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Fishery Bulletin 107(2) 
Month 
Figure 6 
Bottom depths derived from an archival tag attached to a female northern rock sole 
(Lepidopsetta polyxystra) compared with bottom depths derived from the model- 
estimated fish migration path and bathymetric data. The fish was at liberty in 
the eastern Bering Sea from 2003 to 2004 (northern fish). 
feeding strategy, albeit on a larger spatial scale. The 
so called “feeding months” for northern rock sole in the 
eastern Bering Sea reportedly occur during summer 
when they disperse across the shelf after aggregating 
for spawning during winter and spring (Fadeev, 1965; 
Shubnikov and Lisovenko, 1964). This feeding period 
may well be represented by the first five months that 
the northern fish was at liberty ( July-November), dur- 
ing which migration was less frequent and not in a 
consistent direction. 
As suggested by the migration path presented here, 
at least some adult northern rock sole undergo vertical 
excursions for the purpose of tidally assisted horizontal 
migration, as opposed to vertical movements into the 
water column for feeding or spawning. Both juvenile 
and adult northern rock sole feed during daylight hours, 
but rarely during the night (Corcobado Onate, 1991; 
Hurst et al., 2007) when off-bottom swimming occurs. 
Unlike other eastern Bering Sea flatfish species such 
as arrowtooth flounder ( Atheresthes stomias), which 
feed high in the water column (Yang, 1995), northern 
rock sole feed close to the bottom. Northern rock sole 
feed almost exclusively on benthic invertebrates, such 
as polychaete worms and other marine worms (Corco- 
bado Onate, 1991; Lang et al., 1995; McConnaughey 
and Smith, 2000). In addition, northern rock sole likely 
do not leave the bottom to spawn because, along with 
their congener, southern rock sole (L. bilineata), they 
are the only northeast Pacific flatfishes to spawn de- 
mersal adhesive eggs (Matarese et al. 1989; Stark and 
Somerton, 2002). 
If most northern rock sole prefer to undergo vertical 
excursions during the night, winter offers greater op- 
portunity to travel in a preferred direction because of 
the increased hours of darkness. For the northern fish, 
vertical excursions were most frequent during January, 
when it travelled in a southerly direction. Southerly 
directed tidal currents were sometimes available at two 
different periods within a single night because of the 
semidiurnal nature of the tides (e.g., a full clockwise 
rotation of tidal currents every 12 hours). Hunter et 
al. (2004b) noted similar nocturnal behavior for Euro- 
pean plaice in the North Sea during winter when two 
’’transporting tides” sometimes occurred within a night 
because of the longer periods of darkness lasting up to 
15 hours. 
The northern rock sole vertical movements examined 
here appear shorter in both duration and extent (Table 
1) in comparison with other flatfish for which vertical 
behavior has been studied, with the exception of yellow- 
tail flounder ( Limanda ferruginea ) whose excursions av- 
erage 1.5 hours and 6 m off bottom (Cadrin and Moser, 
2006). Some European plaice ( P . platessa ) in the North 
Sea reportedly spend from 6 hours to 12 hours a night 
swimming in midwater during winter months (Hunter 
et al., 2004b). Common sole (Solea solea) in the North 
Sea are thought to use the upper half of the water col- 
umn for selective tidal stream transport during which 
