald shiner (Notropis atherinoides) bridge gaps in open waters.[47]| Species | Family | Primary Habitat | Key Traits |
|---|---|---|---|
| Gizzard shad (Dorosoma cepedianum) | Clupeidae | Reservoirs, large rivers | Planktivorous filter-feeder; high biomass supporter for predators; potential bloom contributor.[46] |
| Fathead minnow (Pimephales promelas) | Cyprinidae | Ponds, lakes, streams | Multiple spawning cycles; hypoxia tolerant; widely stocked. |
| Golden shiner (Notemigonus crysoleucas) | Cyprinidae | Northern lakes, ponds | Largest native minnow; adhesive eggs; effective for cool waters. |
| Bluegill (Lepomis macrochirus) | Centrarchidae | Warm shallows, ponds | Prolific reproducer; spiny protection; common in southern systems. |
| Alewife (Alosa pseudoharengus) | Clupeidae | Great Lakes, coastal rivers | Invasive pelagic schooler; high density impacts; salmon forage.[48] |
| Rainbow smelt (Osmerus mordax) | Osmeridae | Cold lakes | Anadromous/planktivorous; key Great Lakes forage post-introduction.[47] |