Trapnet Mortality and Vital Statistics of Lake Whitefish in Green Bay and Northwestern Lake Michigan
File(s)
Date
1989-08Author
McComb, Thomas N.
Publisher
University of Wisconsin-Stevens Point, College of Natural Resources
Metadata
Show full item recordAbstract
A total of 28,744 lake whitefish were tagged at North
and Moonlight bays (NMB), Whitefish Bay, and Peshtigo Reef,
Wisconsin, and Big Bay de Noc (BBN) and the Bark and Cedar
Rivers (BCR) area, Michigan, and 205,610 lake whitefish were
measured in 1975 through 1985 to determine vital statistics
and fit predictive models. Also, 31 trapnet lifts were
observed and 10 holding net experiments were performed in
Green Bay and Lake Michigan to assess short-term mortality
of sublegal (less than 432 mm long) lake whitefish caught in
trapnets.
Two discrete stocks were associated with spawning areas
at NMB and BBN, and fish from the NMB stock were migratory,
whereas fish from the BBN stock were not.
Data suggested that more than 75% of lake whitefish of
sublegal size (less than 432 mm) survive a trap net capture
and release, although storms occurring while fish are in the
nets may reduce survival. Crowding in the lifter bag and
long sorting times were primary factors reducing survival.
The harvest from the NMB stock was spread over ages 3-
5 primarily, but the harvest from the BBN stock was
predominantly age 3 and 4 fish. The 1972, 1975, 1978, and
1980 year-classes were the most abundant year-classes in the
1975-1985 harvest from the NMB stock. In the BBN stock, the
1976, 1978, and 1980 year-classes were more abundant than
others in the harvest from 1979-1985.
Fish from the BBN stock were longer at annulus
formation than fish from the NMB stock up to age 3; after
that fish from the NMB stock were longer.
Fish from the BBN stock were estimated to be exploited at
a higher rate than those from the NMB stock (51 and 41% for
BBN and NMB, respectively) in 1976-1985, and estimated total
mortality rate was lower for fish from the NMB stock (0.70)
than for fish from the BBN stock (0.80). Fish of age-7 and
older had lower mortality rates than fish of younger ages.
Fishing accounted for 64% of total mortality in the BBN
stock, and 59% in the NMB stock.
Average annual abundance and recruitment estimates were
3 million fish of age 3.5 and older and 1.8 million recruits
for the NMB stock, and 1.4 million fish of age 3.5 and older
and 1 million recruits for the BBN stock. Quotas were
similar to recent annual yields. Equilibrium yield per
recruit for both the NMB and BBN stocks was less at the
current minimum size limit (432 mm) than the initial weight
of recruits, and as the simulated minimum size limit
increased, yields decreased and spawning biomass increased.