Webjørn Melle

The challenge of the herring in the Norwegian sea: Making optimal collective spatial decisions

Norwegian spring-spawning herring, Clupea harengus harengus L., are long-lived multiple spawners subject to strong variation in recruitment success. They tend to adopt low-risk, preferred-conservat...

Summer distribution patterns and biomass estimates of macrozooplankton and micronekton in the nordic seas

Abstract The vertical and horizontal distribution patterns of zooplankton and micronekton were mapped during three research cruises in summers 1993, 1994 and 1995 by pelagic trawl and MOCNESS plankton net sampling. The distribution patterns have been related to the distribution of water masses and the distribution of planktivorous fish such as herring, Clupea harengus. Zooplankton biomass typically revealed a bimodal vertical distribution with high values in the surface layer and at 200-600 m depth. This subsurface maximum contained, among others, several species of macrozooplankton and micronekton such as krill, pelagic shrimps and mesopelagic fish. The dominant krill species Thysanoessa inermis, T. longicaudata and Meganyctiphanes norvegica are widely distributed in the Nordic Seas, extending from the coastal areas of southern Norway in the south to the subarctic and Arctic water masses in the northwest. Though widespread, highest abundances of M. norvegica were restricted to the warmer Atlantic waters....

Migration behaviour of norwegian spring spawning herring when entering the cold front in the Norwegian Sea

Abstract The distribution of Norwegian spring spawning herring when migrating to the feeding areas in the Norwegian Sea in spring was mapped by acoustic surveys in April 1995 and 1996. The schooling behaviour of the herring was recorded by a high-resolution sonar, and the swimming speed and swimming direction were quantified by tracking individual schools for up to one hour. In early April the herring migrated in the Norwegian Sea from the continental shelf off Norway, between 66° and 68°N, and westward to 2°W. When reaching the cold-water front, at about 0°, the herring turned southward along the front. During daytime the herring migrated in large schools at 300–400 m depth; at night they rose to surface and either dispersed or maintained schooling.

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

Abstract 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.

Effects of interactions between fish populations on ecosystem dynamics in the Norwegian Sea – results of the INFERNO project

The Norwegian Sea (NS) is the feeding ground for some of the largest fish stocks in the world, including Norwegian spring spawning (NSS) herring (Clupea harengus Linnaeus, 1758; Figure 1), blue whiting (Micromesistius poutassou Risso, 1827) and the Northeast Atlantic (NA) mackerel (Scomber scombrus Linnaeus, 1758). These planktivorous stocks have substantial spatial and dietary overlap (e.g. Nøttestad et al. 1997; Dalpadado et al. 2000; Kaartvedt 2000), and are often collectively referred to as the ‘pelagic complex’ in the Norwegian Sea. Due to their high abundances, they can potentially have a strong ecological impact on the ecosystem and each other (Skjoldal et al. 2004a). The NSS herring collapsed in the late 1960’s and rebuilt during the 1980’s (Dragesund et al. 1997). Following the herring collapse, high abundances of blue whiting were discovered in the Norwegian Sea (Misund et al. 1998), and it has been speculated that the blue whiting population increased concurrently with the collapse of the NSS herring (Skjoldal et al. 1993), but the evidence remains inconclusive (Daan 1980). Since the late 1980s the abundance of fish in the NS has increased steadily and this has increased the potential for interactions between the planktivorous stocks (Figure 2). This was the background for seeking funding for the INFERNO project ‘Effects of interactions between fish populations on ecosystem dynamics and fish recruitment in the Norwegian Sea’ submitted to the Research Council of Norway (RCN) in 2005. The main hypothesis to be addressed in the INFERNO project was that the planktivorous fish populations feeding in the NS have interactions that negatively affect individual growth, mediated through depletion of their common zooplankton resource. The project was funded and lasted for the period 2006 2009 and nine papers from the INFERNO project and associated research are presented in this thematic issue of Marine Biology Research. Many of the principal investigators of the project worked at the Institute of Marine Research (IMR), but the project also benefitted strongly from interactions and exchange of data and ideas with scientists from Russia (Alexander Krysov and Vladimir Zabavnikov), the Faeroe Islands (Jan Arge Jacobsen) and Iceland (Torstein Sigurdsson and Gudmundur Óskarsson). The international partners have participated actively in the project through project meetings and as co-authors of papers. During the project period the trend of a decreasing zooplankton biomass in the NS continued and the biomass now remains low (Figure 2). The fish biomass peaked in 2004 and has since decreased somewhat, but remains fairly high. The abundance of blue whiting increased until 2004, and the range of the horizontal distribution expanded in a northwesterly direction during this period. Strong year classes of mackerel from 2001 and 2002, together with increasing temperatures, resulted in an increased number of mackerel in the Norwegian Sea (Payne et al. 2012; Utne et al. 2012a). Furthermore, there were rather substantial changes in the migration pattern of herring during the study period and thus high interannual variability in horizontal overlap between the species. There was a relatively high spatial overlap between the species during the 1990s, with a southern centre of gravity (for all three species), but due to the northern displacement of Figure 1. Herring (Clupea harengus) represents an important component of the pelagic complex of the Norwegian Sea. Photographer: David Shale (www.deepseaimages.co.uk). Marine Biology Research, 2012; 8: 415 419

Large scale distrlbution of acoustical scattering layers at the Norwegian continental shelf and the Eastern Norwegian Sea

Abstract Vertical and horizontal distributions offish and micronekton were mapped in a zigzag pattern from Lofoten, northern Norway to Stadt, southwestern Norway, in March 1992. Pearlsides (Maurolicus muelleri) were present whenever depth was sufficient for daytime requirements (i.e. > 150–200 m). They performed diel vertical migrations, characterized by ascent at dusk, followed by nocturnal descent, and a dawn ascent before downward migration to day depth. Nocturnal descent among pearls ide has previously been explained by preference for warm water to speed up digestion, but this could not explain the results in this study as relevant vertical temperature gradients were minor. A deep scattering layer consisting of mesopelagic fish, pelagic shrimps and other macroplankton was present outside the shelf. This layer was located beneath ca 300 m by day, while the tendency to carry out diel vertical migrations varied strongly between water masses. Blue whiting (Micromesistius poutassou) associated with the lay...

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

Abstract 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.

Modeling emergent life histories of copepods

The distribution and population dynamics of zooplankton are affected by the interplay between currents, behaviour and selective growth, mortality and reproduction. Here, we present an individual based model for a copepod where life-history and behavioural traits are adapted using a genetic algorithm approach. The objectives were to investigate the importance of spatial and inter-annual variability in biophysical forcing and different predator densities on the adaptation of emergent life history traits in a copepod. The results show that in simulations with adaptation, the populations remained viable (positive population growth) within the study area over 100-year simulation whereas without adaptation populations were unviable. In one dimensional simulations with fixed spatial position there were small differences between replicate simulations. Inter-annual variability in forcing resulted in increased difference in fitness between years. Simulations with spatial-, but without inter-annual variability in forcing produced large differences in the geographic distribution, fitness and life history strategies between replicate simulations. In simulations with both spatial and inter-annual variability the replicates had rather small variability in traits. Increased predator density lead to increased day depth and avoidance of the lit upper waters. The model can be used for a range of different applications such as studying individual and population responses to environmental changes including climate change as well as to yield robust behavioral strategies for use in fully coupled end to end ecosystem models.