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IntroductionGlobally, anchovy and sardine typically display asynchronous population fluctuations with anchovy dominating during cool periods and sardine dominating during warm periods. However, this anchovy-sardine cold-warm paradigm has recently broken down in the California Current, suggesting that recruitment may not be a simple product of large-scale physical drivers. Instead, consideration of larval fish trophodynamics together with local oceanography is likely necessary to mechanistically relate survival and recruitment to the physical environment.MethodsWe examined otolith-derived metrics of northern anchovy (Engraulis mordax) growth in the context of local oceanography and anchovy in situ prey and zooplankton predators in the northern California Current (NCC).ResultsAnchovy growth was spatially variable and the regions that conferred heighted growth differed with regard to the cross-shelf extent of upwelled waters. When upwelling was restricted to the nearshore environment, anchovy larvae grew significantly faster inshore than offshore. Conversely, when the upwelling front moved farther offshore following sustained upwelling, offshore anchovy larvae grew significantly faster than inshore larvae. Modelling individual anchovy growth revealed that growth was affected by ambient copepod prey availability and gelatinous zooplankton predation pressure, with growth peaking at intermediate prey availability and the highest abundance of predators. Fast growth under high predation pressure may be indicative of the selective loss of slow growing larvae. Notably, larval anchovy abundances were high offshore but diminished immediately inshore of the upwelling front regardless of its cross-shelf position. This suggests that the upwelling front may act as a shoreward boundary for anchovy larvae, affecting their access to the highly nutritious prey base typical of the Oregon continental shelf waters in summer.DiscussionVariation in larval anchovy growth with local oceanographic conditions and fine-scale distributions of prey and predators provides a mechanistic hypothesis of food-web dynamics which will enhance our ability to predict the response of forage fishes to ecosystem variability.

Plastic pollution is an anthropogenic stressor in marine ecosystems globally. Many species of marine fish (more than 50) ingest plastic debris. Ingested plastic has a variety of lethal and sublethal impacts and can be a route for bioaccumulation of toxic compounds throughout the food web. Despite its pervasiveness and severity, our mechanistic understanding of this maladaptive foraging behaviour is incomplete. Recent evidence suggests that the chemical signature of plastic debris may explain why certain species are predisposed to mistaking plastic for food. Anchovy (Engraulis sp.) are abundant forage fish in coastal upwelling systems and a critical prey resource for top predators. Anchovy ingest plastic in natural conditions, though the mechanism they use to misidentify plastic as prey is unknown. Here, we presented wild-caught schools of northern anchovy (Engraulis mordax) with odour solutions made of plastic debris and clean plastic to compare school-wide aggregation and rheotactic responses relative to food and food odour presentations. Anchovy schools responded to plastic debris odour with increased aggregation and reduced rheotaxis. These results were similar to the effects food and food odour presentations had on schools. Conversely, these behavioural responses were absent in clean plastic and control treatments. To our knowledge, this is the first experimental evidence that adult anchovy use odours to forage. We conclude that the chemical signature plastic debris acquires in the photic zone can induce foraging behaviours in anchovy schools. These findings provide further support for a chemosensory mechanism underlying plastic consumption by marine wildlife. Given the trophic position of forage fish, these findings have considerable implications for aquatic food webs and possibly human health.

Forage fish play a central role in the transfer of energy from lower to higher trophic levels. Ocean conditions may influence this energy pathway in the Northern California Current (NCC) ecosystem, and we may expect it to differ between warm and cold periods in the northeast Pacific Ocean. The recent unprecedented warming in the NCC provides a unique opportunity to better understand the connection between ocean conditions and forage fish feeding habits and the potential consequences for predators that depend on them for sustenance. Here we present findings from gut content analysis to examine food sources of multiple forage fishes (northern anchovy Engraulis mordax, Pacific sardine Sardinops sagax, jack mackerel Trachurus symmetricus, Pacific herring Clupea pallasii, surf smelt Hypomesus pretiosus, and whitebait smelt Allosmerus elongatus) off the Washington and Oregon coasts. Analyses were applied to fish collected in May and June during recent warm years (2015 and 2016) and compared to previous collections made during cool (2011, 2012) and average (2000, 2002) years. Results of the diet analysis indicate that fish feeding habits varied significantly between cold and both average and warm periods. Euphausiids, decapods, and copepods were the main prey items of the forage fishes for most years examined; however, gelatinous zooplankton were consumed in much higher quantities in warm years compared to cold years. This shift in prey availability was also seen in plankton and trawl surveys in recent years and suggests that changing ocean conditions are likely to affect the type and quality of prey available to forage fish. Although gelatinous zooplankton are generally not believed to be suitable prey for most fishes due to their low energy content, some forage fishes may utilize this prey in the absence of more preferred prey resources during anomalously warm ocean conditions.

In the northern California Current, Columbia River Chinook salmon Oncorhynchus tshawytscha that return as adults in spring are primarily hatchery-produced, though they include natural-origin fish listed under the US Endangered Species Act. Anomalously warm ocean conditions persisted in the California Current during 2015 (>2.5°C above normal) through the winter period when fish prey resources of juvenile salmon develop and during spring as salmon enter the ocean. The biomass of ichthyoplankton in winter 2015 was the 4th highest of our 18 yr time-series (1998–2015), predicting good food conditions for salmon and high adult salmon returns several years later. The larval composition of 2015 ichthyoplankton included abnormally large amounts of the warm-water taxa northern anchovy Engraulis mordax and rockfish Sebastes spp. When the composition of ichthyoplankton is dominated by warm-water taxa in winter, we would predict poor returns of salmon. May diets of juvenile Chinook salmon collected in coastal waters reflected high proportions of juvenile rockfish, no evidence of northern anchovy, and most closely resembled those of other warm years. June diets also reflected a warm prey community being consumed, predicting poor returns of salmon. Chinook salmon had high percentages of empty stomachs and were small and thin in 2015, with fish weighing 17.6% less than the same-length fish in a cold year (2008). Lower condition of juvenile Chinook salmon related to decreased returns of adult salmon. Overall, all but one biological predictor (biomass of prey) suggests that the prospects for the 2015 ocean-entry smolts were not favorable for survival.

Characterizing predator diets is essential to understanding food web dynamics. We investigated the dietary breadth and variation of the California sea lion (CSL) Zalophus californianus at 3 of the California Channel Islands from 1981–2015. Prey species were identified from hard parts and soft-tissue remains of pyrosomes recovered from fecal samples, revealing a diverse diet of fish and cepha lo pods. Percent frequency of occurrence and percent split-sample frequency of occurrence were used to describe long-term trends, correlations between prey taxa, diet diversity, and diet similarity. The most common of 142 prey taxa identified to species were market squid Doryteuthis opalescens, northern anchovy Engraulis mordax, shortbelly rockfish Sebastes jordani, jack mackerel Trachurus symmetricus, Pacific sardine Sardinops sagax, Pacific hake Merluccius productus, and Pacific mackerel Scomber japonicus. Dietary differences were observed be tween male and female sea lions, and between animals from different islands and seasons. Intra-island diets at 2 locations were more similar than inter-island diets. Important interannual and decadal changes in diet were identified. CSL diet shifted from an anchovy-based diet in the 1980s to a market squid based-diet in the 1990s and 2000s, with other prey taxa being consumed more frequently when consumption of those 2 main prey declined. Prey-switching likely provides flexibility needed in the dynamic California Current Ecosystem, and may allow CSLs to adapt to changes in food supply and availability driven by climate change.

A number of scientific papers in the last few years singled out the influence of environmental conditions on the spatial distribution of fish species, highlighting the need for the fisheries scientific community to investigate, besides biomass estimates, also the habitat selection of commercially important fish species. The Mediterranean Sea, although generally oligotrophic, is characterized by high habitat variability and represents an ideal study area to investigate the adaptive behavior of small pelagics under different environmental conditions. In this study the habitat selection of European anchovy Engraulis encrasicolus and European sardine Sardina pilchardus is analyzed in two areas of the Mediterranean Sea that largely differentiate in terms of environmental regimes: the Strait of Sicily and the North Aegean Sea. A number of environmental parameters were used to investigate factors influencing anchovy and sardine habitat selection. Acoustic surveys data, collected during the summer period 2002-2010, were used for this purpose. The quotient analysis was used to identify the association between high density values and environmental variables; it was applied to the entire dataset in each area in order to identify similarities or differences in the \"mean\" spatial behavioral pattern for each species. Principal component analysis was applied to selected environmental variables in order to identify those environmental regimes which drive each of the two ecosystems. The analysis revealed the effect of food availability along with bottom depth selection on the spatial distribution of both species. Furthermore PCA results highlighted that observed selectivity for shallower waters is mainly associated to specific environmental processes that locally increase productivity. The common trends in habitat selection of the two species, as observed in the two regions although they present marked differences in hydrodynamics, seem to be driven by the oligotrophic character of the study areas, highlighting the role of areas where the local environmental regimes meet 'the ocean triad hypothesis'.

Spatial distributions of marine fauna are determined by complex interactions between environmental conditions and animal behaviors. As climate change leads to warmer, more acidic, and less oxygenated oceans, species are shifting away from their historical distribution ranges, and these trends are expected to continue into the future. Correlative Species Distribution Models (SDMs) can be used to project future habitat extent for marine species, with many different statistical methods available. However, it is vital to assess how different statistical methods behave under novel environmental conditions before using these models for management advice, and to consider whether future projections based on these techniques are biologically reasonable. In this study, we built SDMs for adults and larvae of two ecologically important pelagic fishes in the California Current System: Pacific sardine (Sardinops sagax) and northern anchovy (Engraulis mordax). We used five different SDM methods, ranging from simple (thermal niche model) to complex (artificial neural networks). Our results show that some SDMs trained on data collected between 2003 and 2013 lost substantial predictive skill when applied to observations from more recent years, when ocean temperatures associated with a marine heatwave were outside the range of historical measurements. This decrease in skill was particularly apparent for adult sardine, which showed non-stationary relationships between catch locations and sea surface temperature through time. While sardine adults and larvae shifted their distributions markedly during the marine heatwave, anchovy largely maintained their historical spatiotemporal distributions. Our results suggest that correlative relationships between species and their environment can become unreliable during anomalous conditions. Understanding the underlying physiology of marine species is therefore essential for the construction of SDMs that are robust to rapidly changing environments. Developing distribution models that offer skillful predictions into the future for species such as sardine and anchovy, which are migratory and include separate sub-stocks, may be particularly challenging.

Fluctuations in prey abundance, composition, and distribution can impact predators, and when predators and fisheries target the same species, predators become essential to ecosystem-based management. Because of the difficulty in collecting concomitant predator–prey data at appropriate scales in patchy environments, few studies have identified strong linkages between cetaceans and prey, especially across large geographic areas. During summer 2018, a linetransect survey for cetaceans and coastal pelagic species was conducted over the continental shelf and slope of British Columbia, Canada, and the US West Coast, allowing for a large-scale investigation of predator–prey spatial relationships. We report on a case study of humpback whales (Megaptera novaeangliae) and their primary prey—Pacific herring (Clupea pallasii), northern anchovy (Engraulis mordax), and krill—using generalized additive models to explore the relationships between whale abundance on 10-km transect segments and prey metrics. Prey metrics included direct measures of biomass densities on segments and an original hotspot metric. For each prey species, segments in the upper fifth percentile for biomass density (across all segments) were designated hotspots, and whale counts on a segment were evaluated for their relationship to number of hotspot segments (species-specific and multispecies) within 25, 50, or 100 km. Whale abundance was not strongly related to direct measures of biomass densities, whereas models using hotspot metrics were more effective at describing variation in whale abundance, underscoring that evaluating prey at relevant and measurable scales is critical in patchy, dynamic marine environments. Our analysis highlighted differences in the distribution and prey availability for three humpback whale distinct population segments (DPSs) as defined under the US Endangered Species Act, including threatened and endangered DPSs that forage within the California Current Large Marine Ecosystem. These linkages provide insights into which prey species whales may be targeting in different regions and across multiple scales and, consequently, how climatic variability and anthropogenic risks may differentially impact these distinct predator–prey assemblages. By identifying scale-appropriate prey hotspots that co-occur with humpback whale aggregations, and with targeted, consistent prey sampling and estimations of potential consumption rates by whales, these findings can help inform the conservation and management of humpback whales within an ecosystembased management framework.

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.

Temperature is the main abiotic factor that influences the life cycle of poikilotherms. The present study investigated the thermal tolerance and phenotypic plasticity of several parameters (development time, morphometric measures, bioenergetics) for both embryos and fry of a cold stenothermal fish species, brown trout (Salmo trutta L.) in order to allow for a holistic evaluation of the potential effects of temperature. Five temperatures (4°C, 6°C, 8°C, 10°C, and 12°C) were tested, and the effects of temperature were analyzed at three stages: hatching, emergence, and first food intake. A mean of 5,440 (S.E. ± 573) eggs, coming from seven females and seven males (seven families) captured close to Linkebeek (Belgium), were used for each temperature. Maximum survival of well-formed fry at first food intake and better use of energy budget were found at 6°C and 8°C, temperatures at which the possible contribution to the next generation should therefore be greatest. At 12°C, the experimental population fell dramatically (0.9% survival rate for well-formed fry at first food intake), and fry had almost no yolk sac at first food intake. The present results on survival at 12°C are in accordance with predictions of a sharp decrease in brown trout numbers in France over the coming decades according to climate change projections (1°C to 5°C temperature rise by 2100 for France). At 10°C, there was also a lower survival rate (55.4% at first food intake). At 4°C, the survival rate was high (76.4% at first food intake), but the deformity rate was much higher (22% at first food intake) than at 6°C, 8°C, and 10°C. The energetic budget showed that at the two extreme temperatures (4°C and 12°C) there was less energy left in the yolk sac at first food intake, suggesting a limited ability to survive starvation.