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Harvest was a function of the number of
yearling brown trout stocked the year before from the early 1970s to
1984. From 1985 to 1991, however,
harvest fluctuated and declined to near zero in some years, despite consistent
stocking rates that averaged near 90,000 yearlings per year.
The leading hypothesis for the decline
was that predation rates had increased on the stocked trout. Predation increases were caused by recovery
of walleyes and other piscine predators and fish eating birds.
Vulnerability to predation could be mediated by:
Strain of brown trout
Time of stocking.
We chose to evaluate three strains of brown trout to determine whether there were
differences in post-stocking performance of these strains.
Later in the study we compared shoreline
stocking with offshore stocking.
During the course of the study, we learned that alewife abundance was
lowest in early spring and increased to peak levels as adult spawners entered
Thunder Bay in June.
Thus, the timing
of stocking during spring could effect subsequent survival.
For the study period, we stocked all
yearling brown trout during the June peak in alewife spawner abundance.
Pictured are mature Seeforellen (top) and Plymouth Rock (bottom) strain brown
trout at age 2.
Seeforellen strain was significantly
heavier at age than Wild Rose at ages 3 and 4, but not at age 2.
strain reached lower
average weights at age 2 than the other strains, but sample sizes of age 3+
Plymouth Rock strain were too small to compare with the other strains.
Brown trout appear to be relatively short-lived in Thunder Bay.
94% of all observed returns were ages 1-3.
Age 4 and older fish made up only 0.9% of the observed catch.
No Plymouth Rock strain older than age 3
The comparison of boat and shore stocking
methods was done when alewife numbers were low.
Returns from both test groups were low.
There were no significant differences
between test groups.
Returns of neither test group exceeded 25 per 60,000 stocked.
Return rates declined 89% from that of the strain comparison study.
The more successful strains (Wild Rose and Seeforellen) were used in the Boat vs..
Remember, the stocking window was
established to capitalize on seasonal abundance of spawning alewives.
Annual abundance of adult alewives was
indexed in most years using 1.5 and 2” mesh gillnets.
The catch rate rose from 1990 to 1994, remained relatively high to 1995, but had
declined to below the 1990 level by 1998.
This trend in alewife abundance parallels annual harvest of brown trout in Thunder
Bay, with peak in harvest from 1993-1995, following years of high alewife
Great Lakes fish communities have changed
since the 1960s when alewives were overabundant and surplus production wind
rowed on the beaches. Now surplus
production is pretty well tapped by a variety of predators.
Although population estimates and
consumption rates of predators were not measured, incidence of brown trout in
fish stomachs was highest in years when alewife numbers were low. The chief predator fish of stocked brown
trout were walleyes, but brown trout were observed in stomachs of several fish
As a “last gasp” effort, the MDNR will
evaluate the utility of stocking smaller numbers of larger yearlings in the
fall by comparing their performance with that of spring yearlings.
If this does not work, perhaps it will be time to think of an alternative name
for Alpena’s Brown Trout Festival.