Size of eggs and nauplii of calanoid copepods in the Barents Sea; influence of environmental and maternal factors

During the years 1986 to 1988 eight cruises were conducted in the central and western parts of the Barents Sea. The cruises, together spanning the time period from January to July, covered most developmental stages of the phytoplankton spring bloom. Temperature, salinity, nutrients, chlorophyll a, and zooplankton were generally sampled at each station. Size frequency distributions of eggs and nauplius stages of the copepods Calanus finmarchicus, C. glacialis, C. hyperboreus, Pseudocalanus sp., and Metridia sp., are given. Cluster analysis and a subjective method of classification grouped the sampling stations into environmental regions on the basis of environmental variables. The environmental variables were related to size of eggs (diameter) and nauplii (carapax length) of C. finmarchicus by a linear method of canonical ordination (Redundancy Analysis). A significant overall difference in size of eggs and nauplii between environmental regions was demonstrated. The size of eggs and nauplius stages N1 to N3 were positively correlated with distance from the Norwegian Sea, and the size of the females were positively correlated both with distance from the Norwegian Sea and egg size. Egg size was negatively correlated with temperature. lt was concluded that advection of smaller sized adult females into the Barents Sea gave rise to a population of small sized eggs and young nauplii. The maternal effect may be a direct process where bigger females spawn bigger eggs, or more indirectly where the females spawn bigger eggs at low temperatures. The feeding stages N4 and N6 were positively correlated with chlorophyll content (assumed to represent food supply), and negatively correlated with temperature. Chlorophyll and temperature were inter-correlated, and the contribution of each particular variable to the relationship could not be estimated. Thus, after the nauplii start to feed, food supply and/or temperature have greatest influence on size, and soon mask size patterns due to maternal effects

Navigation mechanisms of herring during feeding migration: the role of ecological gradients on an oceanic scale

The feeding migration of Norwegian spring-spawning herring was studied in relation to prey abundance and environmental factors that may affect their feeding migration. Temperature, salinity, chlorophyll a, nitrate concentration, abundance of Calanus finmarchicus, zooplankton biomass, acoustic data on herring and trawl samples were collected during four basin-scale surveys in the Norwegian Sea from April to August 1995. Herring abundance was positively associated with the overwintering population of C. finmarchicus. We suggest that spatial gradients and temporal dynamics of the seasonal ascent of the C. finmarchicus overwintering generation provide stimuli for and affect the feeding migration of herring. The clockwise migration pattern of herring, observed during the 1990s, can be explained by delayed ascent and development of C. finmarchicus toward the west and north in the Norwegian Sea. We further suggest that herring leave a specific area before their zooplankton prey is depleted. The first generation of C. finmarchicus likely has a minor influence in directing the herring feeding migration due to the low abundance of older stages available as prey. The feeding migration was constantly directed toward colder water, and temperature probably has a secondary effect on herring distribution

Zooplankton reproduction in the Barents Sea vertical distribution of eggs and nauplii of Calanus finmarchicus in relation to spring phytoplankton development

Variable influence of ice melting on water column stabilization causes a large variation in timing of the spring phytoplankton bloom in the central Barents Sea. During two cruises in April 1986 and May-June 1987 situations were encountered that ranged from early bloom stage in Atlantic water not influenced by ice to late bloom stage in meltwater regions. In the present paper we examine the reproduction of Ca/anus finmarchirns (Gunnerus) against the background of this large natural variation in the temporal and spatial distribution of phytoplankton food, with emphasis on the vertical distribution of spawning females, eggs, and nauplii. The vertical distribution of the females indicated a shift from shallow spawning at the early bloom stations to deeper spawning in the pycnocline region at the late bloom stations. The females were generally concentrated in the layers with high concentrations of phytoplankton. The vertical distribution of eggs at the bloom and late bloom stations showed the same pattern as chlorophyll, either uniformly distributed in the upper mixed layer or concentrated as a sharp maximum in the region of the pycnocline and the deep chlorophyll maximum. The distribution of nauplii corresponded closely to the distribution of the eggs at these stations, suggesting little or no sinking of the eggs and hatching of the nauplii at depths with a rich food supply. The eggs at the ea rly bloom stations had a deeper distribution, although spawning apparently took place in the shallow part of the water column. This suggests a high rate of sinking of the eggs. Nauplii hatching from them probably encountered low food concentrations at great depths. The success of early spawning is probably limited through this. The indicated high sinking rate at the ea rly bloom stations in contrast to the low rate of sinking at the bloom and late bloom stations could reflect a difference in egg density owing to different feeding conditions. Increased water viscosity caused by mucus from phytoplankton could also have contributed to the lower sinking rate at high phytoplankton concentrations. Wind-induced vertical mixing did not play a major role in governing egg distributions

Modelling secondary production in the Norwegian Sea with a fully coupled physical/primary production/individual-based Calanus finmarchicus model system

The copepod Calanus finmarchicus is the dominant species of the meso-zooplankton in the Norwegian Sea, and constitutes an important link between the phytoplankton and the higher trophic levels in the Norwegian Sea food chain. An individual-based model for C. finmarchicus, based on super-individuals and evolving traits for behaviour, stages, etc., is two-way coupled to the NORWegian ECOlogical Model system (NORWECOM). One year of modelled C. finmarchicus spatial distribution, production and biomass are found to represent observations reasonably well. High C. finmarchicus abundance is found along the Norwegian shelf-break in the early summer, while the overwintering population is found along the slope and in the deeper Norwegian Sea basins. The timing of the spring bloom is generally later than in the observations. Annual Norwegian Sea production is found to be 29 million tonnes of carbon and a production to biomass (P/B) ratio of 4.3 emerges. Sensitivity tests show that the modelling system is robust to initial values of behavioural traits and with regards to the number of super-individuals simulated given that this is above about 50,000 individuals. Experiments with the model system indicate that it provides a valuable tool for studies of ecosystem responses to causative forces such as prey density or overwintering population size. For example, introducing C. finmarchicus food limitations reduces the stock dramatically, but on the other hand, a reduced stock may rebuild in one year under normal conditions

Vertical distribution of fish and krill beneath water of varying optical properties

The distribution of acoustical scattering layers of fish and krill changed markedly in concert with fluctuating fluorescence (chlorophyll a) in upper waters, possibly due to a varying 'shadow effect'. Beneath clear waters on the outer Norwegian shelf (about 300 m depth), mesopelagic fish (Maurolicus muelleri) were located at approximately 150 to 200 m by day. Krill (mainly Thysanoessa inermis) was primarily found below the mesopelagic fish and above planktivorous demersal fish (Norway pout Trisopterus esmarkii). The vertical distributions changed abruptly across a front into water with less Light penetration associated with increased fluorescence (chlorophyll a). Mesopelagic fish ascended by about 100 m accompanied by a rise of krill. Demersal fish left the benthic boundary zone, with ascending Norway pout foraging in the lower part of the krill layer. We suggest that the intermediate light conditions inside the front provided an 'antipredation window' (sensu Clark & Levy 1988: Am Nat 131:271-290) and thereby favorable feeding conditions for the planktivore. These results indicate that properties of upper layers may impact plankton and fish distributions and their predator-prey interactions throughout the water column on continental shelves

Dyreplankton og dets reproduksjon i Barentshavet - grunnlagsmateriale for vurdering av mulige skader av oljesøl

Rapporten gir datagrunnlag for en vurdering av konsekvensene av oljesøl for næringsgrunnlaget for fisk. Horisontalfordeling, gytetidspunkt og gyteadferd hos dyreplankton og vertikalfordeling av egg og larver er beskrevet. Tidspunkt og områder der sårbare stadier finnes er i noen grad identifisert

Reproduction of zooplankton in relation to initiation of spring phytoplankton bloom in the Barents Sea

The phytoplankton spring bloom development and zooplankton reproduction were investigated in April 1986 in the central and northern Barents Sea. Different stages of phytoplankton bloom development were found, reflecting differences in water column stability and illumination due to ice melting. Eggs and nauplii of Calanus, Pseudocalanus, Microcalanus and Oithona spp. occurred mostly in the upper 50 m without any pronounced maxima in their vertical distributions. The total numbers of eggs and nauplii of Calanus finmarchicus and C. glacialis tended to be higher at bloom stations than at pre-bloom stations, and also to show a general increase during the investigation period. The start of spawning was estimated to early February and was not influenced by the phytoplankton bloom development. Neither did the developmental state of the copepodite population seem to be influenced by the bloom development. The spawning activity, however, seemed to have a maximum coinciding with the early phase of the bloom development

A new compilation of stomach content data for commercially-important pelagic fish species in the Northeast Atlantic

There is increasing demand for information on predator–prey interactions in the ocean as a result of legislative commitments aimed at achieving sustainable exploitation. However, comprehensive data sets are lacking for many fish species and this has hampered development of multispecies fisheries models and the formulation of effective food-web indicators. This work describes a new compilation of stomach content data for five pelagic fish species (herring, blue whiting, mackerel, albacore and bluefin tuna) sampled across the northeast Atlantic and submitted to the PANGAEA open-access data portal (www.pangaea.de). We provide detailed descriptions of sample origin and of the corresponding database structures. We describe the main results in terms of diet composition and predator–prey relationships. The feeding preferences of small pelagic fish (herring, blue whiting, mackerel) were sampled over a very broad geographic area within the North Atlantic basin, from Greenland in the west, to the Lofoten Islands in the east and from the Bay of Biscay northwards to the Arctic. This analysis revealed significant differences in the prey items selected in different parts of the region at different times of year. Tunas (albacore and bluefin) were sampled in the Bay of Biscay and Celtic Sea. Dominant prey items for these species varied by location, year and season. This data compilation exercise represents one of the largest and most wide-ranging ever attempted for pelagic fish in the North Atlantic. The earliest data included in the database were collected in 1864, whereas the most recent were collected in 2012. Data sets are available at doi:10.1594/PANGAEA.820041 and doi:10.1594/PANGAEA.826992

Ecology of mesozooplankton across four North Atlantic basins

A comparative study of the mesozooplankton in four North Atlantic basins is presented. During a trans-Atlantic expedition with R/V G.O. Sars in May and June 2013, the Norwegian Sea, Iceland Sea, Irminger Sea and Labrador Sea was surveyed twice on a round trip from Bergen, Norway to Nuuk, Greenland. Mesozooplankton samples of biomass, species composition and vertical distribution were obtained with WP2 and MOCNESS plankton nets, in addition to in situ data obtained from a Video Plankton Recorder (VPR) and Optical Plankton Counter (OPC) mounted on a submersible towed vehicle. Size-fractionated biomass samples showed that the Norwegian Sea had the highest biomass of small mesozooplankton (180–1000 μm), while Irminger and Iceland Seas had the highest biomass of the medium (1000–2000 μm) and largest (>2000 μm) size fractions, respectively. The Icelandic Sea large fraction biomass was dominated by Amphipods, Chaetognaths, Krill and Calanus hyperboreus. The Labrador Sea had the lowest total mesozooplankton biomass. A total of 9 different species/groups were found to comprise the 5 most numerically dominant species/groups across all basins, with Oithona spp. being the most common genus in all basins. C. finmarchicus was, as expected, found to be the most numerically common species of the Calanus complex in all basins, but the stage composition varied markedly between basins with young copepodite stages dominating only in the Labrador and central Norwegian Seas. In terms of both abundance and biomass, the Iceland Sea had a higher fraction of dominating mesozooplankton distributed below 200 m. The highest average particle density per 25 m interval was registered in the Norwegian Sea during daytime between 25-50 m (OPC data). In the Labrador and Irminger Seas, total estimated particle densities in the upper 50 m were lower and the particle densities peaked at intermediate sizes (1–3 mm). In all basins there were differences in the particle densities estimated between day and night. Based on VPR data, the Irminger and Iceland Seas had the highest density of copepods registered in the upper 200 m, whereas in the Labrador Sea, the highest average copepod densities were registered at depth. Densities of gelatinous organisms were at least an order of magnitude higher in the Labrador and Irminger Seas than in the Iceland Sea.acceptedVersio