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Lake Whitefish in Lake Champlain after Commercial
Fishery Closure and Ecosystem Changes
Seth J. Herbst
a c
a
, J. Ellen Marsden & Stephen J. Smith
b
a
Rubenstein School of Environment and Natural Resources, University of Vermont, 81
Carrigan Drive, Burlington, Vermont, 05405, USA
b
U.S. Fish and Wildlife Service, Lake Champlain Fish and Wildlife Resources Office, 11
Lincoln Street, Essex Junction, Vermont, 05452, USA
c
Department of Fisheries and Wildlife, Michigan State University, 13 Natural Resources
Building, East Lansing, Michigan, 48824-1222, USA
Available online: 27 Dec 2011
To cite this article: Seth J. Herbst, J. Ellen Marsden & Stephen J. Smith (2011): Lake Whitefish in Lake Champlain after
Commercial Fishery Closure and Ecosystem Changes, North American Journal of Fisheries Management, 31:6, 1106-1115
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North American Journal of Fisheries Management 31:1106–1115, 2011
C American Fisheries Society 2011
ISSN: 0275-5947 print / 1548-8675 online
DOI: 10.1080/02755947.2011.641068
ARTICLE
Lake Whitefish in Lake Champlain after Commercial Fishery
Closure and Ecosystem Changes
Seth J. Herbst1 and J. Ellen Marsden*
Rubenstein School of Environment and Natural Resources, University of Vermont, 81 Carrigan Drive,
Burlington, Vermont 05405, USA
Stephen J. Smith
Downloaded by [J. Ellen Marsden] at 07:04 28 December 2011
U.S. Fish and Wildlife Service, Lake Champlain Fish and Wildlife Resources Office, 11 Lincoln Street,
Essex Junction, Vermont 05452, USA
Abstract
Lake whitefish Coregonus clupeaformis were commercially fished in Lake Champlain until the 1913 fishery closure
in U.S. waters. The only study of lake whitefish in the lake had been done in the 1930s. Our goals were to compare
current biological parameters with historical information and to determine distribution and spatial differences in
larval densities, with an emphasis on locating current spawning grounds, to gain insight on the current population
in Lake Champlain. Adult lake whitefish (N = 545) were collected from 2006 to 2010 by using gill nets and trawls
focused in the Main Lake. Larvae were collected extensively lakewide and intensively at Wilcox Cove and Rockwell
Bay with an ichthyoplankton net. Population attributes (size, age, and sex composition; and growth, condition, and
mortality) were typical of unexploited populations, as there was a wide range of length-classes (126–638 mm total
length) and age-classes (1–26 years). Lake whitefish from the Main Lake had a high condition factor, and growth
parameters were comparable with those of fish collected in the 1930s. Lake Champlain lake whitefish had greater
asymptotic lengths than generally documented for the species. Larvae were found at sites throughout the Main Lake,
and larval densities were among the highest recorded for the species (maximum = 2,558 larvae/1,000 m3); however,
no lake whitefish were collected on the two historically documented spawning grounds. Lake whitefish in the Main
Lake demonstrate characteristics of an unexploited population; however, evidence of spawning is absent or rare in
portions of their historic range where habitat has been altered.
Historically, Lake Champlain supported a commercial shoreline seine fishery in the fall, focused in and near Missisquoi Bay
in the north and Larabee’s Point in the south. Overall harvest
and license sales peaked from 1895 to 1912. Lake whitefish
Coregonus clupeaformis were an important part of that commercial fishery and were harvested with shoreline seines during
the fall spawning season. Annually, 41–95 fall seining licenses
were issued; the highest lake whitefish yield was 31,751 kg in
1912, with an average annual lake whitefish yield of 18,537
kg/year (Halnon 1963). Lake whitefish were reported to fre-
quently weigh 3.6 kg, sometimes reaching 5.4 kg (Halnon 1963).
In the early 1900s, concerns arose regarding overexploitation of
lake whitefish. Fishermen and legislators at the time expressed
the opinion that the state of Vermont would obtain greater economic benefits from a strictly recreational fishery. Vermont and
New York prohibited seining in 1885, but Vermont reopened
the fishery in 1892; the commercial harvest was closed in
Vermont waters in 1913 (Wakeham and Rathbun 1897;
Halnon 1963; Marsden and Langdon, in press). The Québec lake
whitefish fishery in Missisquoi Bay continued, however, despite
*Corresponding author: [email protected]
1
Present address: Department of Fisheries and Wildlife, Michigan State University, 13 Natural Resources Building, East Lansing, Michigan
48824-1222, USA.
Received February 22, 2011; accepted August 24, 2011
1106
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LAKE WHITEFISH IN LAKE CHAMPLAIN
substantial decreases in harvest and the number of licenses
through time, with only four licensed fishermen harvesting a
total of 35 kg in 2004. In 2005, Québec fishermen voluntarily
ceased seining because the high effort associated with netting
did not justify the limited harvest (K. Miller, retired commercial
fisherman, personal communication).
Since the closure of the commercial lake whitefish fishery
in U.S. waters of Lake Champlain in 1913, only one study
has focused on lake whitefish. In the early 1930s, Van Oosten
and Deason (1939) described the age structure, size structure,
growth, and condition of lake whitefish collected in the fall
of the year at the two primary commercially harvested locations within the lake. In more recent years, lake whitefish have
been recorded only incidentally during biological surveys conducted periodically from the 1930s to the late 1990s. During the
1970s, a fish population inventory documented lake whitefish
in all areas of the lake except for the two historical commercial
fishing locations (Anderson 1978). The highest lake whitefish
catch rates were in the Main Lake (0.02–0.46 fish/h) and the Inland Sea (0.02–0.52 fish/h in 155-m multipanel gill nets). Lake
whitefish were also present in all annual gillnetting surveys from
1982 to 1998 that were associated with the assessment of lake
trout Salvelinus namaycush populations before and during the
experimental program for control of sea lampreys Petromyzon
marinus (Fisheries Technical Committee 1999).
Currently, little is known about the lake whitefish population
in Lake Champlain. Their spawning grounds, other than those
seined historically by commercial fishermen, are unknown;
few to no data are available on recruitment, growth, condition,
abundance, age distribution, and mortality. In the 80 years
since the 1930 study, Lake Champlain has experienced substantial physical and biological changes. Deforestation during the
1800s, inputs from agricultural land, and shoreline development
have led to increased phosphorus loads and eutrophication,
especially in the northern and extreme southern portions of
the basin (Myer and Gruendling 1979; LCBP 2008). Exotic
species have been entering Lake Champlain at an increasing
rate, particularly through the canal system that connects the
lake to the Hudson River, the Erie Canal, and the Great Lakes.
As of 2009, 48 exotic species had colonized the lake. Of those
invaders, the alewife Alosa pseudoharengus and zebra mussel
Dreissena polymorpha have the highest potential to negatively
affect the lake’s native fish community (Marsden and Hauser
2009; Marsden et al. 2010); the quagga mussel D. bugensis
has not yet invaded the lake. The management goal for lake
whitefish in Lake Champlain is to have multiple spawning
populations, including those in historical spawning areas that
still contain suitable habitat (Marsden et al. 2010); however,
there are no plans to reopen any commercial fishing in the lake.
To address this and other management goals, an analysis of the
current status of the species is needed.
Our goal was to describe the population status of an unstudied lake whitefish population in Lake Champlain almost a
century after the closure of the commercial fishery in U.S. wa-
1107
ters. Specifically, our objectives were to (1) determine where
lake whitefish are currently spawning in Lake Champlain and
whether changes have occurred in lake whitefish use of historic
spawning grounds; (2) quantify larval densities and distribution
during emergence in the spring; (3) quantify size, age, sex composition, growth, condition, and mortality and compare our data
with information collected in the 1930s from the two commercially harvested locations; and (4) examine potential threats to
lake whitefish population health. Spawning grounds were identified by lakewide sampling of larvae; current use of historic
and commercially harvested spawning grounds was identified
by the presence or absence of larvae. Peak larval emergence
was quantified at two locations by sampling with ichthyoplankton nets throughout the hatching period. We estimated growth
parameters using the von Bertalanffy growth model, condition
using the weight–length relationship, and mortality rates using
the catch curve equation; we used Fulton’s condition factor (Fulton’s K) to compare the current condition of lake whitefish with
the condition indicated by historical data.
METHODS
Study area.—Lake Champlain is a long (200 km), narrow (19
km at its widest point), and deep (19.5-m average, 122-m maximum depth) lake with a surface area of 1,130 km2. The lake is
bordered by Vermont (east shoreline) and New York (west shoreline) and by the Canadian Province of Québec (north). Lake
Champlain flows from a narrow river-like basin in the south,
and then north to the outlet, the Richelieu River, which flows
into the St. Lawrence River. Lake Champlain comprises five
basins, separated by geographic and constructed barriers, and
varying in watershed land use (agriculture to forested), trophic
status (eutrophic to oligotrophic), fish populations (warmwater to coldwater species), and geology (Myer and Gruendling
1979). This study focuses on four main areas, the South Lake
near Larabee’s Point, Missisquoi Bay in the north, Proctor Shoal
in the Main Lake, and the west shore of Grand Isle in the Main
Lake (Figure 1). Two of the study sites (Missisquoi Bay and
Larabee’s Point) are similar in terms of physical, biological,
and chemical characteristics. Both areas are shallow (<7 m)
and dominated by a warmwater fish community. Inputs of phosphorus and sediments from surrounding land use in the last
two centuries, dominated by agriculture, have led to eutrophication of these sections of the lake. The Main Lake, on the other
hand, is primarily deep and oligotrophic, supporting warm- and
coldwater fish species; it has been less influenced by riparian
inputs of phosphorus, contaminants, and sediment (Myer and
Gruendling 1979; LCBP 2008).
Fish collections.—Larval lake whitefish were sampled in
2008–2010 lakewide from ice-out until catches declined to zero;
this period began as early as 14 April and extended to the first
week in June. Larvae were collected during the day using an
ichthyoplankton net (75-cm-diameter opening, 600-µm mesh)
towed on the surface behind a boat at approximately 3.5 km/h
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1108
HERBST ET AL.
FIGURE 1. Map of Lake Champlain, with enlarged areas showing study sites.
Adult lake whitefish were sampled in Missisquoi Bay (North Lake), the Main
Lake (Proctor Shoal [PS] and Shelburne Bay), and South Lake (Larabee’s Point
[LP]). Larval sampling was conducted lakewide, but focused sampling was
done only in Main Lake (Wilcox Cove [WC] and Rockwell Bay [RB]). [Figure
available in color online.]
for 10 min/sample; sampling was focused near shore at 2–4-m
water depths. Samples were placed in 70% ethanol at the field
site and taken to the laboratory for measurement and identification. Identification of larval lake whitefish was confirmed
by using Auer’s (1982) key. Larval lake whitefish catches were
standardized to catch per unit effort (CPUE) and reported as
larvae/1,000 m3. Intensive sampling (three tows once per week
from mid-April to early June in 2008 and 2009) was done in
Rockwell Bay and Wilcox Cove (Figure 1) to quantify temporal
changes in larval densities. Mean densities at these sites were
calculated for each day of sampling and then averaged across all
days of sampling. Extensive sampling (single tows during midApril to early June) was done lakewide in 2008 and 2009 to
determine presence or absence of larvae and distribution of lake
whitefish spawning grounds. Offshore larval sampling (from 0.5
km to approximately 4 km from shore, from the surface to depths
of 20–60 m) was conducted in mid-May 2010, west of Wilcox
Cove and Rockwell Bay. Additional sampling was conducted
at Wilcox Cove in 2008 to determine whether larval concentrations varied at different times of the day; triplicate samples were
collected on one date during the day, at dusk, and an hour after
sunset. Larval densities were compared among the three time
periods by using a one-way analysis of variance (ANOVA).
Juvenile and adult lake whitefish were sampled in the fall of
2006–2008 and year-round during 2009–2010 in the Main Lake
near Proctor Shoal (Figure 1). Adult fish were also sampled in
Missisquoi Bay and Larabee’s Point in the spring and fall of
2009. Lake whitefish were collected using a 7.6-m semiballoon
otter trawl with a 6.4-mm stretched-mesh cod end liner and
a chain attached to the footrope, primarily targeting juveniles;
bottom-set gill nets were used to capture adults. We used three
different gill nets, all 1.8 m deep, 70.6–152.4 m long, and including panels of 7.6-, 8.9-, 10.2-, 11.4-, 12.7-, 14.0-, and 15.2-cm
monofilament stretch mesh. Nets were set overnight early in the
study, when we were seeking locations where lake whitefish
could be reliably caught, or for 2–3 h at dusk or dawn to collect diet data for a related study; therefore, we did not obtain
CPUE data comparable with findings at other lakes. Lake whitefish were weighed (nearest g), measured (total length [TL] ±
1 mm), and examined internally to identify sex. A scale sample
was taken from above the lateral line, and otoliths were extracted
and stored in labeled envelopes for age estimation by means of a
combination of sectioning and crack-and-burn methods (Herbst
and Marsden 2011).
Growth and condition.—Growth was estimated by fitting the
von Bertalanffy growth model to mean length-at-age data to estimate growth model parameters (L∞ = asymptotic length, K =
growth coefficient, and t0 = theoretical age at a length of zero;
t0 was estimated freely) for all lake whitefish collected from
the Main Lake during 2006 to 2010 (Ricker 1975). Otolith age
estimates were used for all lake whitefish collected during 2006
to 2010 combined, because otoliths were found to be the least
biased and most precise of three aging structures examined for
lake whitefish in Lake Champlain (Herbst and Marsden 2011),
similar to other stocks (e.g., Barnes and Power 1984; Muir et al.
2008). Growth parameters from the von Bertalanffy model were
estimated separately for each sex (full model) and for both sexes
combined (reduced model). For this analysis, all juvenile (ages
1–3) lake whitefish of unknown sex were added to the data set
for both sexes to avoid biased estimates for K and L∞ . Differences in growth between sexes were tested by using likelihood
ratio tests (Kimura 1980).
Growth was also estimated for lake whitefish collected in
2009 using scale age estimates to compare with historic data
based on scales from lake whitefish in Missisquoi Bay and
Larabee’s Point (Van Oosten and Deason 1939). Historic mean
standard length (SL; mm) data were converted to TL (mm) using
a conversion factor (TL = SL × 1.18) developed for Lake Champlain lake whitefish (Van Oosten and Deason 1939). Differences
in growth between all pairwise combinations of locations (Missisquoi Bay, Main Lake, and Larabee’s Point) were tested using
likelihood ratio tests (Kimura 1980). Growth parameters from
the von Bertalanffy model were estimated separately for each
location (full model; e.g., Missisquoi Bay) and for each pair of
locations (reduced model; e.g., Missisquoi Bay and Main Lake).
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LAKE WHITEFISH IN LAKE CHAMPLAIN
Residual sums of squares were then compared for the full and
reduced models by use of a likelihood ratio test. The full model
was accepted if the residual sums of squares was significantly
different (P ≤ 0.05) from that of the reduced model; otherwise,
the reduced model was accepted, and the growth parameters for
combined locations were used.
Lake whitefish condition was estimated from individuals collected in the fall (September–October) using Fulton’s K (Ricker
1975) for comparison with values estimated for each sex from
lake whitefish collected during the fall in 1930 and 1931 (Van
Oosten and Deason 1939). This technique was used for historical
comparison because the original weight–length data from Van
Oosten and Deason (1939) were not available. Instead, historic
condition was reported only as mean Fulton’s K by sex using SL
(mm), so to make this comparison, we converted our data for TL
to SL (SL = TL × 0.845; Van Oosten and Deason 1939). Differences in body condition, by sex, of individuals collected from
2006 to 2010 in the Main Lake and during the 1930s at the two
historic locations (Missisquoi Bay and Larabee’s Point) were
examined using the 95% confidence intervals (CIs) from Main
Lake fish to determine whether condition values overlapped.
Using TL data collected by the Vermont Fish and Wildlife
Department (VTFWD) during summer (June through August)
assessment of the experimental sea lamprey control program, we
calculated Fulton’s K for lake whitefish between 1982 and 1997
(Fisheries Technical Committee 1999; Marsden et al. 2003).
Only lake whitefish collected in the Main Lake were used for
comparison with lake whitefish collected during this study; lake
whitefish smaller than 350 mm were excluded from the data
set to minimize the length bias associated with Fulton’s K.
Rennie and Verdon (2008) determined that Fulton’s K was sizedependent; hence, given the low numbers of smaller individuals
in the 1982 to 1997 surveys, we limited potential bias by examining condition of similar-sized individuals. A linear model was
fit to the annual data from 1982 to 2010; because there was an
apparent discontinuity in the data, separate regressions were fit
to the periods 1982–1997 and 2006–2010.
Mortality.—Mortality rates were estimated for lake whitefish
collected in the Main Lake during 2006–2010 by using catch
curve analysis (Ricker 1975). To determine the age at which
fish were fully recruited, we visually examined the histogram
of natural logarithm of catch with age and chose the age that
corresponded to the peak leading to the descending limb of the
distribution. We loge transformed the catch curve equation to
estimate the instantaneous total mortality rate (Z) using linear
regression, and we then calculated the annual mortality rate (A;
Ricker 1975).
RESULTS
Larval Collections
In 2008–2009, larval lake whitefish were distributed throughout the Main Lake (Figure 2). Larval lake whitefish were present
at all locations sampled within the Main Lake, but in the Inland
1109
FIGURE 2. Larval lake whitefish sampling locations in Lake Champlain,
2008–2010 (presence = solid circle; absence = cross). Intensive sampling locations (Wilcox Cove and Rockwell Bay) and locations of special concern (Missisquoi Bay and Larabee’s Point) are enlarged, showing the maximum average
( ± SD) larval densities. The number of sample days is given in parentheses for
each year; 3–12 samples were taken on each sampling date.
Sea they were found in very low numbers and at only one location (Figure 2). Larval lake whitefish were also sparse in samples
from the historical commercial fishing location, Larabee’s Point,
with a maximum daily average density of 5 larvae/1,000 m3 from
nine sample days, 2008–2010 (Figure 2). In contrast, the maximum daily average at Wilcox Cove was 2,558 larvae/1,000 m3.
Larval tows in Missisquoi Bay, the other historical commercial
fishing location, yielded no lake whitefish larvae in any of the
three sampling years (Figure 2). The highest densities of larval
lake whitefish were associated with shoreline habitats consisting of cobble or gravel substrate; few to no larvae were found
in areas with wetland characteristics (highly organic substrate
and high macrophyte densities). Larval lake whitefish were also
present in all exploratory offshore samples (range per sample =
12–257 larvae/1,000 m3). At Wilcox Cove, significantly more
larvae were collected at dusk (mean ± SD = 1,583 ± 896
larvae/1,000 m3) than at night (218 ± 52 larvae/1,000 m3; P =
0.02), but there was no significant difference between densities
during the day (961 ± 357 larvae/1,000 m3) and either dusk or
nighttime.
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1110
HERBST ET AL.
FIGURE 3. Larval lake whitefish densities (mean [ ± SD] number of larvae/1,000 m3) sampled during 2009 at Rockwell Bay (upper panel) and Wilcox
Cove (lower panel), Lake Champlain.
Intensive larval sampling conducted at Wilcox Cove and
Rockwell Bay during 2009 captured peak larval emergence
densities of 2,558 and 2,244 larvae/1,000 m3, respectively
(Figure 3). Larval emergence at the two locations began to
rapidly increase on approximately 8 May 2009, which corresponded to water temperatures ranging from 7.8◦ C to 9.4◦ C,
and declined sharply after peaking at both locations. Peak densities were sampled on 13 May in Rockwell Cove and 19 May
in Wilcox Bay. Total length of larval lake whitefish at the two
locations ranged from 10 mm on 22 April to 17 mm on 3 June
2009.
Adult Distribution, Size, Age, and Sex Composition
In total, 545 lake whitefish were collected in gill nets and
bottom trawls conducted from 25 November 2006 through 6
October 2010 during all seasons. Gill nets set in the Main Lake
captured 464 lake whitefish (mean = 0.72 fish/h) with a mean
TL of 496 mm (SE = 3.47, range = 240–658 mm) and a mean
total weight of 1,409 g (SE = 642, range = 100–3,300 g). Gill
nets set in Missisquoi Bay captured nine lake whitefish (mean =
0.19 fish/h), all of which were collected at the southern entrance
to the bay in November 2010. No lake whitefish were collected
in 78.4 h of gillnetting at Larabee’s Point. The bottom trawl
captured 81 lake whitefish (mean = 6.2 fish/h) with a mean TL
FIGURE 4. Lake whitefish (A) length frequency (n = 545 fish) and (B)
age frequency (n = 542 fish) in collections from Lake Champlain, 2006–
2010.
of 301 mm (SE = 11.20, range = 126–511 mm) and a mean
total weight of 377 g (SE = 40, range = 14–1,540 g). Overall,
lake whitefish captured in both gears had a mean TL of 467 mm
(SE = 4.51, range = 126–658 mm; Figure 4) and a mean total
weight of 1,256 g (SE = 30.48, range = 14–3,300 g). The sex
composition, determined from 346 lake whitefish, was slightly
skewed toward females (females = 0.55; males = 0.45).
The age-frequency distribution indicates that multiple ageclasses were sampled in the Main Lake during 2006–2010.
Based on otolith age estimates, age-groups ranged from ages
1 to 26 with a mean age of approximately 9 years (SE = 0.20;
Figure 4). The use of the bottom trawl increased our sample size
of younger individuals; of the 79 fish captured in the trawl, 70%
were age 3 or younger.
Growth and Condition
Lake whitefish collected from the Main Lake during 2006–
2010 did not exhibit sexually dimorphic growth. Female and
male growth parameters based on mean length at otolith age did
not differ significantly (P = 0.23). Combined sexes achieved L∞
of 600 mm TL and a growth coefficient K of 0.20 (Figure 5).
The t0 value was −0.66 with sexes combined and the inclusion
of younger individuals (ages 1 and 2) of unknown sex.
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LAKE WHITEFISH IN LAKE CHAMPLAIN
FIGURE 5. Predicted mean ( ± SD) total length (mm) at age (years) based on
the von Bertalanffy growth model for all lake whitefish collected in Lake Champlain during 2006–2010. Estimated von Bertalanffy growth model parameters
(asymptotic length L∞ and growth coefficient K) and sample size (N) for all
fish (including those of unknown sex) are shown.
Lake whitefish growth estimated from scales of a subset
of 219 individuals collected from the Main Lake during 2009
was not significantly different from historic growth estimated
from 175 fish sampled at Larabee’s Point (P = 0.06) or 120
fish sampled at Missisquoi Bay (P = 0.147). Missisquoi Bay
and Larabee’s Point lake whitefish had significantly different
growth parameters (P = 0.012; Van Oosten and Deason 1939).
Missisquoi Bay lake whitefish collected in 1930 attained the
largest L∞ (635 mm) compared with Larabee’s Point (L∞ =
607 mm) and our Main Lake fish (L∞ = 605 mm). Growth
coefficient K decreased from south (Larabee’s Point: 0.28) to
north (Missisquoi Bay: 0.21); our centrally located Main Lake
site had an intermediate value (0.24).
Body condition of lake whitefish in Main Lake estimated
using Fulton’s K was significantly higher than for lake whitefish collected from Missisquoi Bay in 1930 for both sexes but
was not significantly different from that for fish captured at
Larabee’s Point in 1931 based on the 95% CIs. This comparison
with historic data included only the 170 lake whitefish collected
from the Main Lake in the fall of 2006–2010; no lake whitefish
had been collected during fall in prior studies. Females in our
study accounted for 60% of the total Main Lake sample size
and had a greater mean SL (453 mm; N = 102; SE = 4.49)
than males (444 mm; N = 68; SE = 5.32). Female lake whitefish condition (Fulton’s K = 1.87; 95% CI = 1.83–1.91) was
significantly higher than the condition of males (Fulton’s K =
1.74; 95% CI = 1.68–1.78) based on 95% CI for fish collected
in the Main Lake. Condition of females from the Main Lake
was similar to that of females from Larabee’s Point (Fulton’s K
= 1.84; N = 77), and Fulton’s K-values of fish sampled at both
locations were higher than those of fish sampled at Missisquoi
Bay (Fulton’s K = 1.69; N = 59). The same pattern held true for
males; condition in the Main Lake was similar to that of fish in
Larabee’s Point (Fulton’s K = 1.71; N = 98) and significantly
1111
FIGURE 6. Annual mean ( ± SD) Fulton’s condition factor (Fulton’s K) for
356 lake whitefish collected between 1982 and 1997 in Lake Champlain (Main
Lake) by the Vermont Fish and Wildlife Department and for 449 lake whitefish
collected in the Main Lake and near Grand Isle during the present study (2006–
2010). Only fish having a total length of at least 350 mm were used in the
calculation of mean Fulton’s K.
higher than that of Missisquoi Bay fish (Fulton’s K = 1.62; N
= 61). Condition calculated from TL showed the same pattern,
with females having higher condition (Fulton’s K = 1.13) than
males (Fulton’s K = 1.05).
Annual mean Fulton’s K calculated from TL averaged 1.2
± 0.14 during the 1980s and 1990s and 1.1 ± 0.13 during this
study. The decline in annual mean Fulton’s K from 1982 to 2010
was significant (F = 230.2; df = 1, 805; P ≤ 0.0001). Despite
a decline in the 1990s, the slope of the annual mean Fulton’s K
for the 1982–1997 period was not significantly different from
zero (F = 0.069; df = 1, 356; P < 0.79), whereas condition
declined significantly during this study period (F = 52.7; df =
1, 447; P ≤ 0.0001; Figure 6). This analysis is partly confounded
by the fact that lake whitefish in 2006–2008 were collected in
Grand Isle only in fall, whereas all other fish were collected in
the Main Lake during summer (1982–1997) or during spring,
summer, and fall (2009–2010).
Mortality
Mortality rates were estimated for age-6 and older lake whitefish collected from the Main Lake in 2006–2010; based on the
age-frequency histogram, lake whitefish were fully recruited to
our gear by age 6 (Figure 3). The Z for lake whitefish of ages
6 to 26 was estimated at 0.24 (95% CI = 0.19–0.29), A was
0.21 (95% CI = 0.17–0.24), and annual survival rate (S) was
0.79 (95% CI = 0.75–0.83). Given the absence of a commercial
fishery and an extremely limited sport harvest, A approximates
natural mortality for lake whitefish in Lake Champlain.
DISCUSSION
Lake whitefish in Lake Champlain currently have biological attributes characteristic of a stable, unexploited population. Lake whitefish in 2006–2010 were represented by multiple
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1112
HERBST ET AL.
age-classes and a wide distribution of lengths, with slow growth
and low mortality rates. Larval densities were high throughout
the Main Lake. While data are not available from the period
of exploitation, the current population parameters are similar
to those recorded in a study in the 1930s: Lake whitefish from
the Main Lake had growth parameters and mean Fulton’s Kvalues similar to those of lake whitefish from Larabee’s Point
and Missisquoi Bay in 1930–1931, though Main Lake fish had a
greater condition value than fish from Missisquoi Bay. The only
apparent cause for concern is the low or absent larval densities
at historic commercial fishing sites.
Before this study, knowledge of lake whitefish spawning
grounds in Lake Champlain was limited to historical informational regarding the fall shoreline seining fishery, which harvested lake whitefish in the northern portions of the lake when
the species was preparing to spawn and near Larabee’s Point
in the south (Marsden and Langdon, in press). Historical documents do not indicate why other areas were not fished; we found
that shorelines throughout much of the Main Lake consist of
gravel and cobble, which are suitable and preferred spawning
substrates for lake whitefish (Bégout Anras et al. 1999). We attempted to identify spawning areas by gillnetting for spawning
fish in fall but found spent females on only one date, 20 December 2006; interestingly, Smith (1914) concluded that lake
whitefish spawn after ice formation. Sampling for larval lake
whitefish showed that they were present at all sites with suitable
substrate. Larvae found in the Main Lake could not have drifted
from Missisquoi Bay, as more than 25 km and several islands
and causeways separate the bay and the northernmost of our
sampling sites (Figure 1). Thus, we assume that the paucity of
commercial fishing in the Main Lake was due to preferences
of fishers for fishing access rather than absence of spawning
aggregations.
The current scarcity or absence of larval lake whitefish in
Missisquoi Bay and Larabee’s Point and the low catches of
adults at these sites in the fall may indicate that (1) local populations were lost due to exploitation; (2) populations found in
these areas historically were only staging rather than spawning; or (3) spawning substrates have been degraded. Given that
Van Oosten and Deason (1939) collected large numbers of lake
whitefish in fall seines during 1930 and 1931, 18 years after
commercial harvest ended, overexploitation does not appear to
be the problem. Missisquoi Bay is connected to the rest of Lake
Champlain by a narrow passage, so it seems unlikely that lake
whitefish would move to a cul-de-sac area to stage before spawning elsewhere. Thus, habitat degradation may be an important
factor in these areas. As a result of anthropogenic changes in
land use in the last century, Missisquoi Bay and the South Lake
are now highly eutrophic, having high densities of macrophytes,
silt, and other organic matter that limit available oxygen needed
for egg survival (Myer and Gruendling 1979; LCBP 2008).
Similar changes have been shown to negatively influence lake
whitefish recruitment in other systems (Evans et al. 1996). The
Inland Sea, where larval lake whitefish were also rare (N = 1),
has not been severely altered. Adult lake whitefish were sampled in this basin during gill-net surveys by the VTFWD in
1978 (0.4–0.52 fish/net-hour) and 1993–1996 (Anderson 1978;
Fisheries Technical Committee 1999), so spawning in this basin
may occur in the northern section, where we did not sample.
Larval densities elsewhere in Lake Champlain are among the
highest reported for the species. For perspective, average maximum larval densities in Wilcox Cove and Rockwell Bay (2,558
and 2,244 larvae/1,000 m3) were substantially higher than those
in Chaumont Bay, Lake Ontario (469 larvae/1,000 m3), and
sites throughout Lake Michigan (4–1,922 larvae/1,000 m3) but
were lower than those in Green Bay, Lake Michigan (3,756 larvae/1,000 m3; Hoagman 1973; Freeburg et al. 1990; Mckenna
and Johnson 2009; Claramunt et al. 2010). Wilcox Cove and
Rockwell Bay have spawning substrate suitable for lake whitefish and are protected from wave-generated disturbances, except
for those from the west, which can affect egg survival rates and
recruitment.
Most studies focus on larval lake whitefish sampling near
shore during the day, as larvae are concentrated at the surface
in shallow depths (<3 m) and are seldom captured over adjacent deep water further from shore after hatching (Hart 1930;
Hoagman 1973). Hoagman (1973), for example, captured few
to no larvae in Green Bay, Lake Michigan, at sites 100–150 m
from shore at depths greater than 10 m. In contrast, we collected
mean daily maximum densities of 171 larvae/1,000 m3 at the
surface over depths ranging from 26 to 61 m, relative to the 969
larvae/1,000 m3 captured nearshore on 12 May 2010. These offshore larvae were presumably displaced from nearshore areas
by currents or offshore winds. The frequency and magnitude of
this offshore advection are unknown, as is the survival potential
for these larvae. Offshore movements of larvae may be more
common than generally realized, given that sampling is usually not extended to offshore areas. We collected significantly
higher larval densities at dusk than at night; daytime densities
were lower than at dusk, but high variability and a low number
of replicates precluded finding a significant difference. Assumptions about higher concentrations of larvae at the surface during
the day than at nighttime may be incorrect; Hoagman (1973),
for example, reported higher larval catches during the night than
during the day.
Given the high larval densities throughout most of the lake,
indicative of good reproductive output, are recruitment, growth,
and survival robust? Lake whitefish in Lake Champlain had
a wide size range with multiple length modes, and multiple
age-classes, similar to unexploited populations from several
Canadian lakes (Johnson 1976; Mills et al. 2005). In contrast,
exploited lake whitefish populations are characterized by low
numbers of older individuals and, depending on density effects,
smaller individuals.
Growth of lake whitefish from Lake Champlain was not sexually dimorphic, which was unexpected because lake whitefish
growth frequently differs by sex in both unexploited and exploited lakes (Beauchamp et al. 2004; Cook et al. 2005; Hosack
Downloaded by [J. Ellen Marsden] at 07:04 28 December 2011
LAKE WHITEFISH IN LAKE CHAMPLAIN
2007). Lake whitefish from Lake Champlain had an L∞ value
greater than those of the exploited lake whitefish populations in
the Great Lakes and in 28 inland lakes (Beauchamp et al. 2004)
and greater than those of unexploited populations in Lake Pend
Oreille (Hosack 2007); only lake whitefish from Lake Superior’s
Apostle Island region had a larger L∞ (M. J. Seider and S. T.
Schram, Wisconsin Department of Natural Resources, unpublished data). Growth coefficients K for fish in Lake Champlain
were greater than those of fish from Lake Pend Oreille (females:
0.13; males: 0.15; Hosack 2007) but similar to those of most
other lake whitefish populations. Lake Erie males had a growth
coefficient of 0.32, which was among the highest reported; other
growth coefficients ranged from 0.22 to 0.31 for Lake Erie females, 28 inland lakes populations, and 22 Great Lakes stocks
(Beauchamp et al. 2004; Cook et al. 2005). Changes in lake
whitefish growth have been related to density-dependent factors, with slow growth in years of increased abundance and
biomass (Healey 1980; Wright and Ebener 2005). Abundance
and biomass of lake whitefish in Lake Champlain are unknown,
but given the high L∞ and slow growth rates, we speculate
that density-dependent factors are not limiting growth in Lake
Champlain lake whitefish.
Lake whitefish densities in Lake Champlain do not seem
to be hindering the population’s ability to find available food
resources for somatic growth or reproduction. After the introduction of zebra mussels to Lake Champlain in 1993 (Marsden
and Hauser 2009), we anticipated a diet shift from native prey
to these less energetically valuable exotic mussels, as was seen
in the Great Lakes (Mohr and Nalepa 2005). In the Great Lakes,
this diet shift negatively impacted growth and condition, changes
that ultimately affect the reproductive capabilities of a fish population; however, because of the growth and condition values in
Lake Champlain lake whitefish, we do not anticipate that similar
dietary shifts have occurred in Lake Champlain subsequent to
the introduction of zebra mussels.
Lake whitefish in Lake Champlain have maintained good
condition and high survival despite high wounding rates by
sea lampreys. The energetic cost of sea lamprey parasitism is
generally associated with poor condition, low fecundity, and
high mortality rates; for example, commercial landings of lake
whitefish in Lakes Huron, Michigan, and Superior declined during periods of high sea lamprey abundance and rose after control was implemented (Smith and Tibbles 1980; Spangler and
Collins 1980). In regions of Lake Superior, where sea lamprey
populations have been controlled for several decades, wounding
on lake whitefish averages 0.06–1.0 wounds per 100 fish (Harvey et al. 2008). In contrast, lake whitefish in Lake Champlain
had an average of 10.7 ± 7.5 wounds per 100 fish in the 11
years preceding the beginning of the experimental control period (1980–1990), dropping to an average of 7.3 ± 4.5 wounds
per 100 fish during the experimental control period (1991–1997;
Fisheries Technical Committee 1999). In the current study, conducted during full implementation of sea lamprey control, there
were 2.0 wounds per 100 fish. Despite the high wounding rates
1113
in the 1980s and 1990s, Fulton’s K was robust throughout this
period and a large number of year-classes of lake whitefish are
currently present. Average Fulton’s K during the current study
was lower than in the 1980s and 1990s; mean annual Fulton’s K
declined during the 1990s and 5-year period of the study. However, annual means in the 1990s were based on small sample
sizes (6–20 fish), and predictions about the trajectory of lake
whitefish populations in the lake based on this short time period
and relatively small decline may be premature.
Lake whitefish from Lake Champlain have been exposed to
very little exploitation since the closure of the commercial fishery in U.S. waters in 1912, which explains the high S of 79%,
typical for unexploited populations. Mortality estimates from
catch-curve analysis involve assumptions of consistent recruitment and constant mortality over all ages and over time. These
assumptions are likely to be violated in any natural population. Mortality in Lake Champlain is largely a consequence of
stresses imposed by sea lamprey wounding, maturation, spawning, and senescence, as mortality from fishing is virtually absent. We do not have estimates of sea lamprey-induced mortality
for lake whitefish; however, sea lampreys in Lake Champlain
are smaller, and their attacks on lake trout are less lethal, than
in the Great Lakes (Madenjian et al. 2008), which suggests
that sea lamprey-induced mortality of lake whitefish may also
be lower than in the Great Lakes. With no more than 3 years
of larval sampling at any one site, no data for postlarval lake
whitefish, and low numbers of juveniles, we do not have sufficient data to evaluate recruitment variability. Moreover, no
dominant year-class was found that could be tracked over the
years of the study to evaluate survival. Acquisition of these data
should be a priority for long-term evaluation of lake whitefish
survival.
Van Oosten and Deason (1939) concluded that Missisquoi
Bay and Larabee’s Point had separate lake whitefish populations, on the basis of the biological attributes of the fish they
collected in the 1930s. However, both of these areas are shallow
and too thermally restrictive to support lake whitefish in the
summer; they must have been used by lake whitefish only for
spawning and early larval growth. The virtual absence of larvae and adults in these locations during our study suggests that
lake whitefish spawning is now minimal or absent in Missisquoi
Bay and Larabee’s Point. VanDeHey et al. (2009) found that
lake whitefish in Lake Michigan have small home ranges and
genetically differentiated subpopulations; if similar population
substructuring was historically present in Lake Champlain, then
habitat changes may have eliminated the northern and southern
spawning populations.
Our data indicate that discrete spawning stocks of lake whitefish have potentially been extirpated from the two commercially
fished locations of Lake Champlain, probably as a result of historical changes in riparian land use and increased inputs of
phosphorus. High sediment loads and eutrophication in Missisquoi Bay and Larabee’s Point may have made these sites unsuitable for lake whitefish spawning. Commercial fishing could
1114
HERBST ET AL.
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also have contributed to the decline in Missisquoi Bay, where
harvest continued until the mid-2000s and commercial catches
of lake whitefish declined steadily since the 1960s (Marsden
and Langdon, in press), but the Larabee’s Point population has
not been harvested since the fishery closed in 1914. In the Main
Lake, in contrast, suitable spawning substrate is readily available, larval production is high, and the adult population metrics
are robust and appear to be healthy.
ACKNOWLEDGMENTS
We thank Shawn Good (VTFWD) for access to laboratory equipment. We also thank Elias Rosenblatt, Neil Thompson, Josh Ashline, Kevin Osantowski, Lindsay Schwarting, and
Joanna Hatt for assistance in the field and laboratory, and
Richard Furbush, Joe Bartlett, and Rebecca Gorney for assistance with fish collection. We especially thank the National Oceanic and Atmospheric Administration for funding this
project.
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