Interannual Variability In Abundance Research Articles

Dynamics of an upland stream fish community over 40 years: trajectories and support for the loose equilibrium concept

Previous theoretical models and empirical studies suggested that communities can exist in a "stochastic" or "loose" equilibrium, diverging transiently but eventually returning toward earlier or average structure, in what we call here the "loose equilibrium concept" (LEC). We sampled the fish communities at 12 local stream reaches spaced broadly throughout a relatively undisturbed watershed in the Ozark Mountains of northern Arkansas, USA, in 11 surveys from 1972 to 2012 at a scale of decades, and at a subset of five of these local sites in a total of 16 surveys, allowing tests of the LEC at different spatial and temporal scales. Multivariate analyses of the dynamics of communities over the 40-year period provided support for the LEC at both "global" and "local" scales within the watershed. At the broadest spatial scale, core species numerically dominated the community, and most common species remained so across all decades. In spite of two extraordinary floods, and interannual variation in abundance of some species, the 12-site and five-site global communities and eight of 12 local communities repeatedly returned toward average positions in multivariate space. Trajectories of the global and local fish communities varied relative to model hypothetical trajectories that were based on gradual vs. saltatory changes, and prevalence of returns toward average community structure. Beta diversity among sites was variable across time, but beta partitioning consistently showed that pure spatial turnover dominated over nestedness, because many common species were consistently distributed either upstream or downstream. This study suggests that vertebrate communities in relatively undisturbed environments may display dynamics consistent with the LEC. The LEC, combined with quantification of community trajectory patterns, can help to clarify whether systems are moving about within ranges of conditions that reflect expected noise, or, conversely, have moved so far out of previous bounds, as a result of climate change or human intervention, that they are permanently changed or "novel."

Interannual variations in abundance and biomass of planktonic copepods Oithona in the barents sea

The distribution patterns of the common arctic zooplankton species Oithona similis and Oithona atlantica were investigated in the Barents Sea during warm and temperate years. The maximum abundance and biomass of Oithona spp. (159 x 10(3) ind./m2 and 38.8 mgC/m2, respectively) were recorded in the waters of Atlantic origin. O. atlantica occurred in Arctic waters only during anomalously warm years. It has been found that the quantitative characteristics of O. similis were negatively correlated with salinity and the winter NAO index, whereas the abundance of O. atlantica in Atlantic waters was positively correlated with the temperature anomaly. It is found that the abundance and biomass of Oithona pp. were comparable with the values recorded in other Arctic regions.

Possible link between interannual variation of neon flying squid (Ommastrephes bartramii) abundance in the North Pacific and the climate phase shift in 1998/1999

The relationship between interannual variation in abundance of the autumn cohort of the neon flying squid (Ommastrephes bartramii) and ocean environmental changes in the central North Pacific was examined. We focused on the change in subsurface ocean state during the 1998/1999 climate shift. Changes in catch per unit effort (CPUE) of the neon flying squid derived from long-term driftnet survey was compared to that in ocean environments related to the feeding conditions of the squid. A four-dimensional variational (4D-VAR) ocean data assimilation product was used as an accurate estimate of the dynamic state in the North Pacific. Correlation analysis indicated that the squid CPUE was highly related with the Pacific Decadal Oscillation (PDO) in winter. In January, the correlation field with the entrainment rate (ENT), the proxy for the nutrient-rich water supply entering the mixed layer, showed a good agreement with the main spawning and nursery ground of the autumn cohort (MSNGAC). The nutrient-rich water supply in the MSNGAC in early winter was mainly induced by the deepening of the mixed layer forced by surface latent heat cooling and turbulent mixing, while the basin-scale wind stress curl and the horizontal advection were less affected. These results suggest that the amount of newly supplied nutrient-rich water mass in early winter could affect the primary productivity throughout the winter and the resultant feeding conditions of the juvenile squid. We assumed that this process would determine the stock levels of the neon flying squid in the following summer. We further attempted to reconstruct the changes in neon flying squid CPUE during 1994–2006 by applying regression analysis to several parameters. The result showed that ENT, surface and subsurface temperatures, and the PDO index in February were good predictors for estimating the squid CPUE time series. In addition, the subsurface temperature in the MSNGAC in the preceding autumn was also a good predictor. This indicates that the squid CPUE could be predicted four months earlier than its nursery period, as the same accuracy as that using February data. The accurate prediction in early period could be useful for the squid stock management in the future.

Environmental factors influencing the distribution of Arvicola scherman (Shaw, 1801) at the southwestern edge of its distribution

During 2008, damage to pastures in the eastern mountains of the province of Lugo (NW Spain) was detected, probably due to an increase in populations of Arvicola scherman (Shaw, 1801). Due to the scanty information available on this species in the area we conducted a study with the following main aims: i) to predict the current geographic distribution of the species, ii) identify the environmental variables that may determine their geographic distribution, iii) verify interannual variations in abundance and iv) develop a risk map of populations outbreaks. A total of 280 grassland fields were sampled in which species abundance was evaluated by using presence signs (earth tumuli or mounds), and subsequently an abundance index was estimated. The MAXENT programme developed a potential distribution map using the variables: potential evapotranspiration, water balance, altitude and average annual values of temperature and precipitation. The species was found only in the eastern mountains at altitudes of more than 700 m. Altitude and water balance were the variables that best predicted the presence of the species. Furthermore, a significant difference in the relative abundance between 2008 and 2010 was found, suggesting that the species may experience strong demographic fluctuations in the study area. Probably, the increase in grasslands over the last 40 years may have favoured the expansion and abundance of the species in the study area.

Interannual summer variability in euphausiid populations on the eastern Bering Sea shelf during the recent cooling event (2008–2010)

The recent cooling in the eastern Bering Sea resulted in a series of cold years (2008–2010) marked by extensive ice coverage and late ice retreat. In the present study, we examined the spatial and temporal variability of three predominant euphausiid species Thysanoessa raschii, Thysanoessa inermis, and Thysanoessa longipes in the summers of 2008–2010. Simple box-and-whisker plots as well as generalized additive models (GLMs) were applied to examine habitat selection for three euphausiid species. The coefficients of dispersion were calculated for three species to examine potential changes in spatial patterns. Results showed that T. raschii was broadly distributed in the inner and middle domains. The abundance of T. raschii was related to water temperature, salinity and chlorophyll a concentration. T. inermis was primarily distributed in the middle domain and was related to water temperature and salinity. T. longipes was mostly distributed in the outer domain and was only related to salinity. The proportion of T. raschii, a coastal species in the inner and middle domains, showed large interannual variation with the highest in 2008 and the lowest in 2009 (a >50% decline). This same species did not show large interannual variation in the spatial distribution. T. longipes, a shelf species in the outer domain, did not show large interannual variation in abundance, but the center of mass distribution consistently shifted northward and offshore from 2008 to 2010. T. inermis, a coastal species in the middle and coastal domains, did not show large interannual variation in abundance and spatial distribution. In summary, euphausiid populations showed large spatial and temporal variability among cold years. Different species could be affected by different processes in the southeastern Bering Sea.

A two-stage biomass model to assess the English Channel cuttlefish (Sepia officinalis L.) stock

Abstract The English Channel cuttlefish (Sepia officinalis) is the most abundant cephalopod resource in the Northeast Atlantic and one of the three most valuable resources for English Channel fishers. Depletion methods and age-structured models have been used to assess the stock, though they have shown limitations related to the model assumptions and data demand. A two-stage biomass model is, therefore, proposed here using, as input data, four abundance indices derived from survey and commercial trawl data collected by Ifremer and Cefas. The model suggests great interannual variability in abundance during the 17 years of the period considered and a decreasing trend in recent years. Model results suggest that recruitment strength is independent of spawning–stock biomass, but appears to be influenced by environmental conditions such as sea surface temperature at the start of the life cycle. Trends in exploitation rate do not reveal evidence of overexploitation. Reference points are proposed and suggestions for management of the sustainable utilization of cuttlefish in the English Channel are advanced.

Interannual differences in microplankton drive changes in the feeding and fecundity of Mnemiopsis leidyi in a Long Island estuary

The ctenophore Mnemiopsis leidyi forms seasonal population blooms, which exhibit interannual variation in abundance. Ctenophore egg production and gut contents were examined alongside plankton community structure to identify factors influencing recruitment of M. leidyi in Great South Bay, NY. Ctenophores contained three times as many prey items and produced twice as many eggs in 2008 during a brown tide (Aureococcus anophagefferens) than in 2009, a year without a brown tide. Gut contents demonstrated significant dependence on the copepod Acartia tonsa, which is known to switch from ambush to suspension feeding when flagellates are abundant. Microflagellate abundance was significantly greater in 2008 and corresponded positively with A. anophagefferens, potentially increasing the encounter rates between M. leidyi and A. tonsa by promoting suspension feeding in the latter. Despite the enhanced fecundity, however, ctenophore abundances were five times lower in 2008 and a mismatch was identified between optimum egg production by adults and sufficient microplanktonic prey (dinoflagellates and ciliates) abundance for their larvae, compared to 2009 when the two coincided. We propose that increased microflagellate abundance during the brown tide provided a benefit to ctenophore fecundity by enhancing their encounter rates with A. tonsa, but ultimately larval recruitment was limited by insufficient food availability.

Temporal patterns of settlement, recruitment and post-settlement losses in a rocky reef fish assemblage in the South-Western Mediterranean Sea

Post-settlement events can significantly alter the density distribution of settlers and subsequently the adult population structure. The temporal and inter-annual variability of settlement and the effects of mortality on recruitment were investigated across 2 years in the vicinities of Cabo de Palos–Islas Hormigas Marine Reserve by visual census and light trap sampling. Settlement was seasonal with greater species richness and abundance in summer. Although temporal synchronization was observed between larval supply and settlement, densities of settlers could not be predicted from post-larval abundances. Timing of settlement was consistent between years but with high inter-annual variation in abundance. High mortality (~80 %) and general decoupling between post-larval and settlement stages suggest that early mortality is driving such patterns. Nevertheless, indications of habitat-mediated mortality were found for benthic species such as Symphodus and Diplodus, highlighting the importance of habitat in shaping population demography.

Seasonal and interannual variations in the abundance of jellyfish in a southern coastal waters of Iyo-Nada, Japan: Influence of cyclonic gyre transport

The longterm data on daily abundance of jellyfish (mostly Aurelia aurita) trapped in the intake gates of the Ikata Nuclear Power Plant on the south coast of the central part of Iyo-Nada in the Seto Inland Sea, Japan was examined for a 14-year period (April 1998 to March 2012) in relation to environmental variables, i.e. temperature, salinity, and residual current, of which the last was measured in February, May, August and November. The jellyfish abundance, expressed as wet weight, generally increased in April, reached a maximum in September, markedly decreased from October to November, and was very low or null from December to March. The annual mean jellyfish abundance varied markedly. The interannual variations in the monthly mean jellyfish abundance were not signifi- cantly correlated with water temperature or salinity in any month, while correlations with eastward velocity of the residual current in May and August were generally significantly positive. These results indicate that the jellyfish population at the study site had been introduced from the Hayasui strait about 25 km westward, where jellyfish were more abundant. Since the eastward residual current is considered as a part of the cyclonic gyre in Iyo-Nada during the warm, stratification period, it is inferred that the strength of the cyclonic gyre is the main factor affecting seasonal and interannual variations in jellyfish abundance along the south coast of Iyo-Nada.

Factoring scales of spatial and temporal variation in fish abundance in a subtropical estuary

We employed a new approach linking multivariate and time series analyses to iden- tify common versus unique spatiotemporal components of abundance variation of marine spawn- ing fishes recruiting into a subtropical Western Atlantic estuary. Based on a 10 yr standardized monthly data set, we also investigated patterns of association for local and regional factors with annual and inter-annual variation in abundance of 5 dominant marine estuarine-dependent fishes. The total amount of variation in fish abundance explained by environmental variables was 22.4%. After factoring out shared spatiotemporal variation (0.8%), our analysis showed that tem- poral components had an almost 5-fold greater contribution (28.0%) than spatial components (6.4%) in explaining the variation in abundance of the 5 species. Most of the variation across the temporal scale (58.5%) was associated with annual (from 0.5 to 1.3 yr) rather than multi-year oscil- lations (>2 yr). Such annual patterns were probably associated with adaptations of marine estuar- ine-dependent fishes for exploiting predictable pulses in seasonal productivity typically found in subtropical estuaries. In contrast, inter-annual variation in abundance occurring at a scale of 3 to 7 yr could be attributed to rainfall anomalies associated with El Nino Southern Oscillation (ENSO) events, whereas those occurring at a scale of 2 yr could be influenced by the Southern Annular Mode (SAM). Our findings corroborate the hypothesis that prevalent annual patterns of variation in the abundance of marine estuarine-dependent fishes are associated with predictable productiv- ity pulses linked to the annual temperature regime, whereas inter-annual variations in fish abun- dance are associated with the influence of large-scale climatic phenomena.

What causes the high interannual variability of flatfish (Rhombosolea spp.) in Lake Ellesmere?

Lake Ellesmere, Canterbury, a large coastal lake is mechanically opened to the sea at predetermined water levels. The lake maintains important customary and commercial eel (Anguilla spp.) and flatfish (Rhombosolea spp.) fisheries, although the harvest of flatfish varies considerably between years. The three main flatfish species (black flounders Rhombosolea retiaria, sand flounders R. plebia and yellowbelly flounders R. leporina) recruit in spring, grow rapidly within the lake, and reach the minimum commercial size within two or three years. Maturing fish endeavour to leave the lake to spawn at sea during summer and autumn. There is a weak association between commercial catches and lake openings lagged by two years, but strong associations between catches in the lake and in the Canterbury Bight. The high interannual variability in abundance partly reflects timing and duration of lake openings, the opportunity of maturing fish to emigrate to sea, and the strength of regional recruitment.

A Complete Record from Colonization to Extinction Reveals Density Dependence and the Importance of Winter Conditions for a Population of the Silvery Blue,Glaucopsyche lygdamus

Butterflies in the family Lycaenidac are often the focus of conservation efforts. However, our understanding of lycaenid population dynamics has been limited to relatively few examples of long-term monitoring data that have been reported. Here, factors associated with population regulation are investigated using a complete record of a single population of the silvery blue, Glaucopsyche lygdamus Doubleday (Lepidoptera: Lycaenidae). Adults of G. lygdamus were first observed in an annual grassland near Davis, California, in 1982 and were last seen in 2003. Relationships between inter-annual variation in abundance and climatic variables were examined, accounting for density dependent effects. Significant effects of both negative density dependence and climatic variation were detected, particularly precipitation and temperature during winter months. Variation in precipitation, the strongest predictor of abundance, was associated directly and positively with butterfly abundance in the same year. Winter temperatures had a negative effect in the same year, but had a lagged, positive effect on abundance in the subsequent year. Mechanistic hypotheses are posed that include climatic effects mediated through both larval and adult plant resources.

Feeding ecology of pelagic fish species in the Gulf of Riga (Baltic Sea): the importance of changes in the zooplankton community

The feeding ecology of four pelagic fish species was studied in relation to their prey availability in the Gulf of Riga (Baltic Sea) during the summer 1999-2006. The zooplankton community was dominated by the cladoceran Bosmina longispina, rotifers Keratella cochlearis and K. quadrata and the copepod Eurytemora affinis, with the highest interannual variability in abundance recorded for B. longispina. The last influenced the diet of adult sprat Sprattus sprattus, juvenile smelt Osmerus eperlanus and three-spined stickleback Gasterosteus aculeatus as these were strongly selecting for B. longispina. The fish feeding activity did not match the abundance dynamics of their preferred prey, suggesting that fishes may switch to consume other prey in case the preferred diet was limited. A considerable dietary overlap indicated high potential competition between pelagic fish species. While herring Clupea harengus membras and G. aculeatus were relying on very different food, the diets of young O. eperlanus and G. aculeatus were very similar. Interannual variability in zooplankton composition and abundance significantly affected the diet composition of fishes, but those changes were insufficient to exert a consistent influence upon fish feeding activity and total amounts of zooplankton consumed.

Butterfly abundance in a warming climate: patterns in space and time are not congruent

We present a model of butterfly abundance on transects in England. The model indicates a significant role for climate, but the direction of association is counter to expectation: butterfly population density is higher on sites with a cooler climate. However, the effect is highly heterogeneous, with one in five species displaying a net positive association. We use this model to project the population-level effects of climate warming for the year 2080, using a medium emissions scenario. The results suggest that most populations and species will decline markedly, but that the total number of butterflies will increase as communities become dominated by a few common species. In particular, Maniola jurtina is predicted to make up nearly half of all butterflies on UK Butterfly Monitoring Scheme (UKBMS) transects by 2080. These results contradict the accepted wisdom that most insect populations will grow as the climate becomes warmer. Indeed, our predictions contrast strongly with those derived from inter-annual variation in abundance, emphasizing that we lack a mechanistic understanding about the factors driving butterfly population dynamics over large spatial and temporal scales. Our study underscores the difficulty of predicting future population trends and reveals the naivety of simple space-for-time substitutions, which our projections share with species distribution modelling.

Temporal variability in fish larval supply to Malindi Marine Park, coastal Kenya

1. Larval supply to reef sites influences adult population structure, reef connectivity and conservation potential of marine reserves, but few studies have examined this topic in the Western Indian Ocean (WIO). 2. Fish larval supply to Malindi Marine Park in Kenya was studied using light-traps for a period extending from March 2005 to June 2006. The traps caught pre-settlement fish larvae at two sites spread across the park. Catch rates (number trap−1night−1) were used to represent larval abundance and to test the influence of seasonality and habitat characteristics on larval abundance in the park. 3. Thirty-three species of reef fish larvae in 15 families were sampled. Larval supply to the park was more diverse during the north-east monsoon season (30 species) than in the south-east monsoon season (15 species), with inter-annual variability in abundance. Higher catch rates of larvae occurred in the north-east monsoon month of March in both 2005 and 2006 and the inter-monsoon month of September 2005. 4. Family-specific temporal variation in larval abundance showed dominance of the families Apogonidae and Caesionidae in the park, with higher abundance during the north-east monsoon months. A few families (e.g. Canthigasteridae) showed dominance during the south-east monsoon season. Regression and rank Spearman correlation analyses indicated positive correlation of chlorophyll-a with larval supply while water depth had significant negative correlation with abundance of the Apogonidae and Caesionidae. 5. On a short-term temporal scale larval abundance in the park was highly correlated with the new moon lunar phase more than the full moon. However, on a long-term scale (16 months) larval supply to the park was significant only over a 2-month period and was correlated with environmental productivity more than ambient temperature. These results are useful in understanding the role of larval supply in structuring adult fish populations and the factors that force larval flux at reef sites. Copyright © 2009 John Wiley & Sons, Ltd.

Coastal upwelling circulation and its influence on the population dynamics of Calanus chilensis (Brodski, 1959) off northern Chile (23°S)

Marine pelagic copepods must cope with a highly heterogeneous environment in coastal upwelling systems. In addition to spatial and temporal heterogeneity in temperature and food supply, advective forces may strongly influence their populations. Off northern Chile, the population of Calanus chilensis, a dominant copepod associated with upwelling centres, is subjected to large interannual variations in abundance, not explained by changing temperature regimes occurring upon cold and warm phases of the ENSO (El Niño Southern Oscillation) cycle. During two spring and two summer periods (1998–2000), we studied the abundance, distribution and growth of the copepodids of C. chilensis. The instantaneous mean weight-specific growth rate was estimated as 0.29 day−1 and was not significantly different among cruises. Weights of late stages did not differ either. However, the numerical abundance and biomass substantially differed among cruises, such that daily production of copepodids increased with greater biomasses. Chlorophyll-a was significantly greater in spring 1999 and summer 2000, as compared to spring 1998 and summer 1999, coinciding with greater abundances of copepodids and stronger upwelling. The current field in the upper 200 m indicated that strong upwelling favoured nearshore water retention, whereas weak or moderate upwelling caused more offshore advection. This finding is opposite to the usual view that strong upwelling favours offshore advection. We further concluded that neither food or temperature had much influence on growth, abundance and production of C. chilensis, but instead seasonal and interannual variation in upwelling caused by changes in local winds may be the primary cause for the observed fluctuations in population abundance.

Relationship between zooplankton abundance and the early marine life history of juvenile chum salmon Oncorhynchus keta in eastern Hokkaido, Japan

Interannual variations in abundance, timing of outmigration from rivers, growth rate and condition of juvenile chum salmon (Oncorhynchus keta) were studied in the Nemuro Strait (eastern Hokkaido, Japan) during 1999–2002 to establish a possible relationship to zooplankton abundance. The otolith microstructure of juveniles was examined each year in late June to determine their time and size at sea entry (i.e., outmigration), and to estimate the early marine growth rates. Salmon outmigration peaked in mid- or late May, which coincided, in three of the four study years, with the peak release of juveniles into rivers within the study area. Abundance, growth rate and condition of fish were higher in 2001, when—compared to other years—smaller fish experienced higher growth rates, coinciding with greater zooplankton abundance for that year. Our results suggest that high zooplankton abundance positively influenced juvenile chum salmon growth and the condition of the fish during their early marine life despite their small size at sea entry.

Inter-annual variability in abundance and community structure of zooplankton south and north of Iceland in relation to environmental conditions in spring 1990-2007

An 18 year zooplankton time series from two standard sections differing in hydrographic conditions (Subarctic Water north of Iceland and Atlantic water south of Iceland) was examined in relation to hydrography and phytoplankton dynamics, and large-scale climatic changes in the North Atlantic Ocean, particularly the North Atlantic Oscillation (NAO). The long-term mean zooplankton abundance was higher in south (∼160 000 individuals m−2) than that in north of Iceland (∼120 000 individuals m−2). Abundance fluctuated markedly between years. Copepods (mainly Calanus finmarchicus and Oithona spp.) dominated the zooplankton, comprising >60–70% of the plankton in most years. Among the copepods, C. finmarchicus was more abundant in south of Iceland (∼20–70%) than to the north (∼10–60%). Abundance and community structure were related to environmental variables using regression and multivariate techniques (principal component analysis and redundancy analysis). Temperature and salinity were the most important environmental variables in explaining the differences in species composition in north and south of Iceland, with species and groups like Podon leuckarti and cirripede larvae being relatively abundant to the south, and C. hyperboreus to the north of Iceland. A significant year-to-year variability in community structure was observed both to the south and to the north, with salinity and used nitrate dictating the variability to the south and temperature to the north (Monte Carlo permutation tests, P < 0.05). In neither region was a unidirectional temporal trend in species composition.

Effect of changing land use patterns on the distribution of coho salmon (Oncorhynchus kisutch) in the Puget Sound region

Population increase in the Pacific Northwest of North America over the last century has led to the removal of forests for various purposes. Evidence of salmon response to these alterations in land use is rare owing to a scarcity of fish population data and a high degree of interannual variation in abundance. We examined the relationship between the spatial distribution of spawning coho salmon ( Oncorhynchus kisutch ) and changes in land use from 1984 through 2001 at 84 sites in four rivers draining into northern Puget Sound. Changes in land use over this period were determined from LandSat imagery, county zoning designations, and aerial photographs. Substantial reduction in forest cover occurred in many of the index watersheds during this time. The proportion of salmon using sites subjected to increased urban land use over the study period declined about 75%. Increases were observed at forested sites and those with increased rural residential use. Maintaining salmon populations in rapidly developing areas may require the identification and protection of sites that support large salmon populations and steering development to areas supporting few fish.

Trophic ecology of the swimming crab Polybius henslowii Leach, 1820 in Galician and Cantabrian Seas: Influences of natural variability and the Prestige oil spill

Diet and feeding intensity of the portunid crab Polybius henslowii were studied during the spring of 2004 along Galician and Cantabrian continental shelves (NW Iberian Peninsula) between 78 and 287 m. The area had been affected by the Prestige oil spill in November 2002, with different sectors at different levels of pollution: highest in between Miño River and Cape Finisterre (MF) and from there to Estaca de Bares Point (FE) and lowest from Estaca to Cape Peñas (EP). From 37272 specimens collected in 59 bottom hauls, 342 were analysed for stomach fullness and diet composition. Stomach fullness was lower in FE, but with no significant differences among sectors. Diet was a mixture of pelagic and benthic prey: cephalopods ( Ilex coindetti), brachyurans ( P. henslowii) and polychaetes ( Glycera sp.). Other prey were beloniform eggs and fish remains. Two stress markers were also studied in some specimens: Acetylcholinesterase (a neurotoxicity marker) varied among sampled areas, while lipid peroxidation (an oxidative stress marker) did not. Changes in trophic variables, distribution and density were analysed in relation to depth, surface and bottom T, surface Chl a from satellite imagery and tar aggregate concentration in sediments. Along Galician and Cantabrian coasts prey availability, linked to productivity level, seems to be the main factor affecting P. henslowii diet and distribution. Interannual abundance variability seems to be controlled by large-scale climatic conditions (NAO index). This natural variability masks possible effects induced by the oil spill.