Michigan Department of Natural Resources
Fisheries Research Report No. 2030, 1996
Estimation of Lake Trout (Salvelinus namaycush) Abundance and Mortality Due to Sea Lampreys (Petromyzon marinus) and Fishing
in the Main Basin of Lake Huron, 1984-1993
Shawn Paul Sitar
Abstract.-Sea
lamprey (Petromyzon marinus) parasitism and overfishing have
been cited as the causes of the collapse of lake trout (Salvelinus
namaycush) populations in Lake Huron during the 1950s. The goal
of the ongoing lake trout rehabilitation program is aimed at reducing
sea lamprey abundance, controlling fishing mortality, and restocking
lake trout to establish self-sustaining populations. In order to
rehabilitate lake trout, the magnitude of sea lamprey parasitism and
fishing mortality must be determined in order to gauge progress
towards the goal. With reliable estimates of lake trout deaths due to
sea lampreys and fishery harvest, managers can adjust sea lamprey
control programs and fishing regulations to reach rehabilitation
objectives. I analyzed data on sea lamprey wounding of lake trout,
from 1984-1994, to assess patterns in sea lamprey parasitism
according to length of lake trout, geographic distribution, and year.
Lake trout population models, calibrated by statistical catch-at-age
analysis, were constructed to estimate abundance, fishery harvest,
and numbers killed by sea lamprey during 1984-1993 for the main basin
of Lake Huron.
Sea
lamprey wounding rates on lake trout increased with length of lake
trout and were higher in central Lake Huron than in the south for
lake trout >533 mm. Although sea lamprey wounding of lake trout
varied by year, no overall temporal trends were observed during
1984-1994 in the central and southern main basin of Lake Huron.
Comparisons with northern Lake Huron were not possible because of
insufficient data.
Abundance
of mature lake trout, an index of potential natural recruitment, was
estimated to be highest in southern Lake Huron and lowest in the
north. For lake trout ages most selected by sea lampreys and fishing
(ages 3-10), total annual mortality rates were highest in northern
Lake Huron and have exceeded the Great Lakes Fishery Commission
(GLFC) target maximum total annual mortality rate of 45% in all years
from 1984-1993. Total annual mortality rates in central and southern
main basin of Lake Huron were below the GLFC target maximum during
the same time period. Sea lamprey-induced mortality accounted for
most lake trout deaths in central and southern Lake Huron, whereas
commercial fishing and sea lamprey parasitism both were responsible
for the high number of lake trout deaths in the north. Recreational
fishing was not a significant source of lake trout mortality in the
main basin of Lake Huron.
The
lack of success in re-establishing self-sustaining populations of
lake trout in the main basin of Lake Huron was due in part to the
mismatching of reproductive biomass and spawning habitat. In central
and southern Lake Huron, lack of sufficient spawners and insufficient
spawning habitat are possible reasons that rehabilitation has not
progressed in these areas. In northern Lake Huron, where the amount
of spawning habitat is greatest, excessive sea lamprey-induced and
commercial fishing mortality at premature ages has limited the
abundance of spawners. In order to successfully rehabilitate lake
trout, total mortality rates must be reduced in northern Lake
Huron.