140) and Cooper rivers (Pinopilis Dam at river km
80) which currently share the upriver reach of the
Santee River. In the Santee-Cooper river system.
the occurrence of ripe lower river adults and head-
water adults indicates that the dams divided the
original population, which was likely amphidro-
nious, not anadromous. Although only two divided
populations are known, other rivers may contain or
have contained in the past, an undetected upriver
populationsegmentPersistence of an upriver pop-
ulation segment is likely determined by many fac-
tors. particularly fishing mortality and availability
of spawning conditions. In particular, the Holyoke
Dam was built on the site of a large rapids near
South Hadley Falls, Massachusetts. This was not a
high waterfall, and historical records document
spearfishing for sturgeon in the rapids, evidence
that they were able to pass this potential obstruc-
tion. After Holyoke Dam was built in 1849, how-
ever, the upriver segment of the Connecticut River
population persisted unknown to biologists for al-
most 100 years.
In a divided population, upriver fish can move
downstream past the dam and join lower river fish,
but the reverse is difficult if not impossible Down-
stream movement of juveniles and adults has been
documented in the Connecticut River, where it ap-
pears to be a natural movement pattern timed to
occur with increased river discharge (Seibel 1993,
M. Kieffer & B. Kynard unpublished data). Al-
though some fish from the upriver segment move
downstream pass the dam, the pattern is variable
(as with other movement patterns of the species),
and will take years of study to decipher. Lower river
migrants can enter fishlifts at Holyoke Dam, but
passage is infrequent with 81 fish lifted from 1975 to
I995 (Holyoke Fishlift data). Three adult/year
(range, 0-13) are lifted during April-October with
most fish lifted singly on one day in the spillway lift
(B. Kynard unpublished data). Although this level
of passage insures gene flow, the small number of
fish lifted annually contributes little to total repro-
duction.
All evidence indicates spawning by the Connecti-
cut River upriver segment is the main source of
recruitment for the entire population. A spawning
run of lower river fish annually migrates upstreamlow Mactaquac Darn (river km 137-120, Dadswell
1979).
Adults likely have a behavioral drive to reach a
historical spawning area that is located at about riv-
er km 200 km or farther. When a dam blocks the
spawning migration, females apparently move as
far upstream as they can, then may or may not
spawn in the reach below the dam. Comparison of
maximum upstream spawning locations in rivers
throughout the range shows an almost 1:1relation-
shipofknown or suspected spawning locations and
location of the first dam (Figure 3). Exceptions are
rivers with dams located 300 km or farther up-
stream, i.e., the Savannah River (dam at river km
300). the Delaware River (dam at river 331 ),and the
Altamaha River (dam at river km 441). Only runs in
these rivers likely escaped blockage, either partially
or totally, bydams. Although Connecticut River
fish appear to be an exception (Figure 3), they also
spawn near river km 200 downstream of a dam (the
second dam located at river km 198, not the first
dam at river km 140). Merrimack River fish do not
attempt to move Farther upstream than river km 32,
and do not spawn directly downstream of Essex
Dam at river kin 46 (Kieffer & Kynard 1996). This
different migratory behavior is not likely related to
the small number of fish present because rare Cape
Fear River shortnose sturgeon continue to migrate
as Tar upstream as possible (e.g. to Lock & Dam No.
l at river km96,Moser & Ross 1994). All evidence
from other north-central rivers indicates the pre-
sent spawning pattern of the Merrimack River fish
is not likely the historical pattern. Closer examin-
ation of Kennebec River adults, where ripe males
were captured at river km 58 (Squiers et al.^7 ), may
show that prespawning adults gather at river km 58,
but spawn as far upstream as they can go (e.g., be-
low the dam at river km 69).
A dam built downstream of a spawning reach will
block the migration of anadromous spawners, but it
may divide amphidromous populations into an up-
river segment, with access to the spawning site, and
a lower river segment whose upstream spawning
and foraging migrations are blocked by the dam.
This is likely the situation in the Connecticut River
with Holyoke Dam at river km 140. It also may be
the situation in the Santee (Wilson Dam at river km