228 
Fishery Bulletin 107(2) 
Available Selected 
current current 
Sept 03 
Available Selected 
current current 
Jan 04 
Figure 4 
Monthly plots from September 2003 through April 2004 of predicted near-bottom 
tidal current directions for periods when a female northern rock sole ( Lepidopsetta 
polyxystra ) (the northern fish) underwent nightly vertical excursions (selected current), 
compared to all the available nightly near-bottom tidal current directions (available 
current). Arrow directions indicate the direction of the current and arrow lengths 
indicate the total number of hours spent in each direction. The available current is 
scaled from 0 to 50 hours (see upper leftmost diagram) and the selected current is 
scaled from 0 to 10 hours 
migration path position and the recovery position is 
considerably higher than values of 0.6 BL/s reported for 
European plaice and Japanese flounder (Pai'alichthys 
olivaceus) during tidally assisted migration (Kawabe et 
ah, 2004; Metcalfe et ah, 1990). We offer several expla- 
nations. First, estimates of current speed in the model 
were lower than actual current speeds, a bias that 
would inflate our estimated swimming speed. Second, 
the northern fish may have been capable of maintaining 
a more consistent direction; that is, more aligned to the 
destination of the migration than the current directions. 
We assumed the fish traveled in the average direction of 
the current during each excursion event. However, if the 
fish had a predetermined destination and the ability to 
navigate, it may have actively deviated slightly from the 
average current direction. More efficient overall swim- 
ming directions would have reduced the overall dis- 
tance traveled, and again would have resulted in lower 
swimming speeds necessary to complete the migration 
path. Finally, some vertical excursions may not have 
been identified because of the frequency of collection of 
archival tag data (0.5 hour or 1 hour), or the fish may 
have migrated toward its destination without undergo- 
ing vertical excursions (e.g., it swam while near the bot- 
tom). Evidence of this type of movement was clear for 
the southern fish, which appeared to use tidal currents, 
but its horizontal migration was not limited to periods 
of vertical excursions. The northern fish may also have 
migrated without undergoing vertical excursions, but 
because the bathymetric terrain was fairly flat on the 
central eastern Bering Sea shelf, such movements could 
not be detected. 
Although we demonstrate the preference for night- 
time vertical activity for only two adult northern rock 
