Small Pelagic Fish Populations Research Articles

Fish shrinking, energy balance and climate change

A decline in size is increasingly recognised as a major response by ectothermic species to global warming. Mechanisms underlying this phenomenon are poorly understood but could include changes in energy balance of consumers, driven by declines in prey size coupled with increased energy demands due to warming. The sardine Sardina pilchardus is a prime example of animal shrinking, European populations of this planktivorous fish are undergoing profound decreases in body condition and adult size. This is apparently a bottom-up effect coincident with a shift towards increased reliance on smaller planktonic prey. We investigated the hypothesis that foraging on smaller prey would lead to increased rates of energy expenditure by sardines, and that such expenditures would be exacerbated by warming temperature. Using group respirometry we measured rates of energy expenditure indirectly, as oxygen uptake, by captive adult sardines offered food of two different sizes (0.2 or 1.2 mm items) when acclimated to two temperatures (16 °C or 21 °C). Energy expenditure during feeding on small items was tripled at 16 °C and doubled at 21 °C compared to large items, linked to a change in foraging mode between filter feeding on small or direct capture of large. This caused daily energy expenditure to increase by ~10 % at 16 °C and ~40 % at 21 °C on small items, compared to large items at 16 °C. These results support that declines in prey size coupled with warming could influence energy allocation towards life-history traits in wild populations. This bottom-up effect could partially explain the shrinking and declining condition of many small pelagic fish populations and may be contributing to the shrinking of other fish species throughout the marine food web. Understanding how declines in prey size can couple with warming to affect consumers is a crucial element of projecting the consequences for marine fauna of ongoing anthropogenic global change.

Climate-induced small pelagic fish blooms in an overexploited marine ecosystem of the South China Sea

Marine ecosystems are a primary conservation concern because of the separate and synergistic effects of overfishing and climate change on their productivity and biodiversity. For the purpose of implementing ecosystem-based fisheries management, it is essential to understand how fish stocks respond to climate change in marine ecosystems, especially those that have been overexploited. The Beibu Gulf in the northwestern part of the South China Sea has long been one of China’s major fishing grounds. After the collapse of the demersal ecosystem in 1990 s, small pelagic fishes, including Japanese jack mackerel (Trachurus japonicus) and Japanese scad (Decapterus maruadsi), have occupied a more-important ecological niche in this gulf. However, insufficient research on how small pelagic fish respond to climate change creates great uncertainty for implementing the current management measures. This study used fish data of trawl surveys and environmental data to investigate possible mechanisms leading to dramatic fluctuations in small pelagic fish populations and their possible effects on the Beibu Gulf marine ecosystem. Abnormal blooms of small pelagic fish, especially T. japonicus and D. maruadsi, occurred following four La Niña events (in 2007/2008, 2010/2011, 2011/2012, and 2020/2021). Moreover, the dominant fish species shifted observably and regularly, Shannon–Wiener diversity and Pielou’s evenness decreased significantly in the northeastern coastal waters of the gulf, and the spatial pattern of the fish community was reorganized in this ecosystem. The small pelagic fish blooms in summers probably resulted from a combination of climate change, overfishing, and a summer fishing moratorium, which thereby caused a series of responses in the marine ecosystem. Fluctuations in the small pelagic fish populations after La Niña events seemed more dramatic after the collapse of demersal fish stocks. Therefore, we emphasize the need for implementing ecosystem-based, preventive, and adaptive fisheries management.

Long term oscillations of Mediterranean sardine and anchovy explained by the combined effect of multiple regional and global climatic indices

It is widely known that the abundance and distribution dynamics of populations of small pelagic clupeid fish, such as sardines and anchovies, are affected by large-scale climate variability, which may lead to changeovers to new regimes of small pelagics. However, long-distance climatic oscillations, such as El Niño/La Niña and the Pacific Decadal Oscillation, have been little explored in the Western Mediterranean Sea. We investigated the possible effects of the South Oscillation Index (i.e. the atmospheric oscillation coupled with the El Niño/La Niña) and Pacific Decadal Oscillation on fluctuations in catches of European anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus) in the Western Mediterranean Sea, and their association with regional climate oscillations (i.e. the Atlantic Multidecadal Oscillation, the North Atlantic Oscillation, the Western Mediterranean Oscillation index, and the Arctic Oscillation). The study covered two periods: (a) landings between 1950 and 2016; and (b) abundance, biomass, and physical condition (i.e., relative condition index) between 2004 and 2016. The main large-scale climatic oscillations in the region were studied using General Additive Models to investigate the relationship between a time series of species measures of European sardine and anchovy from Geographical Subarea 06. Results show that the long-term Pacific Decadal Oscillation favours sardine landings, whereas the combined effect of the Western Mediterranean Oscillation Index and the Atlantic Multidecadal Oscillation favours anchovy. We discuss potential links between the present findings and changes in the plankton community caused by prevailing winds in the region driven by long-distance climate oscillations and their impact on the reduction in small pelagic fish populations in the study area.

Effects of sampling intensity and biomass levels on the precision of acoustic surveys in the Mediterranean Sea

Acoustic surveys represent the standard fishery-independent method worldwide for evaluating the biomass and spatial distribution of small pelagic fish populations. Considering the peculiarities of the spatial behaviour of pelagic fishes, the efficiency of the survey design in determining their biomass and spatial distribution is related to its ability to capture the portion of the patches accounting for larger part of the total biomass. Yet, the spatial structure of the patches could be strongly influenced by ecosystem characteristics as well as by changes in total biomass related to a density-dependent mechanism. This is of particular interest for anchovy and sardine which are known for their wide fluctuations and high sensitivity to the environment. In this study, we analysed the efficiency of acoustic surveys, targeting European anchovy (Engraulis encrasicolus) and European sardine (Sardina pilchardus), in 10 different areas of the Mediterranean Sea across three years of different biomass levels. Using the geostatistical Coefficient of Variation (CVgeo) of the average occurrence probability of high/medium density values, we showed different patterns in terms of survey design efficiency among areas and species. Anchovy usually showed lower CVgeo than sardine, but in the Alboran Sea. In 4 out of 20 cases, CVgeo values showed a consistent decrease with increasing biomass while in the remaining cases the CVgeo did not follow any clear pattern suggesting the presence of important environmental effects. Higher survey design efficiency was found in high productive sectors influenced by river run-off, letting us to hypothesize that higher productivity along with the presence of well-localized enrichment mechanisms could favour a spatially consistent distribution and coherent organization of fish population leading to higher precision estimates with a given transect design. While most surveys displayed CVgeo close to 10% or less even at low biomass, evidencing generally good performances of the survey design, a few areas exhibited higher CVgeo, yielding discussions about a potential need to decrease the inter-transect distance, always keeping in mind that survey should be as synoptic as possible.

Changes in Life History Traits of Small Pelagic Fish in the Western Mediterranean Sea

Small pelagic fish (SPF) in the western Mediterranean Sea are key elements of the marine food web and are important in terms of biomass and fisheries catches. Significant declines in biomass, landings, and changes in the age/size structure of sardine Sardina pilchardus and anchovy Engraulis encrasicolus have been observed in recent decades, particularly in the northern area of the western Mediterranean Sea. To understand the different patterns observed in SPF populations, we analyzed key life history traits [total length at age, length at maturity, gonadosomatic index (GSI), and body condition (Kn)] of sardine and anchovy collected between 2003 and 2017, from different fishing harbors distributed along a latitudinal gradient from northern to southern Spain. We used Generalized Linear Models (GLM) to estimate the length at maturity and Generalized Additive Models (GAMs) to test the relationship with environmental variables (seawater temperature, water currents, and net primary productivity). The life history traits of both species presented seasonal, interannual and latitudinal differences with a clear decline in length at age, length at first maturity, and body condition, for both species in the northern part of the study area. In the southern part, on the contrary, life history traits did not present a clear temporal trend. The environmental conditions partially explained the long-term changes in life history traits, but the selected variables differed between areas, highlighting the importance of regional oceanographic conditions to understand the dynamics of small pelagic fish. The truncated length-at-age pattern for both species with the disappearance of the larger individuals of the population could have contributed to the poor condition of small pelagic fish populations in the northern part of the western Mediterranean Sea in recent years. In the south area, recent declines in body condition for sardine and anchovy were observed and could be a possible first sign for future population declines. This study highlights the importance of understanding the trade-off between the energy invested in reproduction, maintenance and growth at seasonal and interannual level to advance our knowledge on how environmental and human pressures influence population dynamics of small pelagic fish at local and regional scales.

Zooplankton mortality effects on the plankton community of the northern Humboldt Current System: sensitivity of a regional biogeochemical model

Abstract. Small pelagic fish off the coast of Peru in the eastern tropical South Pacific (ETSP) support around 10 % of global fish catches. Their stocks fluctuate interannually due to environmental variability which can be exacerbated by fishing pressure. Because these fish are planktivorous, any change in fish abundance may directly affect the plankton and the biogeochemical system. To investigate the potential effects of variability in small pelagic fish populations on lower trophic levels, we used a coupled physical–biogeochemical model to build scenarios for the ETSP and compare these against an already-published reference simulation. The scenarios mimic changes in fish predation by either increasing or decreasing mortality of the model's large and small zooplankton compartments. The results revealed that large zooplankton was the main driver of the response of the community. Its concentration increased under low mortality conditions, and its prey, small zooplankton and large phytoplankton, decreased. The response was opposite, but weaker, in the high mortality scenarios. This asymmetric behaviour can be explained by the different ecological roles of large, omnivorous zooplankton and small zooplankton, which in the model is strictly herbivorous. The response of small zooplankton depended on the antagonistic effects of mortality changes as well as on the grazing pressure by large zooplankton. The results of this study provide a first insight into how the plankton ecosystem might respond if variations in fish populations were modelled explicitly.

Concomitant changes in the environment and small pelagic fish community of the Gulf of Lions

An important decrease in small pelagic fish condition and size has been observed in the most productive ecosystem of the Mediterranean Sea, the Gulf of Lions, since 2008, leading to an important fishery crisis. Previous studies suggested bottom-up control to be the most probable cause for these changes. Here, we investigate whether an environmental change might have caused such a situation. In the absence of zooplankton time series, this study aims at describing temporal changes in key abiotic factors for the planktonic and fish production of the Gulf of Lions, such as SST, meso-scale fronts, wind-induced coastal upwelling, river discharge, water stratification and deep convection and then at understanding potential link on Chl-a concentration as well as small pelagic fish populations. Our results indicate that the environmental conditions have broadly changed in the Gulf of Lion, with a major change in the mid-2000s, affecting the Chla concentration (which showed a regime shift in 2007), but also the SST, the upwelling and frontal activities, the Rhone river discharge (and particularly the N and P nutrients inputs) as well as the deep winter convection. Those changes could have affected the plankton production and consequently the small pelagic fish community that displayed similar patterns of variations as the environmental conditions.

Endogenous, Climate, and Fishing Influences on the Population Dynamics of Small Pelagic Fish in the Southern Humboldt Current Ecosystem

There remains a lack of holistic approaches for analyzing how different density-independent and density-dependent (endogenous) mechanisms interact to drive the dynamics of the small pelagic fish populations of the southern Humboldt Current ecosystem. In this study, we analyzed the drivers of the small pelagic fishes off the coast of Chile from the late 1980s until the early 2010s. We postulated that climate, fishing and endogenous effects drove the biomass dynamics of these populations. Per capita growth rates (R-models) were used to investigate how these factors regulated the dynamics of three anchovy populations and one population of common sardine off the Chilean coast. We found that the dynamics of the anchovy populations located off northern Chile were driven by endogenous components and by the effects of the climate, fishing and the climate-fishing interaction. We proposed that during the study period, the climate conditions favoured the population growth of the anchovies in the north; however, fishing had a negative effect on anchovy biomass, which was facilitated by the climate. The dynamics of the small pelagic fishes off central-southern Chile showed weaker endogenous effects. Indeed, the anchovy population displayed the lowest density-dependent effect, and fishing played the most significant role. The endogenous effect on the common sardine was slightly higher in comparison to that on the anchovies; however, climate (sea surface temperature) seemed to be the main driver of the flourishment in the common sardine biomass following 2006, which supported the previous hypothesis regarding the effect of climate on the species. We discussed that the R-models approach could be used to provide a holistic understanding of the drivers of the biomass dynamics of these populations. The approach provided a framework for integrating climate variability in the population dynamics of these species and moving towards an ecosystem approach to fisheries management. Further steps involve exploring the effects of competition and predation on the population dynamics of these species.

General surface circulation controls the interannual fluctuations of anchovy stock biomass in the Central Mediterranean Sea

The sustainable exploitation of small pelagic fish populations, characterized by short life span and early age at first reproduction, is typically more influenced by the success of annual recruitment rather than by fishing mortality. Recruitment strength, in turn, is related to the high environmental variability characterizing the pelagic fish habitats, able to strongly affect the survival of early stages, from hatching to recruitment. Here, we consider the case study of anchovy (Engraulis encrasicolus) stock in the Strait of Sicily (Central Mediterranean). The interannual fluctuations exhibited over an 18-year long period by this fish population was found to be mainly linked to surface circulation patterns, as far as they are able to control retention/dispersal processes of larval stages. We firstly used Lagrangian simulations to reproduce the fate of anchovy early stages during their planktonic phase. Larval retention indices constructed from the output of the simulations were able alone to explain a large proportion of variance (up to 70%) in yearly biomass of the anchovy population, outclassing the other environmental factors considered in this study. Such results are relevant for fisheries management, for all fish stocks characterized by potentially high vulnerability of early life stages.

Bonga shad (Ethmalosa fimbriata) spawning tactics in an upwelling environment

AbstractSuccessful recruitment in small pelagic fish populations inhabiting upwelling zones is subject to variation in fecundity and is driven by spatial and temporal fluctuations in environmental conditions, that is, mainly sea surface temperature, salinity and food availability. These fluctuations in abiotic factors have stimulated small pelagic fish populations to exhibit specifically adapted spawning tactics. To better understand to what extent a short‐lived exploited fish species such as bonga shad Ethmalosa fimbriata has adapted to an upwelling environment, we have investigated the interrelationship between upwelling intensity as a proxy for productivity and population fecundity by means of a virtual population analysis. We found that females of intermediate size contributed significantly more eggs to the population's fecundity than smaller or larger ones. Our model results further indicate that E. fimbriata exhibits a spawning preference at water temperatures of around 25°C and upwelling intensities of around 2.5 m3 s−1 m−1. Hence, we hypothesize that climate change‐driven increases in sea temperatures and modifications of upwelling‐favourable winds could significantly impact the species’ reproductive biology. To understand how climate change might impact fisheries, spawning tactics of small pelagic fishes are important to assess as well as their recruitment success. Such information is particularly relevant in countries where the fishery is critical at socio‐economic level, to better implement fisheries management addressing multiple stressors.

Trophic ecology of Scopoli’s shearwaters during breeding in the Zembra Archipelago (northern Tunisia)

While breeding, seabirds are central-place foragers requiring resources to sustain high-energy requirements. Therefore, during this period, they are particularly sensitive to food-resource availability, which can vary within and between years. Intra- and inter-annual variations in Scopoli’s shearwater (Calonectris diomedea) trophic ecology were investigated at its largest colony (Zembra Island, 37°07′33″N, 10°48′23″E, Mediterranean Sea). Carbon and nitrogen isotope values were analysed in the blood (adults and chicks) and feathers (adult wing) during pre-laying, incubation, and chick-rearing in 2015 and 2016 to assess variations in stable isotope composition, isotopic niches, trophic levels, and diet inferred from isotope mixing models. Scopoli’s shearwaters showed variations in isotopes throughout the breeding season and among years, with incubation showing the highest δ13C and δ15N values, trophic levels, inter-annual isotopic niche consistency, and the most specific and narrowest isotopic niche. The difference in blood δ13C values between adults and chicks suggested trophic habitat segregation: adults feed inshore, while chicks are fed more oceanic prey. Stable-isotope mixing models based on three potential prey groups revealed that the diet could consist mainly of pelagic fish and crustaceans throughout the breeding season, whereas non-pelagic fish and cephalopods could be consumed more sporadically, mainly during incubation. Feather δ15N values suggested that the adult diets contained more zooplankton in 2014. These results demonstrated the Scopoli’s shearwater trophic ecology plasticity in response to the variable nutritional demands of breeding phases and changes in prey availability. Scopoli’s shearwaters may, thus, be valuable bio-indicators of small pelagic fish populations considered critically depleted in the Mediterranean.

Properly designed effort management for highly fluctuating small pelagic fish populations: a case study in a purse seine fishery targeting chub mackerel

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 617-618:265-276 (2019) - DOI: https://doi.org/10.3354/meps12688 Properly designed effort management for highly fluctuating small pelagic fish populations: a case study in a purse seine fishery targeting chub mackerel Momoko Ichinokawa*, Hiroshi Okamura National Research Institute of Fisheries Science, Fisheries Research Agency, 2-12-4 Fukuura, Kanazawa-ku, Yokohama, Kanagawa 236-8648, Japan *Corresponding author: ichimomo@fra.affrc.go.jpAdvance View was available online September 6, 2018 ABSTRACT: Proper fisheries management is difficult in fisheries targeting small pelagic fish that are subject to climate-induced oceanic regime shifts. A favourable regime tends to induce capacity investments, but the results of these investments turn out to be irreducible overcapacity under the next unfavourable regime. The Japanese purse seine fishery studied here, which targets small pelagic fish, is a typical example of such overcapacity. However, we demonstrate that this fishery represents a best practice of properly designed effort management in recent years through its quantitative evaluation. Generalized linear mixed models (GLMMs) applied to daily logbook data revealed that effort restrictions on daily purse seine operations (in terms of the duration and the total number of operations per ship and day) significantly reduced total fishing efforts on chub mackerel. Stochastic simulations based on the parameters estimated in GLMMs quantified the extent of the potential catch reduction by the effort reductions at approximately 20%. Combined with the effect of another concurrently implemented management measure, i.e. day closures, the net effects of the effort management on the total catch reduction were estimated at 30%. Because the quota uptake percentages in this fishery were 60-80% during the study period, we conclude that the effort management helped avoid overshooting of the quota and consequent seasonal closures. KEY WORDS: Input control · Effort limitations · Race to fish · Catch shares Full text in pdf format Supplementary material PreviousNextCite this article as: Ichinokawa M, Okamura H (2019) Properly designed effort management for highly fluctuating small pelagic fish populations: a case study in a purse seine fishery targeting chub mackerel. Mar Ecol Prog Ser 617-618:265-276. https://doi.org/10.3354/meps12688 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 617-618. Online publication date: May 16, 2019 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2019 Inter-Research.

Climate variability and sardine recruitment in the California Current: A mechanistic analysis of an ecosystem model

AbstractRecruitment varies substantially in small pelagic fish populations. Understanding of the mechanisms linking environment to recruitment is essential for the effective management of fisheries resources. In this study, we used a fully coupled end‐to‐end ecosystem model to study the effect of climate variability on sardine recruitment in the California Current System during 1965–2006. Ocean variability was represented by ROMS hydrodynamic and NEMURO biogeochemical models, and sardine population dynamics was simulated through a full life cycle individual‐based model. Model analysis was designed to elucidate how changes in abiotic and biotic conditions may impact the spawning habitats, early life stage survival, and ultimately recruitment of sardine. Our findings revealed the importance of spatial processes to shape early life stages dynamics. Shifts in spawning habitats were dictated by the spatial variations in temperature and the behavioral movement of adults. Additionally, the spatial match of eggs with warmer temperatures and larvae with their prey influenced their survival. The northward shifts in spawning locations and the accomplishment of good recruitment in warmer years agreed with existing knowledge. Egg production and survival during egg and yolk‐sac larval stages were key factors to drive the long‐term variations in recruitment. Finally, our analysis provided a quantitative assessment of climate impact on year‐to‐year variation in sardine recruitment by integrating multiple hypotheses.

Spatial variability in picophytoplankton, bacteria and viruses in waters of the Great Australian Bight (southern Australia)

The Great Australian Bight (GAB) is a region of great economic and ecological importance to southern Australia, supporting fisheries of national and international importance, and large populations of small pelagic fish and iconic apex predators. There is, however, limited information on the plankton forming the base of the food web and their links to higher trophic levels in the GAB. Bacteria [inclusive of Archaea], picophytoplankton (Prochlorococcus, Synechococcus and picoeukaryotes) and viruses were investigated in the central through to the eastern GAB for the first time, across five shelf edge to offshore transects in the austral Autumn of 2013. Downwelling conditions were evident over the region prior to and during the study period. Prochlorococcus numerically dominated picophytoplankton, exceeding Synechococcus and picoeukaryotes on average, by 5-fold and 30-fold respectively. Synechococcus and picoeukaryotes showed a clear shelf edge to offshore trend with highest abundances occurring at the shelf edge. Bacterial abundances showed low variation over the region, but with highest percent of high DNA bacteria (%HDNA) occurring at the shelf edge. Viral abundances closely followed bacteria, however individual viral groups were positively correlated with picophytoplankton groups, suggesting all picoplankton groups were subject to viral infection in GAB waters. Highest total picoplankton biomass occurred in surface waters at the shelf edge (26.5 ± 1.49 µg C L−1), with picophytoplankton carbon biomass (PB) and bacterial carbon biomass (BB) each accounting for ~50% of the total picoplankton biomass in the upper 60 m of the water column. Total picoplankton biomass was ~2-fold lower offshore waters, with BB comprising the bulk of the biomass (~72%). For the upper 60 m of the water column, picoeukaryotes accounted for ~51% of PB at the shelf edge, compared to ~24% in offshore waters. Based on our estimated C:Chl a ratios, picoeukaryotes accounted for 32.7 ± 3.0% of Chl a at the shelf edge, compared to 9.1 ± 0.6% in offshore waters. Prochlorococcus dominated PB in offshore waters (~67%), accounting for ~28% of Chl a. Multivariate analyses showed that the best predictors of the picoplankton community were a combination of nitrate and salinity (ρ = 0.704), indicating strong environmental forcing of the picophytoplankton community across the GAB. The elevated PB at the shelf edge, due to the dominance of picoeukaryotes, was a regional feature of the central and eastern GAB during the time of this study. We hypothesize that picoeukaryotes play a critical role in supporting higher trophic levels in GAB shelf edge waters. This study highlights the importance of small plankton in food web dynamics in GAB waters.

Predator–prey interactions in the face of management regulations: changes in Mediterranean small pelagic species are not due to increased tuna predation

Recently, the abundance of young Atlantic bluefin tuna (Thunnus thynnus) tripled in the northwestern Mediterranean following effective management measures. We investigated whether its predation on sardine (Sardina pilchardus) and anchovy (Engraulis encrasicolus) could explain their concurrent size and biomass decline, which caused a fishery crisis. Combining the observed diet composition of bluefin tuna, their modelled daily energy requirements, their population size, and the abundance of prey species in the area, we calculated the proportion of the prey populations that were consumed by bluefin tuna annually over 2011–2013. To assess whether tuna could alter the size structure of the three small pelagic fish populations (anchovy, sardine, and sprat (Sprattus sprattus)), the size distributions of the consumed prey species were compared with those of the wild populations. We estimated that the annual consumption of small pelagic fish by bluefin tuna is less than 2% of the abundance of these populations. Furthermore, size selectivity patterns were not observed. We thus concluded that tuna predation is unlikely to be the main cause of major changes in the small pelagic fish populations from this area.

Improved management of small pelagic fisheries through seasonal climate prediction.

Populations of small pelagic fish are strongly influenced by climate. The inability of managers to anticipate environment-driven fluctuations in stock productivity or distribution can lead to overfishing and stock collapses, inflexible management regulations inducing shifts in the functional response to human predators, lost opportunities to harvest populations, bankruptcies in the fishing industry, and loss of resilience in the human food supply. Recent advances in dynamical global climate prediction systems allow for sea surface temperature (SST) anomaly predictions at a seasonal scale over many shelf ecosystems. Here we assess the utility of SST predictions at this "fishery relevant" scale to inform management, using Pacific sardine as a case study. The value of SST anomaly predictions to management was quantified under four harvest guidelines (HGs) differing in their level of integration of SST data and predictions. The HG that incorporated stock biomass forecasts informed by skillful SST predictions led to increases in stock biomass and yield, and reductions in the probability of yield and biomass falling below socioeconomic or ecologically acceptable levels. However, to mitigate the risk of collapse in the event of an erroneous forecast, it was important to combine such forecast-informed harvest controls with additional harvest restrictions at low biomass.

Spatio-temporal patterns and environmental controls of small pelagic fish body condition from contrasted Mediterranean areas

Small pelagic fish are among the most ecologically and economically important marine fish species and are characterized by large fluctuations all over the world. In the Mediterranean Sea, low catches and biomass of anchovies and sardines have been described in some areas during the last decade, resulting in important fisheries crises. Therefore, we studied anchovy and sardine body condition variability, a key index of population health and its response to environmental and anthropogenic changes. Wide temporal and spatial patterns were investigated by analyzing separately data from scientific surveys and fisheries in eight Mediterranean areas between 1975 and 2015.Results showed that anchovy and sardine body condition as well as maximum size in some areas sharply decreased in most Mediterranean areas along years (except in the Northern Alboran Sea). Despite this general pattern, well-marked environmental differences between sub-regions were highlighted by several analyses and variations in body condition were not found to be homogeneous over all the Mediterranean Sea. Further, other analyses revealed that except for the Adriatic where major changes towards a lower body condition were concomitant with a decrease in river runoffs and chl-a concentration, no concomitant environmental regime shift was detected in other areas.Together, these analyses highlighted the current poor body condition of almost all small pelagic fish populations in the Mediterranean. Yet, global environmental indices could not explain the observed changes and the general decrease in condition might more likely come from regional environmental and/or anthropogenic (fishing) effects. A prolonged state of poor fish body condition, together with an observed reduced size and early age-at-maturity may have strong ecological, economic and social consequences all around the Mediterranean Sea.

Reprint of “Atlantic Multidecadal Oscillation (AMO) modulates dynamics of small pelagic fishes and ecosystem regime shifts in the eastern North and Central Atlantic”

Dynamics of abundance and migrations of populations of small pelagic clupeoid fish such as anchovy (Engraulis encrasicolus), sardine (Sardina pilchardus), sardinella (Sardinella aurita), sprat (Sprattus sprattus) and herring (Clupea harengus) in the eastern North and Central Atlantic between Senegal and Norway vary in synchrony with the warm and cool phases of the Atlantic Multidecadal Oscillation (AMO). This is shown by compiling retrospective data on fish catches and anecdotal observations, which in some cases date back to the mid-19th century. The AMO is defined as the de-trended mean of North Atlantic (0–60°N) sea surface temperature anomalies. However, it is not primarily the temperature which drives the dynamics of the small pelagic fish populations. Instead, the AMO seems to be a proxy for complex processes in the coupled atmosphere–ocean system of the North Atlantic. This is manifested in large-scale changes in strength and direction of the current system that move water masses around the North Atlantic and likely involves the North Atlantic Oscillation (NAO), the Atlantic Meridional Overturning Circulation (AMOC), the Mediterranean Overflow Water (MOW) and the subpolar gyre (SPG). The contractions and expansions of the SPG apparently play a key role. This was particularly obvious in the mid-1990s, when the SPG abruptly contracted with the result that warm subtropical water masses moved to the north and east. Small pelagic fish populations in the eastern North and Central Atlantic, including those in the Mediterranean responded quickly by changing abundances and migrating northwards. It seems that the complex ocean–atmosphere changes in the mid-1990s, which are described in the text in detail, caused a regime shift in the ecosystems of the eastern North and Central Atlantic and the small pelagic clupeoid fish populations are the sentinels of this shift.

Atlantic Multidecadal Oscillation (AMO) modulates dynamics of small pelagic fishes and ecosystem regime shifts in the eastern North and Central Atlantic

Dynamics of abundance and migrations of populations of small pelagic clupeoid fish such as anchovy (Engraulis encrasicolus), sardine (Sardina pilchardus), sardinella (Sardinella aurita), sprat (Sprattus sprattus) and herring (Clupea harengus) in the eastern North and Central Atlantic between Senegal and Norway vary in synchrony with the warm and cool phases of the Atlantic Multidecadal Oscillation (AMO). This is shown by compiling retrospective data on fish catches and anecdotal observations, which in some cases date back to the mid-19th century. The AMO is defined as the de-trended mean of North Atlantic (0–60°N) sea surface temperature anomalies. However, it is not primarily the temperature which drives the dynamics of the small pelagic fish populations. Instead, the AMO seems to be a proxy for complex processes in the coupled atmosphere–ocean system of the North Atlantic. This is manifested in large-scale changes in strength and direction of the current system that move water masses around the North Atlantic and likely involves the North Atlantic Oscillation (NAO), the Atlantic Meridional Overturning Circulation (AMOC), the Mediterranean Overflow Water (MOW) and the subpolar gyre (SPG). The contractions and expansions of the SPG apparently play a key role. This was particularly obvious in the mid-1990s, when the SPG abruptly contracted with the result that warm subtropical water masses moved to the north and east. Small pelagic fish populations in the eastern North and Central Atlantic, including those in the Mediterranean responded quickly by changing abundances and migrating northwards. It seems that the complex ocean–atmosphere changes in the mid-1990s, which are described in the text in detail, caused a regime shift in the ecosystems of the eastern North and Central Atlantic and the small pelagic clupeoid fish populations are the sentinels of this shift.

Changes in egg buoyancy during development and its effects on the vertical distribution of anchovy eggs

Small pelagic fish populations exhibit reproductive strategies resulting from past natural selection pressure, by which certain traits become more or less common in a population, allowing them to adapt and become better suited to certain habitats. One such adaptation is the buoyancy of eggs, which is observed as density changes during development. This is an important issue in fisheries and modeling science, as it affects the vertical distribution of eggs and, therefore, egg transport. Recently, individual-based models for anchovies in the Mediterranean have focused on developing adequate biological algorithms to simulate realistic spatial variations of eggs and larvae. Some models that include movements of particles according to Stokes’ law also assume a constant value of egg density during egg development. However, field observations show differences in the vertical distribution of eggs when egg density during development is considered. We address the problem of egg density and its vertical distribution within a biological context. In Mediterranean waters, the incubation time for anchovy eggs during peak spawning is approximately 48–70h; during these first hours, egg density has an influence on the horizontal and vertical trajectories of eggs, as well as their routes and hatching zones. In this study, we introduce an algorithm describing the egg density of European anchovy eggs throughout development. Egg density measurements were carried out in a density gradient column (DGC). We fitted a polynomial model that estimated egg density, as a function of time from fertilization and that was dependent on temperature. Simulations to study the vertical transport of eggs in the Mediterranean were carried out using ICHTHYOP/MARS3D. The vertical distribution of pelagic eggs was determined by a set of interacting biological and physical parameters related to eggs (density, diameter) and ambient seawater (density, viscosity, turbulence), respectively. The egg buoyancy model introduced here was validated and will provide insight for the design of anchovy egg surveys, as the vertical position of the eggs in the water column during development can be inferred by the hydrographic structure of seawater.