Jens Christian Holst

Horizontal distribution and overlap of planktivorous fish stocks in the Norwegian Sea during summers 1995-2006

Abstract The Norwegian Sea harbours several large pelagic fish stocks, which use the area for feeding during the summer. The period 1995–2006 had some of the highest biomass of pelagic fish feeding in the Norwegian Sea on record. Here we address the horizontal distribution and overlap between herring, blue whiting and mackerel in this period during the summers using a combination of acoustic, trawl and LIDAR data. A newly developed temperature atlas for the Norwegian Sea is used to present the horizontal fish distributions in relation to temperature. The centre of gravity of the herring distribution changed markedly several times during the investigated period. Blue whiting feeding habitat expanded in a northwestern direction until 2003, corresponding with an increase in abundance. Strong year classes of mackerel in 2001 and 2002 and increasing temperatures throughout the period resulted in an increased amount of mackerel in the Norwegian Sea. Mackerel was generally found in waters warmer than 8°C, while herring and blue whiting were mainly found in water masses between 2 and 8°C. The horizontal overlap between herring and mackerel was low, while blue whiting had a large horizontal overlap with both herring and mackerel. The changes in horizontal distribution and overlap between the species are explained by increasing stock sizes, increasing water temperature and spatially changing zooplankton densities in the Norwegian Sea.

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