Temporal and spatial variation in predation on juvenile herring (Clupea harengus L.) by Northeast Arctic cod (Gadus morhua L.) in the Barents Sea in 1984–1997

Geir Odd Johansen 0 0 Geir Odd Johansen: University of Bergen, Department of Fisheries and Marine Biology , PO Box 7800, N-5020 Bergen , Norway Previous studies indicate that predation by Northeast Arctic cod is an important cause of natural mortality of juvenile Norwegian spring-spawning herring in the Barents Sea. In this paper broad scale temporal and spatial variation in the predator-prey interaction between these two species in the Barents Sea was analysed. The analysis was based on cod stomach data from this area in 1984-1997. The predator-prey interaction between cod and juvenile herring in the Barents Sea was highly variable in time and space. On a yearly basis the most intense predation occurred in years with strong year classes of herring in the Barents Sea. Intensity of predation increased with decreasing abundance of capelin. Seasonal variation in intensity of predation on juvenile herring was low. Maps of the spatial distribution of cod feeding on herring illustrated a difference between the first and second halves of the year. In late winter and spring herring was consumed by cod in a restricted area in the southern part of the Barents Sea. In late summer, autumn and early winter herring was consumed by cod in a wide area south of the Polar Front. Climatic variation influenced the spatial distribution in the second half of the year. The results were discussed in the context of broad scale ecosystem dynamics in the Barents Sea. 1054-3139/02/040270+23 $35.00/0 - Introduction The Norwegian spring-spawning herring (Clupea harengus L.) has its nursery areas in fjords along the Norwegian coast and in the Barents Sea. After a growth period of about one to two years in the fjords or three to four years in the Barents Sea the juveniles migrate westwards and join the spawning stock in the Norwegian Sea (Dragesund, 1970; Dragesund et al., 1980; Hamre, 1990). High recruitment to the spawning stock and fishery depends on abundant year classes in the Barents Sea. In periods with high spawning stock abundance, the individuals descending from this area usually constitute about 80% of an individual year class when fully recruited to the spawning stock (Holst and Slotte, 1998). The Norwegian spring-spawning herring exhibits very high variation in year-class strength. Strong year classes seem to be correlated with periods of high inflow of Atlantic water into the Norwegian and Barents Sea (Hamre, 1994). Some of this variation may be an effect of the ecological interactions between cod (Gadus morhua L.), capelin (Mallotus villosus Mu ller 1776) and juvenile herring in the Barents Sea (Hamre, 1994). Year classes of Norwegian spring-spawning herring that are abundant at the 0-group stage in the Barents Sea can be strongly reduced during the first years of life (Barros and Toresen, 1998). Barros et al. (1998) found that more than 90% of this inter-cohort variation in mortality can be explained by the ratio between the abundances of capelin and juvenile cod. They suggested that when the capelincod ratio is low, the cod consume more juvenile herring than if the ratio is high, to compensate for the shortage of capelin. Earlier studies on the diet of cod in the Barents Sea (summarised by Bogstad and Mehl, 1997) emphasise capelin as the most important prey for cod in this area on a broad scale. However, studies by Mehl (1989) and Orlova et al. (1995) demonstrate that cod also has the potential of generating high predation mortality on juvenile herring in the Barents Sea on a narrow temporal and spatial scale. For example, the 19841985 year classes were strong at the 0-group stage but suffered heavy predation from young cod and did not contribute to the adult stock as expected (Mehl, 1989). The main purpose of this work is to identify important periods and geographical areas for predation on juvenile herring by cod in the Barents Sea based on stomach content data from cod covering the period 19841997. This is considered as vital information for future studies on cod as a cause of natural mortality of juvenile herring in the Barents Sea. The study will focus on broad scale temporal and spatial trends. The trends will be related to population dynamics and distribution of juvenile herring, and climatic variation in the Barents Sea. The analyses will cover the following aspects: (i) Year-to-year and seasonal variation in the occurrence of herring in cod stomachs. (ii) The relationship between occurrence of herring in cod stomachs and abundance of juvenile herring and capelin. (iii) Identification of the main geographical areas for predation by cod on juvenile herring and its seasonal variation. (iv) Climatic effects on the main geographical areas for predation by cod on juvenile herring. The results will be discussed in the context of broad scale ecosystem dynamics in the Barents Sea, during the period of interest. Materials and methods This study is based on the analysis of stomach content from individual Northeast Arctic cod in the Barents Sea from the period 19841997. The Barents Sea is defined as the area bordered by the continental slope towards the Norwegian Sea (west), a line between Spitsbergen and Franz Josef Land (north), Novaya Zemlya and a line from the northern tip of this island towards Franz Josef Land (east), and the coast of northern Norway and the Kola Peninsula (south) (Figure 1). It is a shallow continental shelf sea with an average depth of 230 m. The water masses are divided into a northern and a southern part by an east-west oceanic front at approximately 7576 N (the Polar Front). Temperature variations depend mainly on the activity and properties of the inflow of Atlantic water. The temperature regime is characterised by alternating periods with high and low temperature of variable duration. In the southern part, which is dominated by Atlantic water masses, the temperature at 50200-m depths fluctuates between 37 C throughout the year (Furevik, 2001). In the northern part, which is dominated by Arctic water masses, the temperature is mostly below 0 C. Details on the physical oceanography of the Barents Sea can be found in Loeng (1989, 1991), Tereshchenko (1996), and Loeng et al. (1997). The biological communities, with their function and productivity, are described by Hamre (1994), Loeng (1989), Sakshaug (1997), and Sakshaug et al. (1994). Stomach content data from cod were extracted from the joint IMR-PINRO stomach database at the Institute of Marine Research (IMR). This database includes stomachs sampled during both Norwegian and Russian regular surveys. Note that most of these surveys are not targeted for stomach sampling. Most of the cod were caught by bottom trawl, mainly in the first quarter of the year and in AugustOctober. A detailed description of the general survey methodology can be found in Jakobsen et al. (1997) and Lepesevich and Shevelev (1997). Note that some of the data are from surveys of pelagic fish and shrimp. The sampling design ha (...truncated)