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Lake Pend Oreille Bull Trout population appears
stable |
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October 26, 2016 |
A moratorium on angling and lake trout
eradication efforts to address the threats of
overfishing and a voracious non-native species
appear to have stemmed the decline of bull trout
in Lake Pend Oreille, according to a recent
study.
The study compares the mean age, growth,
maturity and abundance from surveys done in 2006
and 2008 with a 10-year old study of bull trout.
It found that management had addressed current
threats to the fish – overfishing and an
invasive species—and in doing so, had increased
the likelihood of their long-term persistence in
the lake.
Bull trout have inhabited the Idaho lake since
the ice age and it once supported a
“world-renowned” bull trout fishery, according
to the study. The bull trout population in the
1998 survey was 12,134 and today that population
is holding steady 12,513 fish.
“We found that Bull Trout size and age
structure, mortality, growth, maturity, and
abundance were consistent with that of a
population that was rebuilding,” said lead
researcher Michael Hansen. “Collectively, these
findings suggest that management effectively
addressed current threats to increase the
likelihood of long-term persistence of the Bull
Trout population in Lake Pend Oreille.”
The adfluvial bull trout population – those that
live in lakes and spawn in streams – in Lake
Pend Oreille began to decline with the
construction of hydro-electric dams both
upstream in the Clark Fork River (Cabinet Gorge
Dam) and downstream in the Pend Oreille River (Albeni
Falls Dam).
Lake trout were introduced in 1925 and the
addition of opossum shrimp to the food supply
resulted in a 356 percent increase in the number
of lake trout between 1998 and 2005.
In 1966, the Idaho Department of Fish and Game
placed a moratorium on fishing for bull trout in
tributaries (with the exception of the Clark
Fork River), and a predator removal program for
lake trout using trap and gillnets was initiated
in 2006.
“Demographic Characteristics of an Adfluvial
Bull Trout Population in Lake Pend Oreille,
Idaho,” was published online October 7 in the
North American Journal of Fisheries Management,
http://www.tandfonline.com/doi/full/10.1080/02755947.2016.1209602.
Hansen and co-author Jonathan McCubbins were
with the College of Natural Resources,
University of Wisconsin-Stevens Point, when
conducting the study. Hansen is currently with
the US Geological Survey as a Supervisory
Research Fishery Biologist and McCubbins is a
fishery biologist in Glacier National Park for
the National Park Service. Other co-authors are
Joseph DosSantos, retired, aquatic program
manager with Avista Utilities, and Andrew Dux,
regional fishery manager with the Idaho
Department of Fish and Game.
Although the abundance of bull trout is similar
today as it was in 1998, other attributes –
size, age structure and growth, among others –
have changed.
Bull trout were larger in 1998 at 546
millimeters (21.5 inches) in length than in 2006
– 2008 at 494 mm (19.44 inches), while mean age
was similar at 7.6 years for the earlier survey
and 7.3 years for the later survey.
In the current study, bull trout grew faster to
age four, but slower afterwards, and were
generally longer up to age four and shorter
after age six than in 1998.
Male and female bull trout matured at a similar
age, but females grew faster than males, thereby
maturing at a larger size.
The rise in the abundance of lake trout between
1998 and 2005 coincided with a period when age-4
and older bull trout declined and the
consumption of kokanee, a primary food source
for both lake and bull trout, increased,
resulting in competition between the species for
a diminishing number of kokanee, the study says.
That may have led to the slower growth of ages
four – nine bull trout.
“If Lake Trout suppression allows abundance of
kokanee to increase,” the study says,
“interspecific competition would decline
further, thereby allowing Bull Trout growth to
increase in the future.”
The later survey found bull trout to be younger
at age of maturity and that, the study says, is
consistent with early growth, which would cause
fish to mature at an earlier age, “thereby
reducing the size and age structure.”
The shorter fish and declining age structure
suggests that bull trout “may be compensating
for increasing density of competitor species by
reducing age at maturity through increased early
growth,” the study says.
The shift to smaller fish, while maintaining
abundance, could be due to increased
recruitment, it added, although the number of
redds (nests) declined from 726 in 1998 to 584
in 2008 “as would be expected if age and size
structure shifted downward.”
Still, size and age structure, mortality,
growth, maturity and abundance “are consistent
with that of a population that is rebuilding,”
the study says.
That could be good news for anglers.
“Given the stable population status since the
time of harvest closure, the Bull Trout
population may be able to support limited angler
harvest,” Hansen said. “Providing Bull Trout
harvest opportunity for anglers is a management
goal for the Lake Pend Oreille fishery, and
reinstituting some level of harvest may be
justified if tributary habitat conditions and
population status remains stable in the future. |
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