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

We estimated cumulative dam mortalities for each migratory route for both smolts
and adults. For smolts we used data from Holbrook (2007) that estimated the
proportion of smolts choosing to migrate down the mainstem Penobscot as opposed
to using the Stillwater Branch. We then used these cumulative mortalities to
calculate life stage specific monthly survivals and applied this additional monthly
mortality to life stage specific monthly mortalities. These survivals were then used
as parameters in a population matrix model (Robertson 2005, Sweka 2008) that
estimated the population growth rate (lambda) as well as projected population size.
We then compared these results to a reach without dam-related mortality. We did
not consider amount or productivity of rearing habitat in the reaches, although we
present data from Rago (1986) that do. The purpose of this exercise was to
evaluate the effect of dams on the population dynamics of the sub-populations. We
also ran scenarios under optimistic marine survival conditions, where marine survival
was set in the model to obtain a stable population (lambda = 1), and then damrelated
mortality was inflicted on the population. This allowed us to estimate the
affects of the dams under ocean conditions that should otherwise allow viable subpopulations.
The cumulative effect of dam mortality varied across management reaches in the
Penobscot basin before and after the Penobscot Project dam removal and bypass
(referred to as “dam removal” from here on) (Figure 1). We assumed that the
Howland bypass will be as effective as dam removal, and set dam related mortality
to zero for this site after dam removal. Kenduskeag Stream represents a reference
state with no dams, while the other reaches have varying numbers of dams,
increasing with upstream distance. Note that some reaches improve more than
others with dam removal, particularly the Lower and Upper Piscataquis. However,
all values of lambda are well below one, indicating that all sub-populations are not
viable under current ocean survival conditions.
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