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The present paper is a review of the available literature on the significance of forage fish, the plethora of services they provide, and the threats faced by them. Forage fish are pelagic planktivorous species that operate as conduits of energy between the lower trophic level (plankton) and the upper trophic level (predators). A variety of ecosystem services are provided by them, from serving as prey for higher trophic levels to producing fish meal and oil. Forage fish have a consumption value for humans and cultural importance to many societies. Forage fish have faced constant natural and anthropogenic threats in the past, resulting in numerous fish collapses which subsequently impacted their predators. The economic benefit provided by forage fish has been estimated to be approximately $ 18.7 billion per annum. An introspection of the data on ecosystem services revealed lack of data on regulating and cultural services, eventually leading to a monetary underestimation and their commercial prioritization over the wider benefits they provide.

Small pelagic fish are key planktivores and prey in marine ecosystems, and their population abundances undergo strong temporal and spatial variability. Top-down (predator controlled) and bottom-up (prey-driven) processes during early life history are important for determining forage fish survival and recruitment. We examined biological and environmental factors hypothesized to influence age-0 Pacific herring Clupea pallasi in the Strait of Georgia (SOG), British Columbia, Canada. Primarily bottom-up processes affected interannual variability in age-0 herring abundance and condition, with some evidence of top-down effects on condition. Age-0 herring abundance increased with increasing adult spawning biomass and peaked when most adults spawned about 20 d prior to the peak spring primary production bloom. This timeline would temporally align first-feeding herring larvae with their prey, such as small copepods. Age-0 herring abundance also increased with increasing juvenile salmon abundance, indicating that conditions favourable for herring were also favourable for their predators and competitors. Age-0 herring condition decreased with increasing spawning biomass, increased when most adults spawned closer to the peak spring bloom, increased with increasing temperatures above 8.2°C, and increased then stabilized with increasing prey zooplankton density. Age-0 herring condition had a dome-shaped relationship with predator abundance, indicating that high predator abundances negatively affected fish condition. Study results suggest that density-dependent processes, such as intraspecific competition, may be important in the SOG. A positive correlation between age-0 herring abundance and subsequent age-3 recruit abundance may provide a leading indicator of low recruitment years.

Marine top predators are expected to adjust their foraging behaviour at multiple time scales concomitantly with changes in forage fish availability. Rhinoceros auklets Cerorhinca monocerata rearing chicks at Teuri Island, Japan Sea, fed on anchovy Engraulis japonicus in 2012 and 2013 (anchovy regime) but switched to sand lance Ammodytes spp in 2019 and 2020 (sand lance regime). Here, we studied their at-sea behaviour using the GPS locations of 33 birds and the depth-acceleration records of 26 birds, and compared their foraging behaviour between these prey regimes. At the trip scale, auklets used offshore waters (> 50 m sea depth) and coastal waters in the anchovy regime but used mainland coastal waters (< 50 m sea depth) in the sand lance regime. In the sand lance regime, the birds also conducted more overnight 2- to 4-day trips in 2020 and spent more time flying during 1-day trips as they fed in further areas compared to the anchovy regime. At the dive scale, auklets frequently dove to both < 5 m and 20–30 m depths in the anchovy regime but mainly to < 5 m depth in the sand lance regime. Within each dive, auklets showed a greater number of fast/strong wing stroke events in the anchovy regime than in the sand lance regime. These changes in auklet behaviour reflected the different habitats, depth distribution, and swim speed of the targeted prey species. Our study shows the behavioural flexibility of a wing-propelled flying-diving seabird in response to the inter-annual shifts in the dominant forage fish community. It also indicates the ecological constraints on the mechanisms determining nest productivity in this day-foraging/night-provisioning seabird.

Seabirds consume forage fish, which are keystone species in many marine ecosystems. The Junk Food Hypothesis proposes that high-lipid prey should produce better reproductive performance by seabirds. In the Gulf of Maine, changes in the forage fish community followed rapid warming post-2005 and included a decline in high-lipid Atlantic herring Clupea harengus. We studied 3 species of alcid (Atlantic puffin Fratercula arctica, razorbill Alca torda, common murre Uria aalge) over 23 yr at 3 colonies to assess changes in chick diet and its relationships with reproductive success. Puffin and razorbill chick diet changed over time; puffin diet was highly variable taxonomically, whereas razorbill diets were more consistent, showing proportional changes within fewer taxa. For puffins and razorbills, herring was replaced by sand lance Ammodytes spp. and other taxa with lower energy density. Puffins did not require high-lipid fish to breed successfully, but diet–reproduction relationships became unpredictable following ex tremely warm winters (2013 and 2016). Razorbills and murres provisioning with low-lipid fish showed reduced chick condition and breeding success. We concluded that razorbills and murres need higher-quality diets than puffins, which more frequently exploited lower-lipid food during food shortages. However, puffin reproductive output was much more vulnerable to ocean warming owing to their longer breeding season and more varied diet. Different responses of closely-related species to changes in prey are driven by differences in chick-development strategies with clear implications for using seabirds as environmental indicators.

Climatic factors drive changes in forage fish communities and may influence the productivity of piscivorous predators, but specific mechanisms of response remain poorly known. Between 1984 and 2020, we studied the rhinoceros auklet Cerorhinca monocerata, a seabird breeding in the western North Pacific at Teuri Island, Japan. We tested the hypothesis that climate-mediated prey-switching affects ‘food packaging’ (i.e. the way energy is brought to dependent offspring) and breeding success by quantifying relationships between climate, prey energy density, amount of food delivered, and the growth and survival of chicks. Prey composition switched 4 times: 1988–1992, 1997–1998, 2013–2014, and 2017–2018. All but the last of these switches were associated with (lagged) shifts in seawater temperature/Pacific Decadal Oscillation. Rhinoceros auklets brought multiple fish in each meal-load to chicks, and numbers were inversely correlated with the size of the fish. These relationships varied between fish species. The heaviest meal-loads were achieved when diets were dominated by anchovy Engraulis japonicas, which occurred during warm phases (1992–2013). Chick production, growth rates, and mass at fledgling were also highest during the warm phases. This study shows that climate affects seabird reproduction by shifting the manner in which food is selected relative to changes in forage fish community structure and abundance.

We develop a multi-model approach to explore how abundance of a forage fish (Pacific sardine Sardinops sagax) impacts the ecosystem and predators in the California Current, a region where sardine and anchovy Engraulis mordax have recently declined to less than 10% of contemporary peak abundances. We developed or improved applications of 3 ecosystem modeling approaches: Ecopath, Model of Intermediate Complexity for Ecosystem assessment (MICE), and Atlantis. We also used Ecopath diets to predict impacts to predators using a statistical generalization of the dynamic Ecosim model (Predator Response to the Exploitation of Prey [PREP]). Models that included brown pelican Pelecanus occidentalis at the species level (MICE and Ecopath/PREP) both predict moderate to high vulnerability of brown pelicans to low sardine abundance. This vulnerability arises because sardine comprises a large fraction of their diet, and because other important prey (anchovy) also exhibit large population fluctuations. Two of the ecosystem models (MICE and Atlantis) suggest that California sea lions Zalophus californianus exhibit relatively minor responses to sardine depletion, due to having broader diets and lower reliance on another fluctuating species, anchovy. On the other hand, Ecopath/PREP suggests that sardine declines will have a stronger impact on California sea lions. This discrepancy may in part reflect structural differences in the models: Atlantis and MICE explicitly represent density dependence and age-structure, which can mitigate effects of prey depletion in these models. Future work should identify fisheries management strategies that are robust to uncertainties within and among models, rather than relying on single models to assess ecosystem impacts of management and forage fish abundance.

Forage species availability is a key determinant of seabird success, survival, and population change. In the Northwest Atlantic Ocean, capelin Mallotus villosus, a keystone forage species, experienced a stock collapse in the early 1990s that was a pivotal component of a regional regime shift. Since then, capelin have exhibited delayed protracted spawning, younger spawning age, distribution shifts, and smaller size. As capelin specialists, pursuit-diving common murres Uria aalge have had to adjust to these changes. We show that the masses of capelin provisioned to murre chicks at the species’ largest colony declined steadily from 1990–2017. We predicted that the parental provisioning of lower quality prey would reduce offspring condition, lower parental body mass, and increase foraging effort. Offspring condition declined, and while no negative effects were found on adult body mass, parental murres worked substantially harder in 2016, when capelin were dispersed and availability was low and when offspring and parental mass were the lowest in the time-series. These circumstances suggest that the murres neared a behavioral tolerance of parental effort. Despite the multi-decadal order-of-magnitude reduction in the regional capelin stock, parental murres coped by exploiting local prey availability at persistent shallow-water spawning sites and by increasing foraging effort. Even while the keystone forage fish stock remained at extremely low levels, the murre population increased, a likely consequence of enhanced adult survival due to amelioration of anthropogenic risk factors.

Fisheries can profoundly affect bycatch species with ‘slow’ life history traits. Managing bait type offers one tool to control species selectivity. Different species and sizes of marine predators have different prey, and hence bait, preferences. This preference is a function of a bait’s chemical, visual, acoustic and textural characteristics and size, and for seabirds the effect on hook sink rate is also important. We conducted a global meta-analysis of existing estimates of the relative risk of capture on different pelagic longline baits. We applied a Bayesian random effects meta-analytic regression modelling approach to estimate overall expected bait-specific catch rates. For blue shark and marine turtles, there were 34% (95% HDI: 4–59%) and 60% (95% HDI: 44–76%) significantly lower relative risks of capture on forage fish bait than squid bait, respectively. Overall estimates of bait-specific relative risk were not significantly different for seven other assessed taxa. The lack of a significant overall estimate of relative capture risk for pelagic shark species combined but significant effect for blue sharks suggests there is species-specific variability in bait-specific catch risk within this group. A qualitative literature review suggests that tunas and istiophorid billfishes may have higher catch rates on squid than fish bait, which conflicts with reducing marine turtle and blue shark catch rates. The findings from this synthesis of quantitative and qualitative evidence support identifying economically viable bycatch management measures with acceptable tradeoffs when multispecies conflicts are unavoidable, and highlight research priorities for global pelagic longline fisheries.

Interspecific interactions may be altered as a result of poleward species range shifts caused by climate change. In recent decades, Cumberland Sound, Nunavut, in the Canadian Arctic, has undergone concurrent increases in the availability of the forage fish capelin (Mallotus villosus) and the number of migratory harp seals (Pagophilus groenlandicus) during the open-water period; however, the impacts of these changes on endemic Arctic species, such as ringed seals (Pusa hispida), have received little attention. We coupled stomach contents with stable isotope analysis (δ13C and δ15N) of muscle and liver to determine the extent of potential competition between ringed seals (n = 91) and harp seals (n = 39) in Cumberland Sound. Isotopic niche breadth was greater for ringed seals (95% ellipse: 10.08‰2) than for harp seals (95% ellipse: 8.00‰2), and harp seal isotopic niche overlapped more with ringed seal isotopic niche than vice versa, suggesting asymmetrical competition potential. Although there was high overlap in isotopic niche breadth (range 50.3–91.0%) and prey species consumed (Schoener’s Index 0.60), stomach content analysis revealed differences in prey species proportions and size composition, thereby reducing the degree of realized niche overlap. Harp seals consumed a higher biomass of fish (66.7%) than did ringed seals (31.9%), and harp seals also consumed larger capelin (64–200 mm), polar cod (Boreogadus saida; 28–194 mm), and Liparidae (55–115 mm) than ringed seals (63–154 mm, 20–189 mm, and 16–128 mm, respectively). With climate change and range shifts predicted to continue into the future, our results provide an important baseline for future studies examining interspecific interactions.

As intermediaries between the bottom and top of food webs, forage fish fuel a diversity of coastal consumers and are of socioecological importance throughout the world’s oceans. Many forage fish are migratory, but despite their recognized importance, relatively little is known about their role in providing spatial subsidies, which are the movements of energy, material, and organisms across ecosystems. Until recently, spatial subsidies associated with Pacific herring Clupea pallasii, a dominant migratory forage fish that spawns in subtidal and intertidal zones, received little scrutiny. Building on research that traced links between herring spawns and coastal ecosystems, we used stable isotopes of carbon (δ13C) and nitrogen (δ15N) to assess whether herring spawning events influenced isotopic signatures of 10 macrophyte and invertebrate species across beaches where spawning did or did not occur. Overall, species collected from spawning beaches had significantly greater δ15N levels (general linear mixed model parameter estimate = 1.58 ± 0.17 SE, F 1,370 = 83.77, p < 0.001); no significant effects were detected for δ13C (parameter estimate = 0.03 ± 0.23 SE, F 1,343 = 0.01, p = 0.914). In terms of total nitrogen, macrophytes from spawning beaches had significantly elevated concentrations (parameter estimate = 5.03 ± 0.94 SE, F 1,180 = 28.71, p < 0.001). Using directional statistics, mean angles of isotopic change differed significantly between species collected from spawning and non-spawning beaches (Watson-Williams F-test; F 1,48 = 10.44, p = 0.002). Our study identifies multiple species as recipients of herring-derived nutrients at spawning events, providing additional evidence of the broad ecological influence of Pacific herring.