Northern Anchovy Research Articles

Meta-analysis in zooarchaeology expands perspectives on Indigenous fisheries of the Northwest Coast of North America

Fisheries are of fundamental importance to Indigenous peoples of the Pacific Northwest of North America today and in the past but few archaeological analyses have documented geographic patterning in fisheries across the entire region. This paper adopts meta-analysis methods and GIS-based spatial visualizations to survey the single largest compilation of fine-screened zooarchaeological fisheries data reported to date, including 513,605 fish remains identified at 222 sites from Oregon to southeast Alaska. These systematically collected zooarchaeological data indicate the most ubiquitous and proportionally abundant fish taxa over the late Holocene and reveal previously undocumented spatial patterning, indicating where certain fish taxa are consistently found in high relative proportions. Rather than seeking to evaluate chronological and/or evolutionary change, this study explores the environmental and cultural basis for assessing variability in Indigenous fisheries over millennial time scales. Specifically, we observe Pacific herring and the Pacific salmons to be the two most ubiquitous and proportionally abundant fish taxa across the Northwest Coast followed by flatfishes, sculpins, rockfishes, greenlings, dogfish, and a host of other poorly known taxa that represent consistent fishing effort. We document geographic patterning in the abundance and ubiquity of a range of fish including greater abundance of salmons in northern portions of the study area and outline trends that could represent biogeographic ranges for northern anchovy, Pacific hake, and pollock, among others. We conclude that examining patterning in the ubiquity and rank-order abundance represented by archaeological fisheries data offers significant potential for linking regionally distinct cultural practices noted in the 18th and 19th centuries to much longer human and ecological histories over the Holocene.

Growth and survival of larvae of the northern anchovy Engraulis mordax (Pisces: Engraulidae) reared under starvation conditions

A batch of 2800 larvae of the northern anchovy, Engraulis mordax Girard, was maintained up to twelve days after hatching in the laboratory under supposed starvation conditions (20-300 orgs./1) and variable concentrations of the nanoflagellate Tetraselmis sp. (5,000-10,000 cells/ml). In contrast with previous reports, growth and survival under these conditions were comparable to those observed under optimum feeding conditions. This suggests strategies that may be used by marine fish larvae to avoid starvation and be able to grow in an environment characterized by low food densities. We question the existence of a critical period. The ecological importance of nanoflagellates as a source of food in the marine environment is also suggested.

Recent collapse of northern anchovy biomass off California

New estimates of abundance of northern anchovy (Engraulis mordax) are developed from California Cooperative Oceanic Fisheries Investigations (CalCOFI) data on egg and larval densities for the period 1951–2011. Previous estimates utilizing mean density over the CalCOFI area show a hyperstability bias because of the nearshore concentration of CalCOFI stations and the tendency of the anchovy population to contract into this area when abundances are low. New abundance estimates weight sample egg and larval densities to the local area represented by that sample, and then sum contributions to obtain total abundance. We develop total egg and larval abundance estimates for January and April, form a combined index, and calibrate it to the absolute biomass estimates produced by the Daily Egg Production Method in the early 1980s. Anchovy spawning biomass was very low, 10,000–20,000 Metric Tons (MT), in the early 1950s when CalCOFI sampling began. Abundance increased and fluctuated between 0.5 and 2 million metric tons from 1960 through 1990. After 1990, spawning biomass fluctuated around 200,000 tons, briefly increased during 2005–2006, then declined substantially over four years to below 20,000 tons from 2009 to 2011. CalCOFI ichthyoplankton data collected after 2011 are not yet available for analysis, but continuous underway egg sampling conducted during CalCOFI cruises indicates continued low abundance and very limited spawning through 2015 in both southern and central California. The recent collapse of anchovy abundance occurred in the absence of a significant fishery. Present annual catches of a few thousand tons are small by historical comparison, but the exploitation rate may now be relatively high given the low stock abundance. The decline in anchovy abundance coincides with recent die-offs and reproductive failures of anchovy-dependent predators in the ecosystem (e.g., sea lions and pelicans).

A 500 year climate catch: Pelagic fish scales and paleoproductivity in the Santa Barbara Basin from the Medieval Climate Anomaly to the Little Ice Age (AD 1000–1500)

Based upon observations from the 20th century, populations of lower trophic level, short-lived fish species are thought to be controlled by large scale ocean–atmosphere circulation shifts. Here, we provide a new 500 year reconstruction of northern anchovy and Pacific sardine fish scale deposition rates (SDRs) and relative abundance from Santa Barbara Basin (SBB), through the transition from the Medieval Climate Anomaly into the Little Ice Age (AD 1000–1500). Three intervals of high anchovy biomass (high SDRs) were coincident with increased upwelling diatom abundance in SBB and cool/negative Pacific Decadal Oscillation (PDO) events at 1130–1160, 1280–1310 and 1410–1450. Intervals of absent or low SDR of both sardine and anchovy in SBB sediments occurred in association with increased wave activity (benthic diatoms), California Current diatoms, and the relative dominance of sardine over anchovy. Thus although ocean–atmosphere circulation shifts such as the PDO appear to play a role in shifting the dominance of anchovy over sardine, the biomass of sardines as indicated by SDRs has a more complicated response to environmental change.

Forage species in predator diets: Synthesis of data from the California Current

Characterization of the diets of upper-trophic pelagic predators that consume forage species is a key ingredient in the development of ecosystem-based fishery management plans, conservation of marine predators, and ecological and economic modeling of trophic interactions. Here we present the California Current Predator Diet Database (CCPDD) for the California Current region of the Pacific Ocean over the past century, assimilating over 190 published records of predator food habits for over 100 predator species and 32 categories of forage taxa (species or groups of similar species). Literature searches targeted all predators that consumed forage species: seabirds, cetaceans, pinnipeds, bony and cartilaginous fishes, and a predatory invertebrate. Diet data were compiled into a relational database. Analysis of the CCPDD highlighted differences in predator diet data availability based on geography, time period and predator taxonomy, as well as prominent prey categories. The top 5 forage taxa with the most predators included juvenile rockfish, northern anchovy, euphausiid krill, Pacific herring and market squid. Predator species with abundant data included Pacific hake, common murre, and California sea lion. Most diet data were collected during the summer; the lack of winter data will restrict future use of the CCPDD to understand seasonal patterns in predator diet unless more such data become available. Increased synthesis of historical information can provide new resources to understand patterns in the role of forage species in predator diet. Increased publication and/or accessibility of long-term datasets and data-sharing will further foster the synthesis of information intended to inform the management, conservation and understanding of marine food webs.

Structure and Organization of the Engraulidae Family U2 snRNA: An Evolutionary Model Gene?

The U2 snRNA multigene family has been analyzed in four species of the Engraulidae family--Engraulis encrasicolus, Engraulis mordax, Engraulis ringens, and Engraulis japonicas--with the object of understanding more about the structure of this multigene family in these pelagic species and studying their phylogenetic relationships. The results showed that the cluster of this gene family in the Engraulis genus is formed by the U2-U5 snRNA with highly conserved sequences of mini- and micro-satellites, such as (CTGT)n, embedded downstream of the transcription unit; findings indicate that this gene family evolved following the concerted model. The phylogenetic analysis of the non-transcribed spacer of cluster U2-U5 snDNA in the 4 species showed that the sequences of the species E. encrasicolus and E. japonicus are closely related; these two are genetically close to E. mordax and slightly more distant from E. ringens. The data obtained by molecular analysis of U2-U5 snDNA and their secondary structure, with the presence of the micro-satellite (CTGT)n and mini-satellites, show clearly that the species E. encrasicolus and E. japonicus are closely related and would be older than E. mordax and E. ringens.

Managing small-scale commercial fisheries for adaptive capacity: insights from dynamic social-ecological drivers of change in Monterey Bay.

Globally, small-scale fisheries are influenced by dynamic climate, governance, and market drivers, which present social and ecological challenges and opportunities. It is difficult to manage fisheries adaptively for fluctuating drivers, except to allow participants to shift effort among multiple fisheries. Adapting to changing conditions allows small-scale fishery participants to survive economic and environmental disturbances and benefit from optimal conditions. This study explores the relative influence of large-scale drivers on shifts in effort and outcomes among three closely linked fisheries in Monterey Bay since the Magnuson-Stevens Fisheries Conservation and Management Act of 1976. In this region, Pacific sardine (Sardinops sagax), northern anchovy (Engraulis mordax), and market squid (Loligo opalescens) fisheries comprise a tightly linked system where shifting focus among fisheries is a key element to adaptive capacity and reduced social and ecological vulnerability. Using a cluster analysis of landings, we identify four modes from 1974 to 2012 that are dominated (i.e., a given species accounting for the plurality of landings) by squid, sardine, anchovy, or lack any dominance, and seven points of transition among these periods. This approach enables us to determine which drivers are associated with each mode and each transition. Overall, we show that market and climate drivers are predominantly attributed to dominance transitions. Model selection of external drivers indicates that governance phases, reflected as perceived abundance, dictate long-term outcomes. Our findings suggest that globally, small-scale fishery managers should consider enabling shifts in effort among fisheries and retaining existing flexibility, as adaptive capacity is a critical determinant for social and ecological resilience.

Spatial overlap between forage fishes and the large medusa Chrysaora fuscescens in the northern California Current region

As in many regions of the world, the shelf waters of the western United States have experienced large increases and high interannual variability in jellyfish populations in recent decades. The northern California Current (NCC) is a productive upwelling zone that is home to large populations of medusae, particularly during some years. Seasonal trawl surveys in the NCC over 13 yr have documented a substantial biomass of jellyfish consisting primarily of one species, the sea nettle Chrysaora fuscescens, with abundances generally peaking in late summer. Trophic overlap can be high in the NCC with planktivorous species such as Pacific sardines and herring that consume copepods and other zooplankton. In this study, we examine the spatial overlap and co-occurrence of C. fuscescens and Pacific herring Clupea pallasii, northern anchovy Engraulis mordax and Pacific sardine Sardinops sagax in the NCC using spatial analysis tools to determine the species that have the potential to be most affected by high jellyfish biomass and the geo- graphic areas in which these interactions are likely to occur. Significant spatial overlap of C. fuscescens with these pelagic fishes occurred during certain months and years, although the results were highly variable. There was an overall negative relationship between the abundance of C. fuscescens and the catch of the 3 forage fishes for both June and September. End-to-end food web models show that jellyfish have a greater potential to affect production of pelagic forage fishes than the reverse.

Temporal and sex-specific variability in Rhinoceros Auklet diet in the central California Current system

We used stable isotopes (δ15N and δ13C) and compared prey provided to chicks by each sex to evaluate seasonal and sex-specific diets in Rhinoceros Auklets (Cerorhinca monocerata) in the central California Current system during 2012–2013. Mixing models indicated northern anchovy (Engraulis mordax) were important prey for adults during fall/winter and juvenile rockfishes (Sebastes spp.) were important prey during incubation both years. Adult trophic level increased between incubation and chick-rearing periods in both years. During 2012, δ15N and δ13C of chick-rearing males and females differed significantly; mixing models indicated that females ate more Pacific saury (Cololabis saira) and less market squid (Doryteuthis opalescens) than males. Likewise, females delivered significantly more Pacific saury and less market squid to chicks than males during 2012. Chick growth (g d−1) and chick survival to fledging were significantly lower during 2012 than 2013, likely because chicks were fed lesser quality prey or fed less frequently in 2012. Lesser body mass of females during incubation in 2012 indicated sex-specific diet differences may have been related to female energetic constraints. The observed variability in Rhinoceros Auklet diet underscores the importance of managing multiple prey populations in this system so that generalist predators have sufficient resources through changing conditions.

Diet of a piscivorous seabird reveals spatiotemporal variation in abundance of forage fishes in the Monterey Bay region

Brandt's Cormorant (Phalacrocorax penicillatus) diet was investigated using regurgitated pellets (n=285) collected on 19 sampling days at three locations during the 2006–07 and 2007–08 nonbreeding seasons in the Monterey Bay region. The efficacy of using nested sieves and the all-structure technique to facilitate prey detection in the pellets was evaluated, but this method did not increase prey enumeration and greatly decreased efficiency. Although 29 prey species were consumed, northern anchovy (Engraulis mordax) dominated and speckled sanddab (Citharichthys stigmaeus) also was important in the diet. Few rockfishes (Sebastes spp.) and market squid (Doryteuthis opalescens) were consumed compared with great prevalence in previous studies during the 1970s. El Niño and La Niña during the study provided a unique opportunity to examine predator response to variation in prey availability. Patterns of prey number and diversity were not consistent among locations. Greatest number and diversity of prey occurred at locations within Monterey Bay during La Niña, results not evident at the outer coast location. Short-term specialization occurred but mean prey diversity indicated a generalist feeding mode. This study demonstrated the importance of periodic sampling at multiple locations within a region to detect spatiotemporal variability in the diet of opportunistic generalists.

A shallow-diving seabird predator as an indicator of prey availability in southern California waters: A longitudinal study

We tested the hypothesis that the Elegant Tern (Thalasseus elegans), a plunge-diving predator, is an indicator of changes in the prey community in southern California coastal waters. Shannon diversity (H′) of the tern's diet determined from dropped fish collected variously at the three nesting sites for 18years over a 21-year interval (1993–2013) showed no significant change in diet diversity. Based on a species–accumulation curve, total diet species represented about 70% of an extrapolated asymptotic richness. Abundance patterns of five prey species making up >75% of prey numbers for all years were compared with abundance patterns of the same species in independent surveys obtained from zooplankton tows, bottom trawls and power-plant entrapments. Three of the five species – northern anchovy, kelp pipefish and California lizardfish – showed significant, positive correlations between diet and survey abundances. Even though the tern's diet has been dominated by anchovy and pipefish, its diet is still broad, with prey taxa representing >75% of the 42 species groups making up the California shelf fish fauna. Altogether, our results support the hypothesis that the Elegant Tern, with its flexible diet, is a qualitative indicator, a sentinel, of changes in the prey communities in southern California coastal waters.

Seabird diet predicts following-season commercial catch of Gulf of California Pacific Sardine and Northern Anchovy

The highly productive Gulf of California exhibits high biodiversity and an abundance of small pelagic, schooling fishes, important both to the ecosystem and to fisheries. These fishes show wide fluctuations in abundance due to oceanographic–atmospheric phenomena such as El Niño-Southern Oscillation, constituting a problem for effective fisheries management. In this work we propose several parameters in the diet of three seabird species as useful predictors of the eventual commercial catch per unit effort (CPUE) of Pacific sardine (Sardinops sagax) and northern anchovy (Engraulis mordax) in the Midriff Island Region of the Gulf of California. We found that seabirds are sensitive to fluctuations in the abundances of these fishes, their proportions in the diet becoming suitable predictors of future commercial CPUE. The precision of our prediction is high because the birds feed on pre-recruit fish, and the time between the sampling of the diets and the initiation of the fishing season is short, minimizing stochastic effects on recruitment and subsequent abundance due to environmental fluctuations.

Genetic corroboration of Engraulis mordax larvae in the upper Gulf of California, a previously undescribed spawning habitat

Larvae of the Californian anchovy (Engraulis mordax) in the upper Gulf of California were identified. The finding was achieved with partial sequences of 16S rRNA (479 bp) and cytochrome c oxidase subunit I (580 bp) of mtDNA. High abundance of preflexion larvae (64 % of the sampling stations) in the northeastern upper Gulf, combined with environmental conditions (temperature: 17–18 °C; salinity: 35.8–36.0 g/kg; dissolved oxygen: 5.5–6.5 mL/L; chlorophyll a: 1.5–3.0 mg/m3), support an undescribed spawning habitat of E. mordax in the Gulf of California.

The Effects of Spatial and Temporal Resolution in Simulating Fish Movement in Individual‐Based Models

AbstractMany fisheries management decisions require predictions of spatial dynamics, and simulation of realistic movement is critically important for accurately representing population‐level dynamics with spatially explicit individual‐based models (IBMs). Movement approaches developed to date have been applied across a wide range of spatiotemporal resolutions. We compared four movement approaches or submodels (restricted‐area search, kinesis, event based, and run and tumble) using an IBM (roughly based on Bay Anchovy Anchoa mitchilli and Northern Anchovy Engraulis mordax) that simulated growth, mortality, and movement of a cohort on a two‐dimensional grid. We evaluated the submodels in 2.7‐ × 2.7‐km environments at five resolutions defined by various cell sizes and time steps. We used a genetic algorithm to calibrate each movement submodel over a 300‐generation training phase and then tested the mean movement parameters for a single generation in the training environment and a novel environment. Restricted‐area search, kinesis, and event‐based submodels had higher egg production than a random walk model (baseline, assuming no behavioral movement) across spatiotemporal resolutions in training and novel environments. The run‐and‐tumble submodel also had higher egg production than the random walk model but only under certain conditions. Although restricted‐area, kinesis, and event‐based submodels outperformed random walk at all resolutions, the submodels did not perform equally well across resolutions in terms of egg production and aggregation of model individuals in high‐quality cells (i.e., those with high growth and low mortality). The variability in performance was due to the change in habitat quality experienced by model individuals from one time step to the next. Restricted‐area and event‐based submodels had higher egg production when model individuals experienced small changes in habitat quality, whereas the kinesis and run‐and‐tumble submodels performed better when model individuals experienced larger changes in habitat quality.Received October 16, 2013; accepted March 11, 2004

Changes in forage fish community indicated by the diet of the Brandt's cormorant (Phalacrocorax penicillatus) in the central California Current

The population, productivity and diet of two Brandt's cormorant (Phalacrocorax penicillatus) colonies located in the central California Current were compared. The offshore colony on Southeast Farallon Island has experienced a declining population over time and anomalously low productivity in recent years. The nearshore colony near Point Arguello has been increasing and its productivity has remained stable. The diets of cormorants at the two colonies elucidated by analysis of regurgitated pellets, while different, have shown similar decreases in the consumption of northern anchovy (Engraulis mordax) since 2008, followed by increased consumption of rockfish (Sebastes spp.) and flatfish (order Pleuronectiformes). By using the diet results from another seabird nesting in central California, the rhinoceros auklet (Cerorhinca monocerata), and one from which whole fish can be obtained, we found that the rockfish species assemblage has changed with offshore rockfish species decreasing while nearshore ones have increased. This change in the rockfish species has negatively impacted Brandt's cormorants at the offshore colony by forcing them to make longer foraging trips to meet energy needs of themselves and their chicks; this has led to low breeding success and a declining population at this site. On the other hand, the nearshore colony has abundant nearby food resources, and it has prospered. These results underscore the value of using seabird data from multiple colonies to better understand changes occurring in the marine environment.

Long-term variation in a central California pelagic forage assemblage

A continuous 23year midwater trawl survey (1990–2012) of the epipelagic forage assemblage off the coast of central California (lat. 36°30′–38°20′ N) is described and analyzed. Twenty taxa occurred in ≥10% of the 2037 trawls that were completed at 40 distinct station locations. The dominant taxa sampled by the 9.5mm mesh net included a suite of young-of-the-year (YOY) groundfish, including rockfish (Sebastes spp.) and Pacific hake (Merluccius productus), two clupeoids (Engraulis mordax and Sardinops sagax), krill (Euphausiacea), cephalopods (Doryteuthis opalescens and Octopus sp.), and a variety of mesopelagic species, i.e., Diaphus theta, Tarltonbeania crenularis, “other” lanternfish (Myctophidae), deep-sea smelts (Bathylagidae), and sergestid shrimp. Annual abundance estimates of the 20 taxa were obtained from analysis of variance models, which included year and station as main effects. Principal components analysis of the abundance estimates revealed that 61% of assemblage variance was explained by the first three components. The first component revealed a strong contrast in the abundance of: (a) YOY groundfish, market squid (D. opalescens), and krill with (b) mesopelagics and clupeoids; the second component was associated with long-term trends in abundance. An evaluation of 10 different published oceanographic data sets and CTD data collected during the survey indicated that seawater properties encountered each year were significantly correlated with abundance patterns, as were annual sea-level anomalies obtained from an analysis of AVISO satellite information. A comparison of our findings with several other recent studies of biological communities occurring in the California Current revealed a consistent structuring of forage assemblages, which we conjecture is primarily attributable to large-scale advection patterns in the California Current ecosystem.

Dynamics of larval fish assemblages in the California Current System: a comparative study between Oregon and southern California

Boundary currents influence near-shore ecosystems worldwide, and understanding how fish assemblages change spatially and temporally throughout these systems is important for establishing the scale at which ecosystem-based management (EBM) should be conducted. Because most research on boundary currents in general, and the California Current System (CCS) in particular, have been restricted to only small portions of the systems, it is largely unknown whether fish assemblages in boundary currents form one coherent ecosystem or if changes in assemblage structure are locally independent. We expand the geographic scope of previous analyses on ichthyoplankton assemblages within boundary currents by comparing dynamics in 2 widely separated regions of the CCS: Oregon (~45° N) and southern California (~34° N) in spring and summer from 2004 to 2011. Both region and season affected assemblage structure. Some taxa that were moderately common in California were consistently rare or absent in Oregon (and vice versa), and the presence of most decreased in summer in both regions. However, the assemblages were very similar in some years. Off Oregon, the assemblage most resembled California's when the ocean temperature was relatively high and northern anchovy Engraulis mordax was abun- dant. Assemblage dynamics were well explained by environmental change in Oregon. By con- trast, California's assemblage and environmental variability correlated poorly. Population sizes of taxa common to both regions did not fluctuate coherently in Oregon and California. These find- ings are important for EBM because they indicate that it is not possible to extrapolate results from spatially restricted localities to understand assemblage dynamics throughout the entire CCS.

The effects of a tidal-mixing front on the distribution of larval fish habitats in a semi-enclosed sea during winter

We examined the effect of a tidal-mixing front on the three-dimensional distribution of larval fish habitats (LFHs) in the Midriff Archipelago Region in the Gulf of California during winter. Zooplankton and environmental variables were sampled from 0 to 200 m in 50 m strata. Four LFHs were defined in association with the front, two on the northern side and two on the southern side. The northern LFHs were: (1) the Mainland Shelf Habitat, located from the surface to 100 m depth on the north-east mainland shelf, characterized mainly by the presence of Citharichtys fragilis; and (2) the Wide Distribution Habitat, extending from north-west to south across the front from the surface to 200 m depth, dominated by the ubiquitous Engraulis mordax. The southern LFHs were: (3) the Eddy Zone Habitat, defined nearly on an anticyclonic eddy, with the highest larval abundance and richness from the surface to 100 m depth, dominated by Leuroglossus stilbius; and (4) the Southern Gulf Habitat, associated with low temperature waters from the southern Gulf of California, dominated by southern-gulf species (e.g. Scomber japonicus and Sardinops sagax). Despite the weak stratification and low thermal contrast (~1.5°C) across the south front compared to summer (~3°C), our results demonstrate that the frontal zone may influence the formation of planktonic habitats even during generally homogeneous periods, which may also be relevant in other regions of the world.

A Rapid Method for Differentiating Four Species of the Engraulidae (Anchovy) Family

The four species of the Engraulidae family: European anchovy (Engraulis encrasicolus), Californian anchovy (Engraulis mordax), Peruvian anchoveta (Engraulis ringens), and Japanese anchovy (Engraulis japonicus) studied in this work are very similar morphologically, and it is very difficult to distinguish between them, especially when frozen or processed. We have used the 5S rDNA as a molecular marker to discriminate these four species and used specific primers designed for each species in the nontranscribed spacers (NTS) of these genes. Multiplex PCR was performed with three pairs of primers, and three different sizes were obtained: 597 bp E. encrasicolus, 598 bp E. japonicus, 380 bp E. mordax, and 250 bp E. ringens. For the species E. encrasicolus and E. japonicus, PCR-RFLP was used as an additional technique to distinguish between them because their NTS sequences showed considerable similarity.

Climate–ecosystem change off southern California: Time-dependent seabird predator–prey numerical responses

Climate change may increase both stratification and upwelling in marine ecosystems, but these processes may affect productivity in opposing or complementary ways. For the Southern California region of the California Current Ecosystem (CCE), we hypothesized that changes in stratification and upwelling have affected marine bird populations indirectly through changes in prey availability. To test this hypothesis, we derived trends and associations between stratification and upwelling, the relative abundance of potential prey including krill and forage fish, and seabirds based on the long-term, multi-disciplinary CalCOFI/CCE-LTER program. Over the period 1987 through 2011, spring and summer seabird density (all species combined) declined by ~2% per year, mostly in the northern sector of the study region. Krill showed variable trends with two species increasing and one deceasing, resulting in community reorganization. Nearshore forage fish, dominated by northern anchovy (Engraulis mordax) as well as offshore mesopelagic species, show declines in relative abundance over this period. The unidirectional decline in springtime seabird density is largely explained by declining nearshore fish abundance in the previous season (winter). Interannual variability in seabird density, especially in the 2000s, is explained by variability in krill abundance. Changes in the numerical responses of seabirds to prey abundance correspond to a putative ecosystem shift in 1998–1999 and support aspects of optimal foraging (diet) theory. Predator–prey interactions and numerical responses clearly explain aspects of the upper trophic level patterns of change in the pelagic ecosystem off southern California.