Fish migration in oscillating stratified water masses
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Abstract Time series records from data storage tag (DST) attached on Northeast Arctic cod (Gadus morhua L.) and Greenland halibut (Reinhardtius hippoglossoides) are investigated in connection with hydrodynamic and hydrographic features at specific locations to obtain spatial and temporal variation on fish migration patterns, which may change the availability of fish to survey gears. From spring to autumn, some Greenland halibut, at 500-800 m depth, were exposed to a persistent diurnal fluctuation of temperature between subzero and 5.5±C. Tidally induced topographic trapped waves with diurnal (K1) frequency were indicated in some region along the Barents Sea escarpment, where the transition zone between warm Atlantic water (AW) and cold Norwegian Sea Arctic Intermediate Water (NAIW) is at 500-700 m. An asymmetry in the temperature diurnal pattern showed a gradual increase followed by a rapid drop. This event may be related to the displacement mechanism of stratified water masses together with the fish diurnal migration pattern in synchrony with tidal motion. The pattern indicates that fish migrated seawards off the slope and was pelagic part of the day, thus unavailable to bottom trawl. Up to three weeks in April, some tagged cod had low vertical activity at a depth exposed to semidiurnal tidal fluctuations of the transition layer between 100-200 m, where cold coastal waters flow on top of warm Atlantic waters. Along the southern coast of the Barents Sea, strong semidiurnal tides can generate baroclinic coastal Kelvin waves causing vertical motion of the pycnocline, thus semidiurnal fluctuation of temperature was observed. Keywords: bottom trapped waves, data storage tag, diurnal vertical migration, Kelvin waves, NE Artic cod, NE Arctic Greenland halibut, spawning activity, temperature gradient.
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