Operational Plan for the Restoration of Diadromous Fishes to the ...

During the development of the Operational Plan, DMR considered the likely success
of passive (natural recolonization) versus active (stocking) shad restoration. In
practice there are no examples of American shad populations in large rivers that
have been restored or are being restored solely by natural recolonization.
Restoration programs in Maine, New Hampshire, Massachusetts, Rhode Island,
Virginia, Maryland and Pennsylvania have relied on stocking adults, juveniles, or fry
to increase the abundance of shad. However, there are two examples in Maine
where natural recolonization has resulted in a static shad population. On the Saco
River where adult shad have simply been passed upriver the average run size for 16
years is 1435 fish. On the Narraguagus River, counts of adult shad passing the
single nonhydropower dam in each of three years (1987, 1989, 2007) have never
exceeded 200 fish.
DMR developed two models to explore scenarios that were likely to achieve shad
restoration to estimated historical abundance in the 40-50 year time frame of this
Operational Plan (8-10 shad generations). These models are simple, because there
is a paucity of demographic data for shad populations in Maine as well as for shad
populations along the Atlantic coast.
The first model explored the possibility of achieving the restoration objective by
natural recolonization; it uses a range of starting population sizes and intrinsic rates
of increase from one generation to the next (Table 5). The model indicates that
achieving Measure 7.1 (633,300 adult shad returns in 41-50 years) by natural
recolonization would require either a very high rate of increase for a prolonged
period or a very large starting population. For example, this target could be reached
with a starting population of 2500-5000 fish that on average doubled in abundance
every five years. This level of reproduction might be expected in bacteria, but not in
shad. If the rate of reproduction is reduced to 1.5 or 1.25, the size of the starting
population would have to be 40,000 to 135,000 fish. DMR is not aware of shad
populations in Maine or elsewhere that consistently achieve these levels of
reproduction nor of remnant populations on the east coast that have been greater
than about 1000 fish. DMR has concluded that reliance on natural recolonization is
not a valid management strategy.
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