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

Summary
No specific information is known about the stock structure, size, or spawning
locations of American shad in the Penobscot River or tributaries except that a
remnant population exists. The ASMFS has defined shad stocks based on the river
of origin and this is consistent with the State of Maine’s management definition. The
management definition of a stock reflects the harvest approach and lends support
for homing of shad to natal rivers.
The published literature available on stock identification based either on
meristic/morphology or genetic investigations indicates weak to weakly moderate
differentiation. As with alewife, these results are consistent with homing to natal
rivers but with a stray rate that appears to be large enough to provide effective
genetic interchange between populations (the range of the FST values reported was
0.001 to 0.006 which would correspond with between 41 and 250 genetically
effective migrants per generation using the approximation of FST ≈ 1/(1 + 4mNe)
described in Waples (1998)). These results have to be interpreted very careful given
the extensive use of exogenous fish transfers and/or hatchery supplementation of
shad in the Mid-Atlantic and New England states. Unpublished data from Bentzen
appears to indicate greater differentiation of shad populations in the Bay of Fundy
and the potential that shad gene flow is much lower (a maximum of 3 genetically
effective migrants per generation) than reported in the published literature.
We choose to consider all of the available sources to understand how shad
populations may (or may not) be structured. There are a number of genetic
investigations prior to the development of methods using microsatellites and there
appears to be a general expectation that the newer techniques will provide a
dramatically higher resolution of stock structure. This may not be true in all cases
and Waples (1998) cautions against drawing this general conclusion because the
biological process (migration and genetic drift) should similarly act upon all markers.
As discussed in the alewife section (see above), the interpretation of the genetic
information for management purposes are not straight forward especially in cases of
weak differentiation. This interpretation is further confounded by the lack
concordance of the existing published literature with preliminary results from
Bentzen. More genetic investigations are necessary to better understand the
population structure in Maine, however they should not be relied upon solely to
make management decisions in cases of low differentiation.
Management experience in Maine and elsewhere supports the use of exogenous
transfer of fish and hatchery supplementation to restore extirpated runs. These
management methods are widespread through most of the states with shad
restoration programs along the Atlantic Coast. Shad populations have been reestablished
using these methods together with improved fish passage and habitat
conditions (Merrimack, Pawcatuck, Connecticut and Susquehanna Rivers). Other
states have identified the importance of hatchery supplementation in sustaining
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