221 
Abstract— Depth data from archival 
tags on northern rock sole (Lepidop- 
setta polyxystra ) were examined to 
assess whether fish used tidal cur- 
rents to aid horizontal migration. 
Two northern rock sole, out of 115 
released with archival tags in the 
eastern Bering Sea, were recovered 
314 and 667 days after release. Both 
fish made periodic excursions away 
from the bottom during mostly night- 
time hours, but also during particu- 
lar phases of the tide cycle. One fish 
that was captured and released in 
an area of rotary currents made 
vertical excursions that were cor- 
related with tidal current direc- 
tion. To test the hypothesis that 
the fish made vertical excursions 
to use tidal currents to aid migra- 
tion, a hypothetical migratory path 
was calculated using a tide model 
to predict the current direction and 
speed during periods when the fish 
was off the bottom. This migration 
included limited movements from 
July through December, followed by 
a 200-km southern migration from 
January through February, then 
a return northward in March and 
April. The successful application 
of tidal current information to pre- 
dict a horizontal migratory path not 
only provides evidence of selective 
tidal stream transport but indicates 
that vertical excursions were con- 
ducted primarily to assist horizontal 
migration. 
Manuscript submitted 29 July 2008. 
Manuscript accepted 19 December 2008. 
Fish. Bull. 107:221-234 (2009). 
The views and opinions expressed 
or implied in this article are those 
of the author and do not necessarily 
reflect the position of the National 
Marine Fisheries Service, NOAA. 
Evidence of the selection of tidal streams 
by northern rock sole ( Lepidopsetta polyxystra ) 
for transport in the eastern Bering Sea 
Daniel G. Nichol (contact author) 
David A. Somerton 
E-mail address for contact author: dan.nichol@noaa.gov 
Alaska Fisheries Science Center 
National Marine Fisheries Service, NOAA 
7600 Sand Point Way NE 
Seattle, Washington 98115 
Northern rock sole (Lepidopsetta 
polyxystra ) in the eastern Bering Sea 
off Alaska reportedly migrate from 
summer feeding grounds to deeper 
spawning grounds in winter (Shub- 
nikov and Lisovenko, 1964; Fadeev, 
1965). Although migration routes are 
poorly understood, at least some indi- 
viduals are thought to migrate long 
distances between summer and winter 
grounds. Shubnikov and Lisovenko 
(1964) suggested that some northern 
rock sole migrate from Unimak Island 
in the Aleutian Islands to areas north- 
east of the Pribilof Islands between 
April and July, covering a distance 
of more than 500 km. As a means to 
understand better how this migration 
occurs, we focus here on one poten- 
tial mechanism that northern rock 
sole use, that is, the opportunistic or 
exclusive use of selective tidal stream 
transport. 
Selective tidal stream transport is a 
mechanism by which aquatic animals 
can assist their horizontal migration 
by actively changing their vertical 
position in the water column, timed 
to coincide with tidal currents flow- 
ing in a preferred direction. Selec- 
tive tidal stream transport has been 
documented for a variety of aquatic 
animals (Forward and Tankersley, 
2001; Gibson, 2003) and has been 
extensively documented in the North 
Sea for European plaice (Pleuronectes 
platessa) (Kuipers, 1973; Rijnsdorp 
and van Stralen, 1985; Metcalfe et 
al., 1990; Fox et al., 2006; Metcalfe 
et al., 2006). Even before the use of 
electronic fish tags, it was recognized 
that some flatfish species selectively 
leave the bottom during periods of 
a preferred tidal current direction 
(De Veen, 1967; Harden Jones et al., 
1979). More recent work with archi- 
val tag data, used in combination 
with tide data (e.g., Hunter et ah, 
2004b), has highlighted the impor- 
tance of both tide and diurnal fac- 
tors in flatfish migration. For plaice 
and other flatfish species, the vertical 
movements (i.e., selective tidal stream 
transport) vary diurnally; most excur- 
sions are made away from bottom dur- 
ing the night (De Veen, 1967; Cadrin 
and Westwood, 2004; Hunter et ah, 
2004b; Walsh and Morgan, 2004). 
Juvenile flatfishes (Champalbert et 
ah, 1992; Burrows, 1994), including 
northern rock sole (Hurst and Duffy, 
2005), also show some preference for 
nighttime activity. 
When flatfishes use selective tidal 
stream transport, the timing of verti- 
cal excursions away from the bottom 
can be combined with predictions of 
tidal current velocity to construct 
hypothetical migration trajectories 
(Arnold and Holford, 1995). For this 
estimation to be successful, verti- 
cal excursions need to be accurately 
identified and all directed horizontal 
movement must be restricted to those 
off-bottom periods. For species known 
to use tidal currents, selective tidal 
stream transport may not be used in 
all habitats because the strength or 
direction of the currents may be un- 
suitable for migration. For example, 
when European plaice inhabit areas 
of weak tidal currents in the North 
Sea, they migrate horizontally, stay- 
ing near the seafloor where there is 
