1999. How vertical fish distribution may affect survey results. -ICES Journal of Marine Science, 56: 345-360.At a selected location in the Barents Sea, acoustic observations were made and bottom-and pelagic-trawl data were collected over a 10-day period. A large proportion of fish were found in the acoustic bottom dead zone. Only during a few hours in the daytime were high acoustic values obtained, mainly consisting of large haddock ascending from the bottom. Small haddock and redfish dominated the acoustic recordings at night. The bottom-trawl catches showed greater variability and higher average catch rates during the day than at night, but the diel variations were relatively less pronounced than those of the acoustic recordings. The largest reductions in catch rates from day to night were observed in small haddock and redfish. This was consistent with the observation that these species were pelagic at night. The acoustic observations and the bottom-trawl catch rates were found to be correlated with diel cycles in observed light level and semi-diel cycles in current speed. The results are interpreted in terms of the variable availability and efficiency of the bottom trawl and of the variable availability of the echosounder. 1999 International Council for the Exploration of the Sea

Bivariate time-series of depth (pressure) and temperature with two-hour intervals from 19-data storage tags (DST) attached to adult Northeast Arctic cod (Gadus morhua L.) released from mid-March are analysed. Interplay between migration behaviour, physiological limitation factors, environment, and ecology in the Barents Sea is investigated using geometrical and statistical methods. Thermo-stratification is identified using r(t), the ratio between temperature and depth change over each record interval. Vertical activity, act(a), in relation to physiological limitations to pressure change is measured with the ratio of the daily depth range to the free vertical range. Cycles are detected by spectral analysis. The analysis supports conclusions from large-scale studies. Cod migrate along stable thermal paths until they reach a front area (or feeding ground), where the vertical activity increases and the records of depth, temperature, and r(t) change pattern, level and range. The (semi-) diurnal vertical migration (DVM) occurs seasonally in some fish, mainly in areas with large temperature gradient. In 11 out of 12 tags where DVM is detected, this occurs during summer and autumn. In seven out of 11 tags where semi-diurnal tidal cycles are detected in the temperature series together with a significant reduction in vertical migration, this occurs during April. In some tags diurnal or semi-diurnal cycles appear in both depth and temperature series. International Council for the Exploration of the Sea

Data (trawl, acoustic, CTD) from scientific surveys along the Norwegian coast, in the North Sea, in the Barents Sea, west of the British Isles, and in the Irminger Sea are used. The vertical density distributions of blue whiting, cod, haddock, redfish, saithe, capelin, and herring are described in relation to environmental conditions and physiological limitations. The first four surveys mainly cover banks and shelf areas shallower than 500 m. The last two surveys, aimed at blue whiting and redfish, mainly cover shelf edge and deep-sea areas with depths from 200 to 1300 m and from 440 to 3000 m. In regard to cod some information from data-storage tags is used.For physoclists the relative vertical profile of each acoustic sample i.e. acoustic-area backscattering coefficient (s A ), is expressed in terms of the relative pressure reduction level from seabed up to surface. Thus relative vertical profiles with different bottom depths are normalized and are made compatible for a discussion in terms of the free vertical range (FVR). This restriction to rapid vertical movement is evident in the physoclist species studied. For samples in the shelf area, the profiles show that blue whiting, haddock, saithe, cod, and redfish are mainly distributed within the bottom half of the water column. Some fish adapt to pelagic living especially in areas with high acoustic density and where the bottom is deep. Here a pelagically living fish is defined as an individual fish having a current free vertical range that does not include the seabed.For demersal fish, day and night relative vertical profiles are corrected for unequal day and night losses in the bottom acoustic dead zone, which is the zone near the seabed where echoes from fish cannot be discriminated from the sea bottom echo. Day and night samples are separated by the sun's passing 5 below the horizon. In most years evidence of diurnal vertical migration is found for all investigated species. In many cases of demersal fish there is a higher relative acoustic density (s A -values) in the mid-range of the bottom half of the water column in the daytime as opposed to the night-time. At night there is a degree of separation, one group of fish descends to aggregate near the seabed and another ascends. Inter-annual variations in the diel movement from different parts of the stock are discussed in relation to the inter-annual variations in age composition of the stock.

A definition of multiple bilinear time series models is given. Sufficient conditions are obtained for the existence of strictly stationary solutions conforming to the model, and a brief discussion of the first and second order structure is included.