Anchovy Population Research Articles

Multidecadal, centennial, and millennial variability in sardine and anchovy abundances in the western North Pacific and climate–fish linkages during the late Holocene

Paleorecords of pelagic fish abundance could better define the nature of fishery productivity dynamics and help understand responses of pelagic fish stocks to long-term climate changes. We report a high-resolution record of sardine and anchovy scale deposition rates (SDRs) from Beppu Bay, Southwest Japan, showing multidecadal and centennial variability in the abundance of Japanese sardine and Japanese anchovy during the last 2850years. Variations in the sardine SDR showed periodicities at ∼50, ∼100, and ∼300yr, while variations in the anchovy SDR showed periodicities at ∼30 and ∼260yr. Comparisons between and correlation analyses of the time series of the sardine and anchovy SDRs demonstrate that there is not a consistent out-of-phase relationship during the last 2850years. This indicates that the multidecadal alternations in the sardine and anchovy populations commonly seen in the 20th century did not necessarily occur during earlier periods. The Japanese sardine SDR record shows a long-term decreasing trend in the amplitudes of the multidecadal to centennial fluctuations. This decreasing trend may have resulted from an increasing trend in the winter sea surface temperature in the western North Pacific. The multicentennial variability in sardine abundance during the last millennium is consistent with the variabilities in the abnormal snow index in East Asia and the American tree ring-based Pacific Decadal Oscillation index, suggesting a basin-wide or regional climate–marine ecosystem linkage.

Trophic relationships between anchovy (Engraulis encrasicolus) and zooplankton in the Strait of Sicily (Central Mediterranean sea): a stable isotope approach

The study of the relationships among environmental factors, primary production, zooplankton community, and feeding behavior of fish species represents a key aspect to obtain a deeper knowledge of biological processes acting at ecosystem level. In this context, oceanographic data and biological samples were collected in two different surveys, carried out during late June 2011 and early July 2015 in the Strait of Sicily (Central Mediterranean Sea). Differences in abundance, biomass, and assemblage structure of zooplankton and anchovy (Engraulis encrasicolus Linnaeus, 1758) population were observed between the two surveys. Still, zooplankton biomass was significantly correlated to longitude, chlorophyll a recorded during the surveys (as a proxy of primary production), and oxygen concentrations. Such differences affected the isotopic composition of anchovy, which showed changes in δ15N and δ13C values between the two surveys. Mixing models identified different contributions of food sources in the two sampling periods, i.e., major consumption on large copepods and euphausiids in 2015 with respect to 2011, which was consistent with a greater availability of these preys in the environment in 2015. The obtained results evidenced that E. encrasicolus may be affected by any environmental change that influences the plankton community, which is the basis of the diet of these fishes. Such findings highlighted the importance to further investigate the trophic relationships among different compartments of the food web, as well as the possible environmental influences, in order to obtain a more complete picture of ecosystem functioning and also in the light of an ecosystem-based approach to fisheries management.

Многолетняя динамика репродуктивных показателей европейского анчоуса Engraulis encrasicolus (L., 1758) и их связь с температурными условиями

Европейский анчоус, или хамса (Engraulis encrasicolus L.), — один из наиболее массовых видов рыб в Азово-Черноморском бассейне. Является основным промысловым объектом: доля его вылова составляет около 80 %. Оценка состояния популяции хамсы и её динамики в условиях климатического потепления — одна из актуальных задач современных исследований. Данная статья посвящена изучению многолетней динамики ряда популяционных репродуктивных показателей хамсы и их связи с температурой воды с целью возможного использования для прогноза состояния популяции при изменении условий среды. В основе работы – полученные в 2000–2015 гг. результаты собственных исследований, выполненных в прибрежной акватории Чёрного моря, прилегающей к юго-западному побережью Крыма (район Севастополя — Балаклавы). Для сбора икры хамсы использовали планктонную сеть Богорова — Расса (БР-80/113) в режиме вертикального лова в слое 0–10 м. Взрослые экземпляры получены из ставных неводов. Проанализированы 702 ихтиопланктонные пробы и 941 экз. нерестовых самок (на стадиях зрелости V, VI–IV и VI–V). Среди популяционных репродуктивных показателей были изучены календарные сроки и продолжительность нерестового периода, интенсивность и эффективность нереста, внутривидовой состав родительского стада. Календарные сроки нереста хамсы в 2000–2014 гг. у юго-западного побережья Крыма варьировали от второй декады мая (ранний нерест) до третьей декады июня (поздний нерест); сроки окончания — от третьей декады августа до третьей декады сентября. Общая продолжительность репродуктивного периода составляла 8–14 недель, средняя — 11 недель. В многолетнем плане установлен сдвиг начала нереста на более ранние календарные сроки. Максимальные (пиковые) среднемесячные значения интенсивности нереста в 2012–2013 гг. варьировали от 33.6 до 78.7 % (среднее 51.2 %). Пик популяционного нереста приходится на июль и август; его повторяемость в июле составила 25.0 %, в августе — 66.7 %. В многолетнем плане установлен сдвиг пика нереста на более ранние сроки: в 2000–2005 гг. повторяемость пика нереста в августе составляла 100 %, в 2007–2013 гг. — только 50 %. Абсолютная численность икры (эффективность нереста) в 2000–2014 гг. изменялась от 1.6 до 29.9 экз.·м-2, составив в среднем 10.3 экз.·м-2. Условно выделены низко-, средне- и высокоурожайные годы с численностью икры менее 10, 10–20 и более 20 экз.·м-2 соответственно и со средней эффективностью нереста 5.1, 14.5 и 25.0 экз.·м-2. Эффективность нереста высокоурожайных лет в 1.7 и 4.9 раза превышает таковую средне- и низкоурожайных лет. Установлен многолетний положительный тренд эффективности нереста. В 2000–2011 гг. произошла перестройка внутривидовой структуры хамсы, которая сопровождалась перераспределением численного соотношения в составе нерестовой части популяции представителей азовского и черноморского подвидов в пользу последнего. До 2004 г. относительная численность черноморской хамсы составляла 33.3 %, после 2004 г. — 76.7 %. Одновременно произошло заметное увеличение её вылова. Коэффициент корреляции между относительной численностью черноморской хамсы и выловом составил 0.92. Календарные сроки начала нереста, его интенсивность и эффективность тесно связаны с температурой воды. Нижней температурной границей массового нереста является 17.5 °С. Область «максимально благоприятной нерестовой температуры» находится выше 23 °С. В её пределах реализуется около 2/3 репродуктивного потенциала популяции.

Anchovy (Engraulis encrasicolus) early life stages in the Central Mediterranean Sea: connectivity issues emerging among adjacent sub-areas across the Strait of Sicily

The combined use of field data on anchovy (Engraulis encrasicolus, Linnaeus, 1758) egg distribution in the Central Mediterranean Sea on both sides of the Strait of Sicily (Sicilian–Maltese and Tunisian waters) and Lagrangian simulations were used to assess the pattern of connectivity between these two sub-areas as a result of spawning activity. The field data were collected during ichthyoplankton surveys carried out in summer 2008 and 2010. The simulation runs showed considerable (up to 20%) rates of particle exchange in both directions (from Tunisian to Sicilian–Maltese waters and vice versa). However, considering the typical high mortality rates of anchovy early stages, the actual larval exchange rates across the Sicily Strait are supposed to be significantly lower (<1%), supporting the hypothesis that the anchovy population sub-units in the Strait of Sicily can be considered as separate fish stocks for the evaluation of their optimum exploitation rates.

Genetic variability of anchovy in the Azov-Black Sea basin

The intraspecific structure of the European anchovy (Engraulis encrasicolus) was studied on the basis of mitochondrial cytochrome b gene (cytb) fragment variability in 84 individuals from seven localities of the Black Sea and the Sea of Azov. The data on haplotype and nucleotide diversity and the values of neutrality tests suggested expansive growth of anchovy populations in the Azov-Black Sea basin. All samples from anchovy populations demonstrated a high level of haplotype diversity (Hd = 0.962). Two dominant haplotypes were identified, the frequencies of which were not directional, and they were present in all localities. Sequence analysis of the mitochondrial cytb gene fragment showed no differentiation between the Sea of Azov and the Black Sea subspecies.

Modeling food web effects of low sardine and anchovy abundance in the California Current

Populations of sardine, anchovy, and other forage species can fluctuate to low levels due to climate variability and fishing, leading to indirect effects on marine food webs. In the context of recent declines of sardine (Sardinops sagax) and anchovy (Engraulis mordax) in the California Current, we apply an end-to-end Atlantis ecosystem model that is spatially explicit, includes trophic interactions, and allows high and low recruitment regimes (production of juveniles). Our simulations suggest that depleted sardine populations, whether caused by fishing or natural cycles, may lead to declines in predator groups such as dolphins and large piscivorous flatfish (e.g. California halibut Paralichthys californicus). Birds exhibited more moderate declines, and California sea lions (Zalophus californianus) exhibited relatively weak declines. The Atlantis ecosystem model also predicted indirect positive effects of sardine depletion, primarily for prey species such as zooplankton. Overall our model predicted moderate declines in most predators during simulated severe declines in sardine and anchovy, illustrating the important buffering role provided by forage species other than sardine and anchovy. This ‘buffered response’ is weaker than what would be suggested by another ecosystem model (Ecosim), as predicted by diet information and a global synthesis of Ecosim models (the PREP equation). One limitation of the Atlantis model is that it did not include processes that might give rise to localized depletion of sardine at scales relevant to central place foragers, such as birds and pinnipeds. This analysis will contribute to a collaborative multi-model approach that evaluates the role of sardine in the California Current.

Transcriptome analysis deciphers evolutionary mechanisms underlying genetic differentiation between coastal and offshore anchovy populations in the Bay of Biscay

Morphometry and otolith microchemistry point to the existence of two populations of the European anchovy (Engraulis encrasicolus) in the Bay of Biscay: one in open seawaters, and a yet unidentified population in coastal waters. To test this hypothesis, we assembled a large number of samples from the region, including 587 juveniles and spawning adults from offshore and coastal waters, and 264 fish from other locations covering most of the species’ European range. These samples were genotyped for 456 exonic SNPs that provide a robust way to decipher adaptive processes in these populations. Two genetically differentiated populations of anchovy inhabit the Bay of Biscay with different population dynamics: (1) a large offshore population associated with marine waters included in the wide-shelf group, and (2) a coastal metapopulation adapted to estuarine environments in the Bay of Biscay and North Sea included in the narrow-shelf group. Transcriptome analysis identified neutral and adaptive evolutionary processes underlying differentiation between these populations. Reduced gene flow between offshore and coastal populations in the Bay of Biscay appears to result from divergence between two previously isolated gene pools adapted to contrasting habitats and now in secondary contact. Eleven molecular markers appear to mark divergent selection between the ecotypes, and a majority of these markers are associated with salinity variability. Ecotype differences at two outlier genes, TSSK6 and basigin, may hinder gamete compatibility between the ecotypes and reinforce reproductive isolation. Additionally, possible convergent evolution between offshore and coastal populations in the Bay of Biscay has been detected for the syntaxin1B-otoferlin gene system, which is involved in the control of larval buoyancy. Further study of exonic markers opens the possibility of understanding the mechanisms of adaptive divergence between European anchovy populations.

Parallel genetic divergence among coastal-marine ecotype pairs of European anchovy explained by differential introgression after secondary contact.

Ecophenotypic differentiation among replicate ecotype pairs within a species complex is often attributed to independent outcomes of parallel divergence driven by adaptation to similar environmental contrasts. However, the extent to which parallel phenotypic and genetic divergence patterns have emerged independently is increasingly questioned by population genomic studies. Here, we document the extent of genetic differentiation within and among two geographic replicates of the coastal and marine ecotypes of the European anchovy (Engraulis encrasicolus) gathered from Atlantic and Mediterranean locations. Using a genome-wide data set of RAD-derived SNPs, we show that habitat type (marine vs. coastal) is the most important component of genetic differentiation among populations of anchovy. By analysing the joint allele frequency spectrum of each coastal-marine ecotype pair, we show that genomic divergence patterns between ecotypes can be explained by a postglacial secondary contact following a long period of allopatric isolation (c. 300kyrs). We found strong support for a model including heterogeneous migration among loci, suggesting that secondary gene flow has eroded past differentiation at different rates across the genome. Markers experiencing reduced introgression exhibited strongly correlated differentiation levels among Atlantic and Mediterranean regions. These results support that partial reproductive isolation and parallel genetic differentiation among replicate pairs of anchovy ecotypes are largely due to a common divergence history prior to secondary contact. They moreover provide comprehensive insights into the origin of a surprisingly strong fine-scale genetic structuring in a high gene flow marine fish, which should improve stock management and conservation actions.

Biocomplexity in Populations of European Anchovy in the Adriatic Sea.

The sustained exploitation of marine populations requires an understanding of a species' adaptive seascape so that populations can track environmental changes from short- and long-term climate cycles and from human development. The analysis of the distributions of genetic markers among populations, together with correlates of life-history and environmental variability, can provide insights into the extent of adaptive variation. Here, we examined genetic variability among populations of mature European anchovies (n = 531) in the Adriatic (13 samples) and Tyrrhenian seas (2 samples) with neutral and putative non-neutral microsatellite loci. These genetic markers failed to confirm the occurrence of two anchovy species in the Adriatic Sea, as previously postulated. However, we found fine-scale population structure in the Adriatic, especially in northern areas, that was associated with four of the 13 environmental variables tested. Geographic gradients in sea temperature, salinity and dissolved oxygen appear to drive adaptive differences in spawning time and early larval development among populations. Resolving adaptive seascapes in Adriatic anchovies provides a means to understand mechanisms underpinning local adaptation and a basis for optimizing exploitation strategies for sustainable harvests.

No loss of genetic diversity in the exploited and recently collapsed population of Bay of Biscay anchovy (Engraulis encrasicolus, L.)

The European anchovy, Engraulis encrasicolus, in the Bay of Biscay suffered a collapse in census population size (Nc) starting in 2002, from which it did not recover until 2010. The slow recovery raised concern over sustainability, potential reduction in adaptive potential, and vulnerability to local extirpation. Long- and short-term effective population size (Ne), Ne/Nc ratio, and other genetic parameters were estimated to evaluate demographic signals of population decline. A total of 349 neutral single-nucleotide polymorphisms (SNPs) were screened in 330 anchovy individuals from the Bay of Biscay distributed across a 20-year period. We show that Nc fluctuations have not significantly affected short-term Ne, and therefore, genetic diversity has remained stable throughout the recent collapse. This study illustrates that Ne estimates should be incorporated into management plans. Our results on short-term Ne suggested that the anchovy in the Bay of Biscay has not faced any recent severe threat of losing evolutionary potential due to genetic drift. However, differences between short- and long-term Ne estimates suggested that the Bay of Biscay anchovy population may be currently much smaller than in the historical past.

Coupling Demographic and Genetic Variability from Archived Collections of European Anchovy (Engraulis encrasicolus).

It is well known that temporal fluctuations in small populations deeply influence evolutionary potential. Less well known is whether fluctuations can influence the evolutionary potentials of species with large census sizes. Here, we estimated genetic population parameters from as survey of polymorphic microsatellite DNA loci in archived otoliths from Adriatic European anchovy (Engraulis encrasicolus), a fish with large census sizes that supports numerous local fisheries. Stocks have fluctuated greatly over the past few decades, and the Adriatic fishery collapsed in 1987. Our results show a significant reduction of mean genetic parameters as a consequence of the population collapse. In addition, estimates of effective population size (Ne) are much smaller than those expected in a fishes with large population census sizes (Nc). Estimates of Ne indicate low effective population sizes, even before the population collapse. The ratio Ne/Ne ranged between 10−6 and 10−8, indicating a large discrepancy between the anchovy gene pool and population census size. Therefore, anchovy populations may be more vulnerable to fishery effort and environmental change than previously thought.

Escape responses of the Japanese anchovy Engraulis japonicus under elevated temperature and CO2 conditions

An increasing number of papers have been published on the effects of ocean acidification and warming on fishes over the last several years. However, there is little information on how these environmental changes affect the swimming behavior of fish. This study examined the escape response under elevated CO2 concentration and temperature of the Japanese anchovy Engraulis japonicus. Following acclimation to four conditions (CO2 400/1000 ppm, temperature 15/19 °C) for 1 month, the fish were tested for escape response through kinematic analysis of startle reactions to a mechanical stimulus. The response was recorded with a high speed video camera of 500 frames per second. The result showed turning rate was significantly higher at 19 °C than at 15 °C. Neither CO2 nor temperature affected the kinematic parameters analyzed (the escape trajectory, swimming velocity, acceleration, escape direction, or frequency of single and double bends), with the exception of the turning rate that was significantly higher at 19 °C than at 15 °C. However, we must clarify how future oceanic environmental changes affect escape responses of schooling fish and prey-predator interactions under more rigorous experimental conditions, to elaborate our prediction capacity for the trajectory of anchovy populations and thereby assess possible implications for anchovy fisheries.

Interannual differences in growth and hatch‐date distributions of early juvenile European anchovy in the Bay of Biscay: implications for recruitment

AbstractIn order to understand better the recruitment variability in European anchovy in the Bay of Biscay, it is important to investigate the processes that affect survival during the early life stages. Anchovy juvenile growth trajectories and hatch‐date distributions were inferred over a 3‐year period based on otolith microstructure analysis. Otolith growth trajectories showed a characteristic shape depending on their hatch‐date timing. Earlier‐born juveniles had notably broader maximum increments than later born conspecifics, resulting in higher growth rates. This observation suggests that early hatching would be beneficial for larval and juvenile growth, and, therefore, survival. The estimated juvenile hatch‐date distributions were relatively narrow compared with the extended anchovy spawning season (March–August) in the Bay of Biscay and indicated that only individuals originated mainly from the summer months (June–August) survived until autumn. Hatch‐date distributions were markedly different among years and seemed to influence the interannual recruitment strength of anchovy. We conclude that years characterized by juvenile survivors originating from the peak spawning period (May and June) would lead to considerable recruitment success. Downwelling events during the peak spawning period seem to affect larval survival. Furthermore, size‐dependent overwinter mortality would be an additional process that regulates recruitment strength in the anchovy population in the Bay of Biscay.

Hydrographic conditions driving sardine and anchovy populations in a land-locked sea

The aim of this paper is to establish a relationship between long-term variability in sardine and anchovy populations in the Adriatic Sea and ocean dynamics and processes that occur over interannual and decadal timescales in the Adriatic-Ionian basin. Basis for such analysis are annual time series of sardine and anchovy landings and recruits at age 0 and annual time series of environmental parameters observed at a representative Adriatic station between 1975 and 2010. Pearson correlations and robust Dynamic Factor Analysis (DFA) were applied to quantify the connections between fisheries and environmental parameters. Variations and trends in fisheries series were best explained by changes in near-bottom temperature and salinity, being an appropriate proxy for tracking changes in water masses' dynamics and hydrographic conditions in the basin. It seems that a prolonged period of decreasing sardine population was characterized by low oxygen availability and environmental conditions in the deep Adriatic waters, triggered by an extraordinary basin-wide event called the Eastern Mediterranean Transient. A collapse in anchovy population has been observed after an exceptional cooling event followed by dense water formation.

Last 150-year variability in Japanese anchovy (Engraulis japonicus) abundance based on the anaerobic sediments of the Yellow Sea Basin in the western North Pacific

Relatively short historical catch records show that anchovy populations have exhibited large variability over multi-decadal timescales. In order to understand the driving factors (anthropogenic and/or natural) of such variability, it is essential to develop long-term time series of the population prior to the occurrence of notable anthropogenic impact. Well-preserved fish scales in the sediments are regarded as useful indicators reflecting the fluctuations of fish populations over the last centuries. This study aims to validate the anchovy scale deposition rate as a proxy of local anchovy biomass in the Yellow Sea adjoining the western North Pacific. Our reconstructed results indicated that over the last 150 years, the population size of anchovy in the Yellow Sea has exhibited great fluctuations with periodicity of around 50 years, and the pattern of current recovery and collapse is similar to that of historical records. The pattern of large-scale population synchrony with remote ocean basins provides further evidence proving that fish population dynamics are strongly affected by global and basin-scale oceanic/climatic variability.

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).

The Pacific decadal oscillation and changes in anchovy populations in the Northwest Pacific

The Pacific decadal oscillation (PDO) has been identified as an important driver for fluctuation of fish populations on the eastern Pacific Ocean. However, the relationship between changes in fish population and climate oscillations on a decadal time scale in the Northwest Pacific is still unclear due to the lack of reliable long-term time series data. In the present study, a time series of Engraulis japonicus (Japanese anchovy) populations was reconstructed using the density of sedimentary fish scales in the Yellow Sea basin. E. japonicus populations and the PDO index showed similar patterns on a decadal time scale. Our results confirm that climate is a major driver of ecological changes on a decadal time scale, and indicate that suitable sea water temperature might be the main forcing of anchovy migration. This work also demonstrates the potential of predicting changes in fish population based on climate records.

Does spatial variation in environmental conditions affect recruitment? A study using a 3-D model of Peruvian anchovy

We used a 3-dimensional individual-based model (3-D IBM) of Peruvian anchovy to examine how spatial variation in environmental conditions affects larval and juvenile growth and survival, and recruitment. Temperature, velocity, and phytoplankton and zooplankton concentrations generated from a coupled hydrodynamic Nutrients–Phytoplankton–Zooplankton–Detritus (NPZD) model, mapped to a three dimensional rectangular grid, were used to simulate anchovy populations. The IBM simulated individuals as they progressed from eggs to recruitment at 10cm. Eggs and yolk-sac larvae were followed hourly through the processes of development, mortality, and movement (advection), and larvae and juveniles were followed daily through the processes of growth, mortality, and movement (advection plus behavior). A bioenergetics model was used to grow larvae and juveniles. The NPZD model provided prey fields which influence both food consumption rate as well as behavior mediated movement with individuals going to grids cells having optimal growth conditions. We compared predicted recruitment for monthly cohorts for 1990 through 2004 between the full 3-D IBM and a point (0-D) model that used spatially-averaged environmental conditions. The 3-D and 0-D versions generated similar interannual patterns in monthly recruitment for 1991–2004, with the 3-D results yielding consistently higher survivorship. Both versions successfully captured the very poor recruitment during the 1997–1998 El Niño event. Higher recruitment in the 3-D simulations was due to higher survival during the larval stage resulting from individuals searching for more favorable temperatures that lead to faster growth rates. The strong effect of temperature was because both model versions provided saturating food conditions for larval and juvenile anchovies. We conclude with a discussion of how explicit treatment of spatial variation affected simulated recruitment, other examples of fisheries modeling analyses that have used a similar approach to assess the influence of spatial variation, and areas for further model development.

Application of GAMs and multinomial models to assess the spawning pattern of fishes with daily spawning synchronicity: A case study in the European anchovy (Engraulis encrasicolus) in the central Mediterranean Sea

Information on the breeding behaviour of fish such as spatial segregation phenomena during spawning events is important both for understanding their reproductive ecology but also for correctly designing sampling schemes in stock assessment. The daily spawning pattern of the European anchovy, Engraulis encrasicolus, was assessed using a novel methodology that included the analysis of the hourly distribution of imminent and recent spawning stages using a combination of generalised additive models (GAMs) and of multinomial models for ordinal categorical data. To do so we used data from nine years of daily egg production method, DEPM, surveys, carried out in the Strait of Sicily. The spatial distribution and occurrence of actively spawning individuals were also analysed. Results showed that the actively spawning phase period (ASPP), i.e., the period from the onset of oocyte hydration to spawning ranged approximately 4.5h between ca. 18:30 and 23:00 while a daily spawning peak was identified at 22:30. The sex ratio was skewed during the ASPP such that females exceeded males by ca. 20% at mid ASPP. During the ASPP the proportion of Day-0 females (i.e., females that spawned or would spawn the day of sampling) decreases with increasing bottom depth while outside the ASPP Day-0 spawners did not show any particular bathymetric pattern. In the study area, anchovies exhibit segregative spawning behaviour which drives spawners to select shallower waters, similar to other anchovy populations in the Mediterranean while in the Atlantic and in other clupeoid populations the opposite bathymetric trend has been observed.

The role of environmental controls in determining sardine and anchovy population cycles in the California Current: Analysis of an end-to-end model

Sardine and anchovy are two forage species of particular interest because of their low-frequency cycles in adult abundance in boundary current regions, combined with a commercially relevant contribution to the global marine food catch. While several hypotheses have been put forth to explain decadal shifts in sardine and anchovy populations, a mechanistic basis for how the physics, biogeochemistry, and biology combine to produce patterns of synchronous variability across widely separated systems has remained elusive. The present study uses a 50-year (1959–2008) simulation of a fully coupled end-to-end ecosystem model configured for sardine and anchovy in the California Current System to investigate how environmental processes control their population dynamics. The results illustrate that slightly different temperature and diet preferences can lead to significantly different responses to environmental variability. Simulated adult population fluctuations are associated with age-1 growth (via age-2 egg production) and prey availability for anchovy, while they depend primarily on age-0 survival and temperature for sardine. The analysis also hints at potential linkages to known modes of climate variability, whereby changes in adult abundance are related to ENSO for anchovy and to the PDO for sardine. The connection to the PDO and ENSO is consistent with modes of interannual and decadal variability that would alternatively favor anchovy during years of cooler temperatures and higher prey availability, and sardine during years of warmer temperatures and lower prey availability. While the end-to-end ecosystem model provides valuable insight on potential relationships between environmental conditions and sardine and anchovy population dynamics, understanding the complex interplay, and potential lags, between the full array of processes controlling their abundances in the California Current System remains an on-going challenge.