Sardine Catches Research Articles

Fisheries shocks provide an opportunity to reveal multiple recruitment sources of sardine in the Sea of Japan

The abrupt decline in sardine catches in the Sea of Japan and the East China Sea (SJ-ECS) in 2014 and 2019 and the recovery in the following years call into question the current assumption that sardines in the SJ-ECS form a self-recruiting subpopulation. To test this hypothesis, we analysed otolith stable oxygen and carbon isotope profiles (δ18O, δ13C) of age-0 and age-1 sardines from 2010 and 2013–2015 year-classes captured in the SJ-ECS, as geographic markers for nursery areas. Age-0 sardines generally showed a significant ontogenetic decrease in otolith δ18O from larval to juvenile stages. However, the majority of age-1 captured in spring 2011, 2015 and 2016 showed non-decreasing otolith δ18O profiles, suggesting that the age-0 off the Japanese coast were not the main source of recruitment. Different migration groups were thus indicated: the “locals” growing up off the Japanese coast and the migrating “nonlocals”. The isotope profiles of the “nonlocals” overlapped with those of age-0 captured in the subarctic North Pacific, suggesting that they may be migrants from the Pacific, or perhaps an unobserved northward migration group in the SJ-ECS. Our results highlight the considerable uncertainty in the population structure assumed in current stock assessment models for Japanese sardine.

Towards sustainable exploitation of European pilchard (Sardina pilchardus) stocks off West African waters

European pilchard (Sardina pilchardus), herein referred to as “sardine” or “European sardine”, is the most productive and economically important small pelagic fish in the Northwest African waters. Thus far, very little is known about its stock status in the West African region despite the rapid decadal increase in its catch production. The present study provides avenues vital to improving guidance for sustainable species management. In this region, sardine is divided administratively into three stocks (northern: stock N, central: stock A + B, and southern: stock C). Thus, the present study used two Surplus Production Models (SPM) to evaluate sardine's stock status. Sardine's catch and scientific survey biomass data obtained from FAO-CECAF's (Fishery Committee for the Eastern Central Atlantic (CECAF)) latest reports were used in this analysis to fit the SPMs and investigate the effectiveness of the available catch-per-unit-effort (CPUE) trends in setting the total allowable catch (TAC) for sustaining the different sardine stocks in the region. Results from the SPMs indicate that the sardine stock in the north is fished at optimum levels. Meanwhile, the SPMs output based on the scientific survey biomass indices indicated a healthy state for the central and southern sardine stocks. Similarly, the fishery CPUE indices for the central and southern stocks showed similar biomass trends compared to the survey biomass index, indicating healthy stock status. Therefore, in cases where survey biomass index data is unavailable to run stock assessments, this study suggests that sardine fishery CPUEs could be used in SPMs to monitor biomass trends over time and may also provide stock status proxies. Additionally, with rapid climate and other environmental changes, sardine stocks must be regularly monitored with consistent and comparable scientific methodologies and updated fisheries time series data.

Ichthyocenosis of the upper epipelagic layer in the Pacific waters of Russia during japanese sar-dine «renaissance» in the beginning of the 21<sup>st</sup> century

Dynamics of integral parameters for the species structure in ichthyocenosis of the upper epipelagic layer in the Pacific waters off the Kuril Islands in 2019–2023 is discussed in compare with previously published data since 2004 in context of a new bloom of japanese sardine Sardinops melanostictus. Results of trawl surveys are clustered using the method of multi-dimensional scaling. Species composition in the trawl catches in 2004–2013, before the time of mass expansion of subtropi-cal migrants, as japanese sardine and chub mackerel Scomber japonicus, was distinguished by domina-tion of mesopelagic fishes of families Myctophidae and Microstomatidae and pink salmon Oncorhynchus gorbuscha. From 2014 to the present, the dominant positions in the ichthyocenosis were held by japanese sardine and chub mackerel, with a decreasing role of the latter. In the last pentade (2019–2023), the species structure of pelagic ichthyocenosis in the Pacific waters of Russia has not undergone fundamental changes. At the same time, the tendency has increased for uneven distribution of biomass between the species of fish community in favor of japanese sardine. The polydominance index decreased significantly (P < 0.05, t-test 4.33) from 4.85 ± 0.44 in 2004–2013 to 2.37 ± 0.21 in 2019–2022, and the index of evenness of species structure decreased from 0.41 ± 0.04 to 0.15 ± 0.02 (P < 0.05, t-criterion 5.25). Growth of domestic and foreign catches of japanese sardine in the second decade of the 21 century is a sign of this species bloom beginning.

Bayesian joint longitudinal models for assessing the exploitation rates of sardine stock in the Mediterranean Sea

AbstractThe European sardine is a pelagic species of great ecological importance for the conservation of the Mediterranean Sea as well as economic importance for the Mediterranean countries. Its fishing has suffered a significant decline in recent years due to various economic, cultural and ecological reasons. This paper focuses on the evolution of sardine catches in the Mediterranean Sea from 1985 to 2018 according to the fishing Mediterranean country and the type of fishing practised, artisanal and industrial. We propose three Bayesian longitudinal linear mixed models to assess differences in the temporal evolution of artisanal and industrial fisheries between and within countries. Overall results confirm that Mediterranean fishery time series are highly diverse along their dynamics and this heterogeneity is persistent throughout the time. Furthermore, our results highlight a positive correlation between artisanal and industrial fishing. Finally, the study observes a consistent decreasing time trend in the quantity of fish landings. Although the causes of this feature could be also linked to economic motivations (such as a reduction in demand or the reorientation of fleets towards more commercially beneficial species), it may indicate a potential risk to the stock of this species in the Mediterranean Sea.

Combining output control and fishing pressure limitations improves the management of the Japanese sardine Sardinops melanostictus

The stock biomass of the Japanese sardine Sardinops melanostictus in the Pacific Ocean shows large fluctuations in response to environmental variations and fishing activities; this is reflected in annual catch amounts. Harvest policy in Japan determines the current allowable biological catch (ABC) by using catch data and marine resource indices. The total allowable catch (TAC) for sardine is based on the ABC, but there is a time delay from the collection and analysis of data to its implementation. We have developed a new harvest control rule (HCR) into which fishing pressure limitations are incorporated. The rule calls for fishing to stop when fish landings reach the TAC or the fishing effort and the fishery catchability reach the limit. We have evaluated the performance of the proposed HCR against the implementation error for fishing pressure and the observation error for fishing mortality. We conducted simulations using two stock recruitment relationships and three historic starting years to investigate the effects of rule initiation timing. The results indicate that this HCR could help to facilitate a recovery of the stock biomass and improve annual catches of the Japanese sardine. Especially when the observation error is large, the HCR is effective in maintaining stock biomass, increasing the average annual catch, and stabilizing the overall annual catch.

From west to east: Heterogeneity in the life history traits of a small pelagic fish (Sardina pilchardus) throughout the Mediterranean

Small pelagic fish are key elements of marine trophic networks and are of great importance to the total landing of marine species. Over the last decades, a decrease in biomass and catches of European sardine has been described, especially in the Mediterranean Sea, as an indication of a drop in stock condition. Multiple causes, including direct and indirect anthropogenic factors, seem to affect this ecologically and economically important species. To address this issue, it is important to identify the current variability in sardine status and understand the strategies for energy allocation, which are intimately related to reproductive potential and recruitment. We analyzed the somatic condition through tissue and mesenteric lipid measurements, relative condition (Kn) and hepatosomatic (HSI) indices, and reproductive condition with the gonadosomatic index (GSI) in four subareas of the Mediterranean (Northern Alboran, Northern Spain, the Northern Adriatic, and the Aegean Sea) and an outgroup from the Atlantic. The analyses were performed within the gonadal development cycle since translocation to reproduction may mask the state of actual energy reserves for comparison. The results revealed marked differences in health status among subareas, highlighting the low condition of Northern Spain sardines throughout the annual cycle. The uniform condition throughout gonadal development in the Northern Adriatic suggests that resource availability modulates the reproductive strategy since in this locality, sardines would have high dependence on immediate reserves, not behaving as strict capital breeders. Moreover, similarities between Alboran and Atlantic stocks are discussed, highlighting the marked energy replenishment of the latter after reproduction compared to the Mediterranean stocks. The Aegean stock was the fastest to reach its maximum after spawning, presenting low condition values from the developing to actively spawning period, but with the greatest recovery at the regressing phase, coinciding with a peak in productivity. Finally, the role of the liver in sardine energy storage is analyzed considering stock heterogeneity, potentially indicating that HSI is intimately related to feeding activity. Given the divergences among stocks, we propose concrete measures for managing this resource (i.e., the need for adapting the close seasons), which could be applied to other species in similar contexts.

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.

Coupling oceanic mesoscale events with catches of the Pacific sardine (Sardinops sagax) in the Gulf of California

Adult fish abundance depends on survival of early life stages, which are susceptible -among other factors- to environmental variability. Ocean mesoscale events (i.e. eddies, costal upwelling, thermal fronts, cold filaments) can be mechanisms of enrichment, concentration, retention and dispersal in marine environments with effects on survival of marine species early life stages. The Pacific sardine (Sardinops sagax) is an important fishery resource in the Gulf of California (GC), and it can rapidly respond to environmental fluctuations leading to dramatic changes in its abundance and fishery availability. This research analyzed the relationship of frequency and duration of different mesoscale processes with catches of S. sagax in the GC. Generalized additive models were applied to evaluate the effects of mesoscale events on the reproduction period, assuming they would reflect on catches several months later. The results show that eddies mostly affected the seasons with anomalous catches, where optimal duration of both types of eddies were responsible for the significantly high catches, while very high cyclonic and very low anticyclonic eddy duration led to significantly low catches. During the seasons with mean catches, high frequency of coastal upwelling and low persistence of Tropical surface sea water (TSW) northward intrusion were responsible for higher catches, while lower ones were associated to higher persistence of TSW intrusion and lower coastal upwelling frequency. A decreasing trend of coastal upwelling and filaments was detected, while the opposite occurred with TSW intrusion. These aspects should be considered in climate change scenarios because if sea surface temperature continues to increase, its implications in mesoscale processes could have consequences in the Gulf of California trophic dynamics.

Knowledge on the Biological and Fisheries Aspects of the Japanese Sardine, Sardinops melanostictus (Schlegel, 1846)

Japanese sardine (Sardinops melanostictus) is a significant small pelagic fish and a valuable resource that plays an essential ecological role in the marine ecosystem. It is present in the far Eastern Asian maritime waters, including the Pacific Ocean, Sea of Japan, and the East China Sea. Encircling nets, particularly purse seines, are the most used fishing equipment to catch this species. Their fishing grounds are located entirely in coastal areas. Japanese sardine catches have shown varying trends over the last five decades, with a high frequency of captures occurring in the 1980s before collapsing in the early 1990s. The economic and ecological importance of this species has prompted much research, which provided additional information about their spawning migration, distribution, fisheries, and biology. This research was mostly undertaken in the Sea of Japan and its adjacent waters spanning in the north Pacific Ocean. Despite all this research and the importance of this species in its habitats and in commercial fisheries, there is a lack of a recent review presenting the status of global fisheries and biological information for this species. This paper summarizes and updates information on the global geographical distribution, biological aspects, trends in catches, stock fluctuations and assessment, and management measures of the Japanese sardine population. This paper also summarizes information related to the influence of environmental factors on the occurrence of this species and also identifies information gaps. Further research directions are also discussed in this work, which may help improve the knowledge of Japanese sardine and establish rational management measures for their conservation.

Effect of a Seasonal Fishery Closure on Sardine and Mackerel Catch in the Visayan Sea, Philippines

The implementation of seasonal fishery closures (SFC) can be controversial due to the frequent lack of clear objectives, monitoring and empirical evidence of management success. In the Philippines, an SFC implemented for the conservation of important fishery commodities in the Visayan Sea has been ruled a success after stricter implementation of this fishery policy in 2012. However, a comprehensive, detailed, and robust analysis of this fishery policy is lacking. Using a difference-in-differences (DID) framework, we estimated the effect of SFC on the interannual and seasonal catch for sardine and mackerel. We expanded our analysis to other species not regulated under the SFC policy. We also conducted semi-structured interviews (N= 235), focus group discussions (N= 9) and key informant interviews (N= 37) involving municipal fisheries stakeholders in the surrounding municipalities around the Visayan Sea, and representatives from the government and non-government agencies, to complement our analyses. Seasonal analyses of catch data show a significant increase in sardine catch at the end of the seasonal closure among SFC-participating provinces. However, overall, the SFC had no significant effect on sardine interannual catch among the provinces participating in the SFC. We also found no significant effect of the SFC on interannual and seasonal catch for mackerel. Furthermore, our findings show no significant changes in fishing pressure to other aquatic species. Interview results corroborate our DID findings for mackerel, but not for sardine. The varying perceptions on the outcomes of the SFC policy can be attributed to several challenges such as lack of implementing guidelines, lack of alternative livelihoods for the affected stakeholders, persistence of illegal fishing, and uneven implementation of the SFC. Since the management objective of this SFC was to conserve the regulated species, alternative management measures may be needed to achieve this goal. This could entail more consistent enforcement, improved cooperation and communication between fisheries managers and stakeholders, fish size or gear restrictions, and identification and conservation of key habitats needed to restore overexploited species.

Effect of environmental variables on the number of births at California sea lion (Zalophus californianus) rookeries throughout the Gulf of California, Mexico

Abstract The 13 California sea lion breeding colonies in the Gulf of California (GoC), Mexico are each characterized by different population growth trends, including a variable number of births. Despite being located in a highly productive marine area, both the species’ population and the number of births have declined significantly over the past few decades. The goal of this study was to assess the effects of certain environmental variables on the number of births at colonies in three regions (northern, central and southern) of the GoC (1995–2018). Data on diet, small pelagic fish catches by fisheries, the density of small pelagic fish schools, remotely sampled values of sea surface temperature and chlorophyll‐a concentration and El Niño Southern Oscillation events were collected in order to evaluate whether these variables had an effect on the number of births using correlations and generalized linear models. No significant relationships were found between the environmental variables and the number of births. However, the number of births decreased when the sea surface temperature anomalies exceeded 1°C in the northern and central regions of the GoC. Although no significant relationships were found between the small pelagic fish catches and the number of births, positive trends were observed between sardine catches and the number of births during certain periods in the central region. A positive correlation between the density of small pelagic fish schools and the number of births suggests a possible relationship between prey availability and reproductive success in the central region. A Programme of Action for the Conservation of the Species should be promoted by the Federal Government and carried out in order to implement specific management actions. Specifically, surveillance efforts should increase, fishing gear conversion plans should be established and sea lion exclusion devices should be introduced, among other measures.

Environmental controls on marine productivity near Cape St. Francis, South Africa

Abstract. This study considers ocean–atmosphere influences on marine productivity over the shelf near Cape St. Francis, South Africa. Multiday estimates of chlorophyll fluorescence in the period 2006–2017 with an area outlined by 34.5–33.75∘ S and 24–26.5∘ E provide the basis for evaluation using data from high-resolution reanalyses. Correlations with the mean annual cycle of chlorophyll fluorescence were significant for salinity, linking marine productivity and the coastal hydrology. A strengthened Agulhas Current induces cyclonic shear that lifts water at the shelf edge. Composite high-chlorophyll-fluorescence events were dominated by a large-scale midlatitude atmospheric ridge of high pressure. The resultant easterly winds caused offshore transport and the upwelling of cool nutrient-rich water in multiday events at the beginning and end of austral summer. Environmental controls on interannual fluctuations of the commercial fishery were also explored. Southwestward currents and diminished heat fluxes favored the squid catch, while anchovy and sardine catches were linked with upper northerly wind, consistent with large-scale weather patterns that underpin coastal upwelling and river discharge. Productivity lags a few days behind cyclonic wind and current shear and the upstream coastal hydrology, which shares a common atmospheric driver.

Análisis de la serie de captura de sardina Monterrey, Sardinops sagax en el Golfo de California, México

Background: The Pacific sardine is a small pelagic fish which supports the most important fishery at national scale. Goals: This paper analyzes the catch information generated by the commercial fleet operating in the Gulf of California from 1969 to 2016 in order to identify the catch trends and the presence of the harmonic components that is related to environmental variability. Methods: Fast Fourier Transformation (FFT) was applied to the time series in order to establish the dominant frequencies. The Singular Spectrum Analysis (SSA) method was used to separate the trend and harmonic components. Each harmonic component was correlated to the Southern Oscillation Index (SOI), the Pacific Decadal Oscillation (PDO) and the Solar Activity data. Results: The FFT showed dominant frequencies at 7 and 11 years. An increasing linear trend was identified from 1969 to 2008 with a decreasing linear segment from 2009 to 2016, as well as two harmonic components of different frequencies (7 and 11 years respectively). The trend and the two harmonic components explain 87.49% of the total variance of the capture. The first and second harmonic components correlated positively with SOI (r= 0.2921, r= 0.3870 respectively), while the second harmonic component correlated negatively with solar activity (r= -0.5313). There were no correlations with the PDO. Conclusions: The fluctuations of Pacific sardine in the Gulf of California are mainly due to three factors: increased catch, the influence of the El Niño events and the influence of the solar activity cycle.  

Seasonal and Interannual Variations of Sardine Catches along the Omani Coast

Seasonal and Interannual Variations of Sardine Catches along the Omani Coast

An overview of small-scale fisheries in the Northern Portuguese coast

An overview of small-scale fisheries in the Northern Portuguese coast

Catch and yield trends of the Portuguese purse seine fishery (2006-2018)

Catch and yield trends of the Portuguese purse seine fishery (2006-2018)

Contribution of ocean variability to climate-catch models of Pacific sardine

Small-pelagic fishes are economically and ecologically important as they make up a large proportion of wild-caught fisheries and provide an important link between planktonic organisms and higher trophic level organisms such as piscivorous fishes, squid, seabirds and marine mammals. In the California Current Ecosystem (CCE), the distribution and abundance of Pacific sardine (Sardinops sagax) is influenced by both natural climate variability and exploitation. This study uses Generalized Additive Models to evaluate the effects of environmental forcing at multiple spatial scales (i.e. local, regional and basin) on sardine catch recorded at British Columbia (CAN), California (USA) and Bahía Magdalena (MEX). We find that the Pacific Circulation Index, sea surface temperature, upwelling strength, and primary productivity were the most influential factors explaining variability of sardine catch in the northeastern Pacific Ocean. Our CCE-wide analysis shows that regional scale environmental variability best explained catch rates of the Pacific sardine for California and Bahía Magdalena while basin-scale environmental variability did so for catch rates for British Columbia.

Role of physical processes in determining the nature of fisheries in the eastern Arabian Sea

We use available data and model simulations to make a case for physical forcing of the spatial variation in the nature of fisheries off the west coast of India in the eastern Arabian Sea (EAS). The approach is heuristic, but builds the case on the basis of model simulations and well-established implications of specific physical processes for the marine ecosystem. We address three questions.First, we show that it is differences in the physical forcing lead to a carnivore-dominated fishery in the northeastern Arabian Sea (NEAS) and a planktivore-dominated fishery in the southeastern Arabian Sea (SEAS). In the NEAS, the growing season for phytoplankton is longer because the upwelling during the summer monsoon (June–September) is followed by convective mixing during the winter (November–February) monsoon. Detrainment, again leading to a phytoplankton bloom, follows the convective-mixing season. The long duration of the growing season permits the existence of a longer food chain, leading to a carnivore-dominated fishery. In the NEAS, the phytoplankton also tend to be small owing to weaker upwelling during the summer monsoon and limitations imposed by silicates, turbulence, and light during the winter monsoon. In the SEAS, the stronger upwelling implies larger phytoplankton that can be directly fed on by fish like the oil sardine. The growing season in the SEAS is also limited to the summer monsoon, predisposing the ecosystem towards a shorter food chain.Second, we show that it is the differences in physical forcing that lead to the weaker fishery in the central-eastern Arabian Sea (CEAS) compared to the SEAS. Though the growing season in both SEAS and CEAS is comparable, the upwelling is weaker in the CEAS, implying a limitation of nitrate and a dominance of small phytoplankton in contrast to the SEAS, where large phytoplankton dominate. Hence, the phytoplankton biomass is less in the CEAS compared to the SEAS and the region cannot support a fishery comparable to the SEAS.Third, we show that it is the difference in physical forcing that leads to a sharp decline in the catch of oil sardines from the SEAS to the CEAS even as the catch of mackerel does not change much. Not only does the stronger upwelling in the SEAS compared to the CEAS imply a larger size of the phytoplankton in the former, but the phytoplankton biomass is also higher in the SEAS. The zooplankton standing stock, as estimated using the backscatter measured by the acoustic Doppler current profilers (ADCPs) deployed on moorings, is determined more by the vertical movement of the depth of the 20 °C isotherm (D20). Hence, the standing stock of zooplankton tends to be lowest when upwelling peaks, i.e., during the summer monsoon. This is also the time when the phytoplankton biomass peaks. The phytoplankton surplus is greater in the SEAS compared to the CEAS because of the stronger upwelling, which implies higher primary productivity and a shallower D20, in the SEAS. This phytoplankton surplus can be fed on by the oil sardine, which prefers diatoms. In contrast, the higher zooplankton standing stock in the CEAS competes with the oil sardine off Karnataka and Goa, where the mackerel, which feed on zooplankton, is the dominant fishery. The weaker upwelling implies lower primary productivity and a deeper D20 (higher zooplankton standing stock) in the CEAS.This limitation of the mature habitat by the physical forcing and the seasonal reversal of the currents limits the spawning regimes of these fish because the carnivorous (planktivorous) fish like the Bombay Duck (oil sardine) can spawn only in those regions from where they can make it to their mature habitat in the NEAS (SEAS).

CAPTURAS DA SARDINHA-VERDADEIRA, Sardinella brasiliensis (STEINDACHNER)(OSTEICHTHYES: CLUPEIDAE) E DA FAUNA ACOMPANHANTE, NO ESTADO DO RIO DE JANEIRO (BRASIL)

Este trabalho trata das capturas da sardinha-verdadeira, Sardinella brasiliensis (Steindachner) e da fauna acompanhante, ao longo da costa do estado do Rio de Janeiro (Brasil), com base nos desembarques registrados em mapas-de-bordo no período de 1993-1997. Os índices de abundância média da sardinha-verdadeira e da fauna acompanhante fora 6,1 t/lance e de 2,2 t/lance, determinando uma razão de captura igual a 1,0:0,4. O ciclo anual das capturas compreende períodos de maior produção e abundância nos meses de abril-junho (outono) e de menor produção e abundância nos meses de julho-setembro (inverno). Há um certo paralelismo entre os ciclos de produção e abundância da sardinha-verdadeira e de sua fauna acompanhante. Nos cardumes, as principais espécies associadas são savelha, Brevoortia aurea (Spix), sardinha-boca-torta, Centengraulis edentulus (Cuvier) e sardinha-laje, Opisthonema oglinum (Le Sueur). Os predadores mais importantes são cavalinha, Scomber japonicus (Houttuyn), xerelete, Carangoides crysos (Mitchill) e bonito-pintado, Euthynnus alletteratus (Rafinesque). O bonito-barriga-listrada, Katsuwonus pelamis (Linnaeus), é um importante predador da sardinha-verdadeira, as escapa dos cercos das traineiras.

Feeding ecology of the common dolphin (Delphinus delphis) in Western Iberian waters: has the decline in sardine (Sardina pilchardus) affected dolphin diet?

Predator diet is expected to respond to changes in the abundance of important prey items. We investigated whether common dolphin diet has changed with changes in the pelagic fish community off the Iberian Peninsula in recent years, in particular with reference to the decline of sardine (Sardina pilchardus) and the increase of Atlantic chub mackerel (Scomber colias) and scads (Trachurus spp.), and estimated how much fish was removed by common dolphins in coastal waters of mainland Portugal. Based on analysis of stomach contents of stranded animals from 2010 to 2013 (n = 150), we related diet composition to sardine abundance, dolphin size and sex, season and region. Despite the decline in sardine stocks in recent years, sardines were the most important prey of common dolphins, as was previously reported in the 1990s, followed by chub mackerel and scads. However, small sardines have disappeared from the diet, consistent with recent poor recruitment to the stock. The relative importance of these pelagic species in the diet increased significantly with increasing dolphin length. The dietary importance of chub mackerel also varied regionally, with higher intake on the southern coast. Finally, we compared the estimated biomass of fish removed by dolphins with that taken by fisheries. We found that even with the severe quota restrictions on sardine catches, total biomass removed by fisheries was over five times greater than the estimated removal by common dolphins. Our results confirm the previously reported preference for energy-rich prey, especially sardine, suggesting that common dolphins in Portuguese waters specialise in feeding on this species.