Meganyctiphanes Research Articles

Functional feeding response of Nordic and Arctic krill on natural phytoplankton and zooplankton

Abstract Krill species play a pivotal role in energetic transfer from lower to upper trophic levels. However, functional feeding responses, which determine how food availability influences ingestion rates, are still not well defined for northern krill species. Here, we estimated and compared the functional feeding responses on natural communities of phytoplankton and mesozooplankton of two coexisting species, Meganyctiphanes norvegica and Thysanoessa raschii. We tested the influence of the presence of phytoplankton on the ingestion rate and the selectivity of both krill species when feeding on zooplankton prey. We performed a series of feeding experiments using increasing concentrations of natural phytoplankton (64 taxa; 2 to >50 μm) and mesozooplankton (28 taxa; ~100–2000 μm) assemblages and the latter in presence and absence of phytoplankton. Results revealed that both krill species exhibited a Holling type III feeding response on phytoplankton. However, T. raschii was able to exploit efficiently the highest phytoplankton concentrations. Our experiments highlighted that the presence of phytoplankton modified the functional feeding response on mesozooplankton preys of M. norvegica, but not that of T. raschii. Similarly, the presence of phytoplankton influenced the feeding selectivity on mesozooplankton preys, although both species showed contrasting selectivity patterns. In addition, we estimated the energy needs in relation to the daily rations. T. raschii satisfied its energy needs by feeding either on high phytoplankton concentrations or on low mesozooplankton densities, whereas M. norvegica did not cover its metabolic costs efficiently by feeding on phytoplankton only, even at high phytoplankton concentrations.

First evidence of anticancer and antimicrobial activity in Mediterranean mesopelagic species

Mesopelagic organisms form huge biomass aggregations, supporting important pelagic trophic webs and several top predators. Although some studies on the occurrence, biology and ecology of these organisms are available, to date there are no investigations on their potential use for anticancer and antimicrobial biotechnological applications. The aim of this study was to screen extracts of seven mesopelagic species for possible anticancer (Lung cell line A549, skin cell line A2058, liver cell line HepG2, breast cell line MCF7 and pancreas cell line MiaPaca-2) and antibacterial (Gram-negative bacteria Escherichia coli and Klebsiella pneumoniae, the Gram-positive bacteria methicillin resistant/sensitive Staphylococcus aureus, and Mycobacterium tuberculosis) activities. Results showed that only two species were active, the lanternfish Myctophum punctatum and the Mediterranean krill Meganyctiphanes norvegica. In particular, M. punctatum showed strong activity against the A549 and MCF7 cells, while M. norvegica was more active against HepG2 cells. Regarding antibacterial assays, both species were active against methicillin resistant S. aureus. Fractionation and LC/MS dereplication of the fractions showed that the main compounds found in extracts of both species were EPA, DHA and ETA. For some of the detected compounds anticancer and/or antibacterial activity are already known, but this is the first time that such activities have been found for mesopelagic species.

Organophosphate contaminants in North Atlantic fin whales

Pollution of the marine environment by litter composed of plastics is a growing concern. Chemical additives such as organophosphate flame retardants (OPFRs), which are added to plastics to improve their qualities, are in focus because they allegedly cause adverse effects on marine fauna. Here we analyse OPFR levels in the muscle of fin whales because, as a mysticete, this cetacean obtains its food by filter-feeding and is thus highly vulnerable to marine litter. Moreover, the fin whale performs long-range migrations from low-latitude areas in winter to high-latitude areas in summer, a trait that makes it a potentially good large-scale biomonitor of pollution. We also analyse OPFR levels in its main prey, the krill Meganyctiphanes norvegica, to assess transfer through diet. The samples analysed consisted of muscle tissue from 20 fin whales and whole-body homogenates of 10 krill samples, all collected off West Iceland. From the 19 OPFRs analysed, we detected 7 in the fin whale and 5 in the krill samples. Tri-n-butyl phosphate (TNBP), Isopropylated triphenyl phosphate (IPPP) and Triphenylphosphine oxide (TPPO) were the most abundant compounds found in both species. Mean ∑OPFR concentration, expressed on a lipid weight basis, was 985 (SD = 2239) ng g−1 in fin whale muscle, and 949 (SD = 1090) ng g−1 in krill homogenates. These results constitute the first evidence of the presence of OPFRs in the tissues of fin whales. Furthermore, they seem to support the non-significance of bioaccumulation of OPFRs through lifespan and of biomagnification trough the food web.

Arctic and Nordic krill circuits of production revealed by the interactions between their physiology, swimming behaviour and circulation

The two species Thysanoessa raschii and Meganyctiphanes norvegica dominate the krill biomass of the Estuary and Gulf of St. Lawrence (EGSL) ecosystem. The EGSL is the southernmost limit of seasonal sea-ice cover in the North Atlantic and constitutes a dynamic frontier between Arctic and Atlantic water masses. In order to better understand the complex mechanisms underpinning the spatio-temporal dynamics of the production and distribution of these keystone forage species in the EGSL, we coupled in a Lagrangian framework an individual-based model (IBM) of krill physiology to a three-dimensional regional circulation model and a NPZD-type biogeochemical model. Our results show that the spatio-temporal production dynamics of both krill species was principally controlled by their food dynamics. Our numerical analyses highlighted the potential areas of krill production within the EGSL during spring and summer. We confirmed that the St. Lawrence Estuary was the most productive area, and we show that it subsidizes the central and southern Gulf during most of the growing season. The Estuary is part of a larger cyclonic circuit of krill production and transport that runs along the shores of most of the EGSL and the slope of the Laurentian Channel.

Diet and trophic structure of fishes in the Barents Sea: The Norwegian-Russian program “Year of stomachs” 2015 – Establishing a baseline

There is a long history of investigations of fish diet in the Barents Sea. The focus has been on commercially important fish species and their food consumption, while diet and interactions of other fishes have been studied only sporadically. In 2015, a large-scale stomach sampling program was carried out for fish species caught on routine monitoring surveys in the Barents Sea during different seasons of the year, supplemented with samples collected from Russian commercial fisheries. A total of 27,657 stomachs from 70 fish species (including two genera) were analysed, providing a baseline on fish diet in the Barents Sea which can serve as a reference for future studies related to climate change. We summarize methodological aspects and diet composition for the studied species. Cluster analysis grouped the fishes in nine trophic groups based on similarities in diet among fish species, while principal component analyses revealed the position of the species and trophic groups along axes reflecting degrees of piscivory, planktivory, and benthivory. The three most distinctly separated groups were piscivores, a group of benthivores feeding on polychaetes, and planktivores feeding on small crustaceans. The latter could be further split into two groups: fishes of Atlantic origin feeding on copepods and euphausiids, and fishes of Arctic origin feeding on hyperiid amphipods. Warming in the Barents Sea were associated with redistribution of water masses, species and increasing biomass of krill and jellyfish. A boreal Meganyctiphanes norvegica, not observed in the northern Barents Sea before, were found in diet of three Arctic fishes (2% of stomachs only). Gelatinous plankton, mainly Ctenophora, were observed in the diet of 1430 individuals from 15 fish species, including two species which have not been reported to eat gelatinous plankton in the Barents Sea before. This work updates our knowledge about trophic structure and interactions in the Barents Sea, providing a baseline for further investigations.

Accounting for seasonal and composition‐related variability in acoustic material properties in estimating copepod and krill target strength

AbstractEstimation of abundance or biomass, using acoustic techniques requires knowledge of the frequency dependent acoustic backscatter characteristics, or target strength, of organisms. Target strength of zooplankton is typically estimated from physics‐based models that involve multiple parameters, notably including the acoustic material properties (i.e., the contrasts in density and sound speed between the animal and surrounding seawater). In this work, variability in the acoustic material properties of two zooplankton species in the Gulf of Maine, the copepod (Calanus finmarchicus) and krill (Meganyctiphanes norvegica), was investigated relative to changing season as well as, for the copepod, temperature and depth. Increases in the density and sound speed contrasts of these species from fall to spring were observed. Target strength predictions based on these measurements varied between fall and spring by 2‐3 dB in krill. Measurements were also conducted on C. finmarchicus lipid extract at changing temperature and pressure. The density contrast of the extract varied negatively with temperature, while the sound speed contrast changed by more than 10 % over the temperature and pressure ranges that the organism expected to occupy. C. finmarchicus target strength predictions showed that the combined effect of temperature and pressure can be significant (more than 10 dB) due to the varying response of lipids. The large vertical migration ranges and lipid accumulation characteristics of these species (e.g., the diapause behaviour of Calanus copepods) suggest that it is necessary for seasonal and environmental variability in material properties to be taken into account to achieve reliable measurements.

Inter-regional variation in feeding patterns of skipjack tuna (Katsuwonus pelamis) inferred from stomach content, stable isotope and fatty acid analyses

Foraging habits of skipjack tuna, SKJ (Katsuwonus pelamis), were investigated in three Spanish marine regions (Balearic Sea, Alboran Sea and Gulf of Cadiz) using stomach content (SCA), stable isotope (SIA), and fatty acid (FA) analyses. The three methodological approaches yielded significant differences among locations. All the studied areas appear to serve as important foraging grounds for SKJ, but the diet composition significantly varied among them. The predominant prey species in the respective locations were the North Atlantic krill, Meganyctiphanes norvegica, in the Balearic Sea, the flyingfish, Exocœtus volitans, in the Alboran Sea, and the anchovy, Engraulis encrasicolus, in the Gulf of Cadiz. Regional differences were also found in δ13C and δ15N values; furthermore, the analysis of standard ellipse areas (SEAc) based on isotopic data showed that the broadest niche corresponded to SKJ from the Gulf of Cadiz, with no significant isotopic niche overlap between areas. Consistent with the SCA and stable isotope results, the FA composition varied among areas. Docosahexaenoic acid (DHA, 22:6n-3) was the most abundant FA in the SKJ muscle, showing its highest values in samples from the Gulf of Cadiz; this FA was the most effective in segregating SKJ geographical groups. In conclusion, this study provides evidence that SKJ in eastern Atlantic and western Mediterranean waters can easily adapt their diet to local prey availability. Furthermore, stable isotope signatures and FA profiles of muscle tissue prove to be reliable trophic markers that allow separation of populations occurring in different marine areas.

Seasonal and large-scale spatial variability of the energy reserves and the feeding selectivity of Meganyctiphanes norvegica and Thysanoessa inermis in a Subarctic environment

Krill, as forage species, play a pivotal role in marine food webs by linking lower to upper trophic levels. Despite their ecological relevance, processes affecting their physiological condition and their feeding selectivity remain not well known. Here, we hypothesize that significant spatial fluctuations of environmental conditions in a Subarctic region affect the energy reserve content and feeding strategies of krill. To test this hypothesis, we assessed the fatty acid content and composition of two dominant northern krill species Meganyctiphanes norvegica and Thysanoessa inermis on a seasonal basis over more than 1000 km in the Estuary and Gulf of St. Lawrence (eastern Canada) and for the first time during the ice covered winter. M. norvegica exhibited large spatial variation of its energy reserve lipids, which were similar to seasonal variations found in a previous study in the same environment. In contrast, lipid content of T. inermis was much less affected by the spatial variations of environmental conditions but showed high seasonal variability. Feeding preferences assessed using fatty acid trophic markers highlight that the zooplankton standing stock could be more critical than phytoplankton to fulfill the metabolic requirements of these krill species and to build lipid energy reserves. In addition, diet comparison between both species highlights that trophic niche partitioning is present in the Gulf of St. Lawrence, however, modulated by spatial differences in food supply and oceanographic conditions.

Krill spatial distribution in the Spanish Mediterranean Sea in summer time

Abstract We documented krill distribution in the Spanish Mediterranean Sea for the first time using acoustic methods, highlighting the method’s suitability to study marine communities restricted to specific areas with patchy aggregation behavior. The 2009–2017 acoustic time series analysis revealed that krill distribution, mainly located on the continental shelf edge, was driven by the presence of fronts and submarine canyons. On the other hand, areas of persistent krill distribution included from Cape La Nao to the eastern part of Almeria Bay, although an interannual northwards increase of krill presence had been detected in 2015–2017 likely related to the position of the Balearic front. We provide information on the aggregation characteristics and biological parameters of three krill species, Nyctiphanes couchii, Nematoscelis megalops and Meganyctiphanes norvegica. N. couchii and N. megalops formed patchy pelagic aggregations in the neritic and oceanic zone, respectively, and they were the most common species in the net tows. By contrast, M. norvegica formed a large demersal aggregation on the continental shelf edge and was only found in 2017; nevertheless, its 861-kg catch represented a unique milestone in the Mediterranean. Finally, krill species shared distribution area with Maurolicus muelleri; thus, coexistence between them are also described.

Mesopelagic Sound Scattering Layers of the High Arctic: Seasonal Variations in Biomass, Species Assemblage, and Trophic Relationships

Mesopelagic sound scattering layers (SSL) are ubiquitous in all oceans. Pelagic organisms within the SSL play important roles as prey for higher trophic levels and in climate regulation through the biological carbon pump. Yet, the biomass and species composition of SSL in the Arctic Ocean remain poorly documented, particularly in winter. A multifrequency echosounder detected a SSL north of Svalbard, from 79.8°N to 81.4°N, in January 2016, August 2016, and January 2017. Midwater trawl sampling confirmed that the SSL comprised zooplankton and pelagic fish of boreal and Arctic origins. Juvenile beaked redfish dominated the fish assemblage in January and Arctic cod in August. The macrozooplankton community mainly comprised the medusa Cyanea capillata, the amphipod Themisto libellula, and the euphausiids Meganyctiphanes norvegica in August and Thysanoessa inermis in January. The SSL was located in the Atlantic Water mass, between 200-700 m in August and between 50-500 m in January. In January, the SSL was shallower and weaker above the deeper basin, where less Atlantic Water penetrated. The energy content available in the form of lipids within the SSL was significantly higher in summer than winter. The biomass within the SSL was >12-fold higher in summer, and the diversity of fish was slightly higher than in winter (12 vs. 9 species). We suggest that these differences are mainly related to life history and ontogenetic changes resulting in a descent towards the seafloor, outside the mesopelagic layer, in winter. In addition, some fish species of boreal origin, such as the spotted barracudina, did not seem to survive the polar night when advected from the Atlantic into the Arctic. Others, mainly juvenile beaked redfish, were abundant in both summer and winter, implying that the species can survive the polar night and possibly extend its range into the high Arctic. Fatty-acid trophic markers revealed that Arctic cod mainly fed on calanoid copepods while juvenile beaked redfish targeted krill (Thysanoessa spp.). The relatively high biomass of Arctic cod in August and of redfish in January thus suggests a shift within the SSL, from a Calanus-based food web in summer to a krill-based food web during winter.

Implementation of an end-to-end model of the Gulf of Lions ecosystem (NW Mediterranean Sea). I. Parameterization, calibration and evaluation

An end-to-end model named OSMOSE-GoL has been built for the Gulf of Lions, the main French Mediterranean fishing area. This spatialized dynamic model links the coupled hydrodynamic and biogeochemical model Eco3M-S/SYMPHONIE (LTL – low trophic level model) to OSMOSE (HTL – high trophic level model). It includes 15 compartments of living organisms, five from the LTL model (i.e. nanophytoplankton, microphytoplankton, nanozooplankton, microzooplankton and mesozooplankton) and ten from the HTL model (northern krill, southern shortfin squid, European pilchard, European anchovy, European sprat, Atlantic horse mackerel, Atlantic mackerel, blue whiting, European hake and Atlantic bluefin tuna). With the exception of northern krill and European sprat, all HTL species are commercially exploited and undergo fisheries mortality pressure. The modeled species represent more than 70% of annual catches in this area. This paper presents the parameterization, calibration and evaluation of this model with satellite data for phytoplankton and with biomass, landings, diet and trophic level data for HTL groups. For most species, the diets in output of OSMOSE-GoL are similar to field and literature data in terms of dominant prey groups and species. However, some differences were observed. Various reasons may explain the mismatch between the modeled diet and field data. Benthic prey sometimes observed in the stomach content of the HTL predators were not modeled in OSMOSE-GoL. Field studies were carried out at specific periods and locations, while our data concern the period 2001–2004 and the entire modeled domain. Inter- and intra-annual variations in spatial distribution and density of prey may also explain these differences. The model estimates trophic level values similar to those cited in the literature for all the HTL compartments. These values are also close to the trophic levels estimated by a previous Ecopath model for the same area and period. Even though some improvements are still possible, this model may already be of use to explore fishery or Marine Protected Areas scenarios for socio-ecosystem management issues.

Trophic niche partitioning of dominant North‐Atlantic krill species, Meganyctiphanes norvegica, Thysanoessa inermis, and T. raschii

AbstractDifferent krill species have a pivotal position in many marine food webs by both preying upon several trophic levels and being forage species for consumers. Within these food webs, different krill species coexist, though it remains unclear what mechanisms allow for the coexistence, for instance, of northern krill species in subarctic environments. Here, we hypothesize that the stable coexistence of sympatric krill species is based on trophic niche partitioning related to seasonal trade‐offs between their respective food preferences, their energy needs, and prey availability. To test our hypothesis, we analyzed the diets, feeding selectivity, and lipid composition of three coexisting northern krill species (Meganyctiphanes norvegica, Thysanoessa inermis, and Thysanoessa raschii) throughout 1 yr using a multimarker approach. We assessed lipid classes, fatty acids, and stable isotope signatures of krill and potential food sources (27 groups, from phytoplankton to lipid‐rich copepods) to elucidate seasonal variation of niche utilization. Results revealed strong trophic niche separation occurring at a very fine trophic scale (species level) throughout the year. The three krill species showed different degrees of food specialization rather than being purely opportunistic as classically proposed. Feeding on copepod prey was important to accumulate energy reserves for overwintering and subsequent rebuilding of energy reserves. Energy reserve utilization might reduce potential competition for the limited available resources, especially under low food conditions.

Individual‐based modeling explains the contrasted seasonality in size, growth, and reproduction of the sympatric Arctic (Thysanoessa raschii) and Nordic krill (Meganyctiphanes norvegica) in the St. Lawrence Estuary, eastern Canada

AbstractThe Nordic krill Meganyctiphanes norvegica and Arctic krill Thysanoessa raschii both dominate the krill community within the Estuary and Gulf of St. Lawrence system where they are central forage species for its pelagic ecosystem. We developed a species‐specific physiological individual based model that implements the critical physiological processes of growth, molting, and reproduction of female adults as responses to environmental forcing. Key innovations of our approach were the decoupling between the molting schedule and growth, as well as considering two distinct sources of prey (phytoplankton and mesozooplankton). Our simulation results revealed that the details of the feeding process were critical for an accurate representation of the production dynamics of adult individuals from both species. Their specific feeding preferences on phytoplankton and mesozooplankton resulted in distinct species‐specific phenological patterns that reproduced observations. The present study highlights the importance of detailed knowledge of diet and feeding behavior of krill species to improve our understanding of population responses in a rapidly changing environment.

Humpback whale in the bay of Algeciras and a mini-review of this species in the Mediterranean

The presence of humpback whales (MN) (Megaptera novaeangliae) in the Mediterranean Sea has increased for the last 150 years. The reasons why they enter this sea are still unknown and therefore, a review adding new records of the species in the Mediterranean Sea has been considered. Repeated sightings of a young individual have been detected in the bay of Algeciras (Strait of Gibraltar) such that coincided, with stranding events of vast amount of northern krill (Meganyctiphanes norvegica) and species of lantern fish (Myctophum punctatum). Furthermore, data of stranded animals were also added.For all these reasons, the historical presence of this species in the Mediterranean – bypassing its Atlantic migratory routes –, and the theory of why the species occasionally visits this sea has been argued in this paper. Mediterranean sightings map for the species have been developed and overlapped with primary production areas and chlorophyll concentrations in the same sea, finding coincidences between areas of high concentration of productivity and presence of humpback whales. All the individuals detected in the Mediterranean Sea were juveniles or sub-adults (never mother–calf pair), supporting that humpback whales enter these waters for feeding reasons . These solitaire animals may come from their trophic migrations movements rather than the return migration from breeding areas belonging to the North Atlantic populations. Given the increasingly frequent occurrence of these species in the Mediterranean waters, environmental monitoring and conservation programmes for humpback whales should be developed.

Effects of high pCO2 on the northern krill Thysanoessa inermis in relation to carbonate chemistry of its collection area, Rijpfjorden

Polar oceans are predicted to be the first marine environments affected by ocean acidification (OA). Thysanoessa inermis is one of the most abundant krill species in northern waters of the Atlantic and a key species in the food web of this ecosystem. Yet, we know very little about potential OA effects on this species. We studied the effects of elevated pCO2 on T. inermis in a laboratory experiment by exposing individuals for 11 weeks to low and high pCO2 (450 and 1200 µatm, respectively, n = 12 per pCO2 treatment). Survival, growth, and moulting frequency was monitored during the experiment, and feeding and oxygen consumption rates (n = 3–5 per pCO2 treatment) were measured at the end of the experiment. No significant effects of high pCO2 on survival, growth, moulting, oxygen consumption, and feeding rate were observed, indicating that T. inermis is tolerant to predicted high OA levels. We also explored physical and chemical properties of waters near the collection area of krill, Rijpfjorden (Svalbard 80° North) during the polar summer (July–August). In situ measurements showed large temperature and salinity gradients from surface to bottom and pCO2 and pH ranged, respectively, 161–417 µatm and 7.99–8.37. Even though substantial spatial variability in pCO2 could be observed, krill in this area is not confronted yet with the investigated high pCO2 levels.

Feeding dynamics of Northwest Atlantic small pelagic fishes

Small pelagic fishes represent a critical link between zooplankton and large predators. Yet, the taxonomic resolution of the diets of these important fishes is often limited, especially in the Northwest Atlantic. We examined the diets, along with stable isotope signatures, of five dominant small pelagic species of the northeast US continental shelf ecosystem (Atlantic mackerel Scomber scombrus, Atlantic herring Clupea harengus, alewife Alosa pseudoharengus, blueback herring Alosa aestivalis, and Atlantic butterfish Peprilus triacanthus). Diet analyses revealed strong seasonal differences in most species. Small pelagic fishes predominantly consumed Calanus copepods, small copepod genera (Pseudocalanus/Paracalanus/Clausocalanus), and Centropages copepods in the spring, with appendicularians also important by number for most species. Krill, primarily Meganyctiphanes norvegica, and hyperiid amphipods of the genera Hyperia and Parathemisto were common in the stomach contents of four of the five species in the fall, with hyperiids common in the stomach contents of butterfish in both seasons and krill common in the stomach contents of alewife in both seasons. Depth and region were also found to be sources of variability in the diets of Atlantic mackerel, Atlantic herring, and alewife (region but not depth) with krill being more often in the diet of alewife in more northerly locations, primarily the Gulf of Maine. Stable isotope data corroborate the seasonal differences in diet but overlap of isotopic niche space contrasts that of dietary overlap, highlighting the differences in the two methods. Overall, the seasonal variability and consumer-specific diets of small pelagic fishes are important for understanding how changes in the zooplankton community could influence higher trophic levels.

Drivers of euphausiid distribution and abundance in the Northeast U.S. Shelf Large Marine Ecosystem

AbstractEuphausiids, or krill, often dominate the biomass of zooplankton communities in high latitude marine ecosystems and are recognized as a key component of food webs. Compared to other ecosystems, however, there is scant information regarding the broad-scale distribution of euphausiids in the Northeast U.S. Shelf Large Marine Ecosystem (NEUS-LME). Using long-term (1977–2011) zooplankton survey data, we examine the drivers of the broad-scale spatial distribution and inter-annual variability in euphausiid abundance throughout the entire NEUS-LME. Our results show strong seasonal patterns in both the abundance and spatial distribution of euphausiids, which were commonly associated with waters exceeding 100 m, the continental shelf break (SB), and warmer and more saline bottom waters. Inter-annual patterns in euphausiid abundance were positively correlated with the Atlantic Multidecadal Oscillation (AMO) and bottom salinity, and inter-annual variation was most conspicuous at the outer continental shelf and SB. Assemblage analyses indicated that Euphausia krohnii, Thysanoessa gregaria, T. longicaudata, and Nematoscelis megalops dominated in waters associated with the outer shelf and SB, while Meganyctiphanes norvegica was more important in the Gulf of Maine. These results improve our understanding of the distribution and abundance of euphausiids and provide important information for effective ecosystem-based fisheries management for the NEUS-LME.

Behavioural responses of krill and cod to artificial light in laboratory experiments.

Most fishes and crustaceans respond to light, and artificial light sources may therefore be an efficient stimulus to manipulate behaviours in aquatic animals. It has been hypothesised that the catch efficiency of pots could be increased if prey, for example krill, can be attracted into the pots providing a visual stimulus and a source of live bait. To find which light characteristics are most attractive to krill, we tested the effects of light intensity and wavelength composition on Northern krill’s (Meganyctiphanes norvegica) behavioural response to an artificial light source. The most attractive individual wavelength was 530 nm (green light), while broadband (425–750 nm) white light was an equally attractive light source. The intensity of the emitted light did not appear to have a direct effect on attraction to the light source, however it did significantly increase swimming activity among the observed krill. The most promising light stimuli for krill were tested to determine whether they would have a repulsive or attractive effect on cod (Gadus morhua); These light stimuli appeared to have a slightly repulsive, but non-significant, effect on cod. However, we suggest that a swarm of krill attracted to an artificial light source may produce a more effective visual stimulus to foraging cod.

Predation and food–weather interactions drive colony collapse in a managed metapopulation of Arctic Terns (Sterna paradisaea)

Seabirds are considered bioindicators of bottom-up ecosystem processes, owing to seabirds’ dependence on marine prey. However, ground-nesting seabirds are susceptible to predation, which can limit their use as bioindicators. Machias Seal Island (MSI) supported the largest colony of Arctic Terns (Sterna paradisaea Pontoppidan, 1763) in the Gulf of Maine metapopulation, but prolonged breeding failure led ∼90% of terns to abandon the colony in 2006. We analyzed 12 years of food, weather, and predation data using logistic regression models to determine which had the strongest influence on breeding success. Food–weather interactions were important; under low rainfall, more euphausiids (northern krill, Meganyctiphanes norvegica (M. Sars, 1857)) in the diet increased breeding success, but euphausiids had a negative effect as rainfall became moderate or high. Predation by Herring Gulls (Larus argentatus Pontoppidan, 1763) increased following the cessation of lethal predator control; we identified a predation threshold of 25%, beyond which terns could not breed successfully. The collapse of MSI’s tern colony can be attributed entirely to gull predation. The breeding success of terns at MSI cannot be used as a bottom-up ecosystem bioindicator without accounting for predation. Managers of ground-nesting seabirds should consider predation and food as equally valid potential causes of population or reproductive declines.

Specific Composition and Distribution of Euphausiids in the Zone of the North Atlantic Subtropical Convergence

The euphausiid community was studied between the surface and upper bathypelagics northeast of the Azores Islands (46°–37° N, 14°–29° W) during the hydrological summer (August to September) of 1984 using a midwater trawl. In total, 11 species of euphausiids were identified. The two most common species, Meganyctiphanes norvegica and Thysanopoda acutifrons, were found in 43.8 and 40.8% of the samples, respectively. In terms of biomass, M. norvegica dominated the catches, its densities reaching up to 160 kg per trawling hour. The region is dominated by North Atlantic arctic–boreal and boreal species, followed by pan-oceanic meso- and bathypelagic, as well as tropical–subtropical and widespread tropical euphausiid species. The study area, which is situated in the zone of the Atlantic subtropical convergence, is subjected to seasonal migrations of the Subpolar Frontal system. Owing to this, a quasi-stationary ecotone is established, where arctic–boreal species are spatially replaced first by tropical–subtropical and then by widespread tropical elements. There are grounds to believe that the quasi-stationary ecotone lasts both for many years and from season to season. In the latter case, seasonal succession of species takes place, but the ecological structure of the ecotone remains almost constant.