Full text of DOI:10.1016/S0380-1330(02)70576-5

J. Great Lakes Res. 28(2):193–201
Internat. Assoc. Great Lakes Res., 2002
The Relationship Between the Abundance of Smallmouth Bass and
Double-crested Cormorants in the Eastern Basin of Lake Ontario
Brian F. Lantry1,*, Thomas H. Eckert1, Clifford P. Schneider1, and Jana R. Chrisman2
1New York State Department of Environmental Conservation (NYSDEC)
Cape Vincent Fisheries Station
PO Box 292, 541 E. Broadway
Cape Vincent, New York 13618, USA
2P.O. Box 814
Cape Vincent, New York 13618, USA
ABSTRACT. Available population and diet data on double-crested cormorant (Phalacrocorax auritus)
and smallmouth bass (Micropterus dolomieui) numbers, demographics, and exploitation rates were syn-
thesized to examine the relationship between cormorant and smallmouth bass abundance in the U.S.
waters of the eastern basin of Lake Ontario. It was found that after the number of cormorants nesting on
Little Galloo Island in New York exceeded 3,500 pairs in 1989, survival of young smallmouth bass, not
yet of legal size for the sport harvest (< 305 mm), began to decline. Despite production of strong year
classes in 1987 and 1988, abundance of smallmouth bass measured from gill net surveys declined to its
lowest level by 1995 and remained there through 1998. Stable or increasing catch and harvest rates in
other local fisheries along the U.S. shore suggested that declines in smallmouth bass abundance in the
eastern basin were not related to water quality. Stable or increasing growth rates for smallmouth bass
age 2 and older since the 1980s further indicated that food resource limitation was also not the cause for
declines in abundance. Comparisons of estimates of size and age-specific predation on smallmouth bass
by cormorants with projected smallmouth bass population size indicated that much of the increased mor-
tality on young smallmouth bass, could be explained by cormorant predation.
INDEX WORDS: Smallmouth bass, cormorants, piscivory, Lake Ontario, age-specific predation.
INTRODUCTION
(1999a), using census data collected since 1985
along the U.S. shore of Lake Ontario from the Nia-
gara River to Henderson, found that while trends in
effort aimed at bass increased steadily lake-wide,
catch rates fluctuated without trend in all areas of
the lake but the east. Of nine sites lake-wide, the
only location where a decline in harvest rate oc-
curred was Henderson in the eastern basin.
Chrisman and Eckert (1999), using New York
State Department of Environmental Conservation
(NYSDEC) index gill net data collected since 1976,
documented that during the 1990s the bass popula-
tion in the eastern basin declined to the lowest
abundance level observed in over two decades.
They also found that the long-term cyclic abun-
dance pattern for bass was reflective of a popula-
tion in which most individuals were from just two
strong year classes produced in 1973 and 1983.
Strong year classes produced in 1987 and 1988 re-
The smallmouth bass (Micropterus dolomieui,
hereafter: bass) fishery of the eastern basin of Lake
Ontario was one of the premier fisheries of its type
in New York State (Brown 1975, Krester and Klatt
1981). Since the mid 1990s anglers and business
owners complained that the quality of bass fishing
in the eastern basin had deteriorated. Recent creel
surveys corroborated the declines in fishing quality
(Eckert 1999a, McCullough and Einhouse 1999). In
their eastern basin creel survey, McCullough and
Einhouse (1999) found that total harvest, harvest
rate, and catch rate in 1998 were down 54 to 80%
from previous surveys completed in 1978 (Panek
1981) and 1984 (Anonymous 1989). Eckert
*Corresponding author. Current address: U.S. Geological Survey, Lake
Ontario Biological Station, 17 Lake St., Oswego, New York 13126,
USA. E-mail: bflantry@usgs.gov
193

194
Lantry et al.
cruited to the adult population at rates 4 to 5 times
lower than the 1973 and 1983 year classes and
failed to curb population decreases during the
1990s. Chrisman and Eckert (1999) also found that
the modal age of index catches shifted from adult
bass fully recruited to the gear to immature fish not
yet of legal harvest size or fully vulnerable to the
index nets. This was indicative of a population with
a mortality bottle neck before recruitment to the
fishery.
occurred or was likely, analysis methods were cho-
sen to specifically lessen the projected impact of
cormorants on bass (biasing bass population esti-
mates high). Then the potential for other obvious
system changes (productivity, introduced dreissenid
mussels, and piscivory by walleye, Stizostedion vit-
reum vitreum) were examined to cause bass popula-
tion declines.
METHODS
Study Area
At the same time the bass population was declin-
ing, nearby double-crested cormorant (Phalacoco-
rax auritus, hereafter: cormorant) populations were
increasing (Weseloh and Ewins 1994). The number
of cormorant nests on Little Galloo (U.S.) and Pi-
geon islands (Canada) increased from 22 in 1974 to
5,839 in 1998 and from 40 in 1979 to 1,818 in
1998, respectively (Weseloh and Ewins 1994; R.
Miller and D. V. Weseloh, pers. com., NYSDEC
Delmar, N.Y. and Canadian Wildlife Service,
Downsview Ontario, Canada). Diet investigations
indicated that from 1993 to 1997, cormorants from
these colonies consumed between 130,000 to
750,000 bass annually from eastern Lake Ontario
(Ross and Johnson 1999, J. H. Johnson, and R. M.
Ross, pers. com., U.S. Geological Survey Cortland,
New York and Wellsboro, Pennsylvania). Although
the declines in bass abundance and predation on
bass by cormorants were clearly documented, two
important pieces of information were not well un-
derstood: 1) the size and age of bass consumed by
cormorants; and 2) whether losses to cormorant
predation increased the rate of bass mortality. Re-
cently, two studies demonstrated that cormorants
were targeting age-2 to -5 bass in the eastern basin
of Lake Ontario (Adams et al. 1999, Schneider and
Adams 1999). With this new information, age-
specific predation by cormorants on bass could be
estimated and its effect on the bass population
could be simulated.
In this analysis the available data were synthe-
sized to evaluate whether recent changes in bass
population dynamics (abundance, mortality, recruit-
ment, year-class production, etc.) were driven by
cormorant predation. Specifically evaluated were:
1) mortality of bass in the size range consumed by
cormorants; and 2) whether observed levels of cor-
morant predation could cause the declines recorded
in the bass population and fishery. Because of the
limitations of available data, the main objective for
this analysis was to demonstrate the potential for
cormorant predation to induce observed changes.
Where data borrowing, data variability, or data bias
Smallmouth bass data from index gillnets (Chris-
man and Eckert 1999, Eckert 1999b), mark-recap-
ture estimates (McCullough 1988), and eastern
basin creel surveys (Panek 1981, Anonymous 1989,
McCullough and Einhouse 1999) were collected
from U.S. waters of Lake Ontario north and east of
Stony Point (Fig. 1). Data from the NYSDEC Fish-
ing Boat Census (Eckert1999a) were collected
along the U.S. shore from the Niagara River east-
ward to Association Island Cut near Henderson.
Data for cormorants originated from two colonies:
Little Gallou and Pigeon islands. Little Galloo Is-
land (LGI) is located in U.S. waters of Lake On-
tario approximately 16 km west of the town of
Henderson Harbor, New York. Pigeon Island is lo-
cated in Canadian waters approximately 20 km
northwest of LGI and about 10.5 km west of the
mouth of the St. Lawrence River.
Smallmouth Bass Abundance Indices
Eckert (1999a) indicated that lake-wide declines
in nutrients were not accompanied by similar de-
clines in harvest rates of smallmouth bass. He in-
ferred that harvest rate reflected abundance, and
hence declines in bass abundance, similar to that
observed from the eastern basin, were not ongoing
lake-wide. To examine the relationship between
stock abundance and harvest, data from the CPUE
summed within years for bass age-5 and older from
the NYSDEC warm-water fisheries assessment
index gill nets (Chrisman and Eckert 1999) were
plotted against harvest rate data from the eastern
basin creel surveys (Panek 1981, Anonymous
1989, McCullough and Einhouse 1999) and for the
Henderson site from the NYSDEC fishing boat
census (Eckert 1999a). Linear regression was used
to quantify the relationship between harvest rates
from the fishing boat census and the CPUE from
index gill nets. CPUE data for age-5 bass were in-
cluded because mean length (1976 to 1998) was

Smallmouth Bass and Cormorants in Lake Ontario
195
FIG. 1. Locations of the Little Galloo Island and Pigeon Island double-crested cormorant colonies in
the eastern basin of Lake Ontario (inset). Location covered by the McCullough and Einhouse (1999) east-
ern basin creel survey (U.S. Waters north and east of Stony Pt.—inside the dotted line). General locations
of the four geographical regions (West; West—Central; East—Central; East) and nine localized creel sur-
vey sites from 1998 NYSDEC Fishing Boat Census (stars; Eckert 1999a).
about 295 mm and as many as 30% or more were
legal sized (305 mm) at this age (Chrisman and
Eckert 1999).
strength and relative abundance (Chrisman and
Eckert 1999, Eckert 1999b). To assess the mortal-
ity of bass below the age appropriate for catch-
curve analysis (age 3 to 5) an index of relative
mortality was used. The relative mortality index
(RM) was a ratio equivalent to the CPUE of bass at
a young age (n0; e.g., age 3) divided by the CPUE
Changes in Mortality of Smallmouth Bass
Trends in mortality of adult smallmouth bass
were examined by Chrisman and Eckert (1999).
Because of incomplete vulnerability to capture in
index gill nets, Chrisman and Eckert (1999) could
not use catch-curve analysis to compute mortality
for bass aged 2 to 5. Throughout their 23-year
index gill netting data set, CPUEs for bass age 3, 4,
and 5 provided a consistent measure of year-class
of bass in the same cohort at some older age (n0+t
;
e.g., age 6):
RM = n0 / n0+t
(1)
The relative mortalities were examined of bass at
successive ages from age 3 to 6, and between pre-

196
Lantry et al.
TABLE 1. Annual totals of smallmouth bass
consumed by double-crested cormorants colonies
on Little Galloo and Pigeon islands, Lake Ontario.
Data courtesy of J. H. Johnson and R. M. Ross,
U.S. Geological Survey Cortland, N.Y. and Wells-
boro, PA.
adult bass, ages 5 to 13, from the Hardscrabble area
(Fig. 1) was 72,850 fish (1,489—marked; 439—
postmarking recaptures; 8—marked recaptures;
95% confidence limits: 38,339 to 163,900) or 11.8
bass/ha (McCullough 1988, Schneider et al. 1997).
The thermocline is normally located below 20 m
and therefore most of the warm-water habitat is lo-
cated above this contour. The area in the eastern
basin between the 0 to 20-m depth contours is about
37,650 ha. Expanding McCullough’s (1988) density
estimate yields a total of 444,270 adult bass for this
depth range. To partition the bass population esti-
mate into adult age groups, a mean annual survivor-
ship value of 0.75 calculated from catch curves of
CPUE data collected annually from 1976 to 1989
during the NYSDEC warm-water gill net survey
(Chrisman and Eckert 1999) was used. To project
the population estimate backwards to ages 1 to 5 a
mean annual survivorship value of 0.38 calculated
from values for young bass listed by Carlander
(1977: p.187) was used. This yielded a density of
91.8 age-2 to -5 bass / ha (Table 2). Using the
weight-at-age data from index gill netting (Chris-
man and Eckert 1999), a standing crop was calcu-
lated from the population estimate of 33.96 kg/ha
without including age-0 bass. This is within the
center of the range of standing crop estimates for
bass in lakes (20 to 40 kg/ha) reported in Carlander
(1977). When the mark-recapture estimate was
done in 1986, the CPUE from index gill netting for
adult bass was 1.5 times greater than the 1993 to
1994 mean and almost 5 times greater than the
1998 value. Further, the mark-recapture population
estimate was performed in an area of optimal habi-
tat that anglers consider some of the best fishing
grounds. By applying this estimate to the entire
eastern basin within the 20-m contour, including
optimal and sub-optimal habitats, and using a mean
adult mortality rate calculated for 1976 to 1989, a
maximum estimate or an overestimation of the re-
cent abundance of bass within U.S. waters of the
eastern basin is obtained. Hence, this bass popula-
tion estimate should provide a conservative descrip-
tion of the impact of cormorant predation.
Year
Little Galloo
Pigeon
Total
1993
1994
1996
1997
1998
320,000
355,000
114,000
672,000
464,000
55,000
58,000
18,000
76,000
144,000
375,000
413,000
132,000
748,000
608,000
recruits (ages 3 to 5) and bass fully recruited to the
sport fishery (age-6 and older). Single factor analy-
sis of variance (ANOVA) and linear regression
were used to compare trends in smallmouth bass
relative mortality to cormorant abundance indexed
by nest counts from the LGI colony.
Abundance of Smallmouth Bass and
Losses Due to Cormorant Predation
Available data were synthesized to estimate age
specific abundance for smallmouth bass in the U.S.
waters of the eastern basin of Lake Ontario. Esti-
mates of cormorant predation on smallmouth bass
in the eastern basin were from data for Little Galloo
and Pigeon Island colonies (Table 1, data courtesy
of J. H. Johnson and R. M. Ross, U.S. Geological
Survey Cortland, New York and Wellsboro, Penn-
sylvania) and the age-structured cormorant diet data
for 1993 to 1994 and 1998 (Schneider and Adams
1999, Adams et al. 1999). Calculations represented
an annual time-step with initial bass abundances
from age-specific population estimates and final
bass abundance from removal of individuals due to
cormorant predation in the absence of other sources
of mortality. Predation by cormorants was simu-
lated for two time periods, 1993 to 1994 and 1998,
because age-specific consumption of bass was
available for those periods.
Cormorants can dive to depths in excess of 22 m
(Palmer 1962) and in eastern Lake Ontario, typi-
cally feed within 20 km of the colony (Neuman et
al. 1997) making nearly all bass habitat in the U.S.
waters of the eastern basin accessible to cormorant
predation. To estimate bass abundance the McCul-
lough (1988) estimate was expanded to cover all
bass habitat in the U.S. waters of the eastern basin.
The 1986 mark-recapture population estimate for
RESULTS
Smallmouth Bass Abundance Indices
Graphical comparison between the three observa-
tions from the eastern basin creel survey (Panek
1981, Anonymous 1989, McCullough and Einhouse
1999) and the index gill net data (Chrisman and
Eckert 1999, Eckert 1999b) suggested that harvest

Smallmouth Bass and Cormorants in Lake Ontario
TABLE 2. Estimated numbers and biomass of
197
smallmouth bass per hectare for water depths of 0
to 20 m in the eastern basin of Lake Ontario.
Numbers of adult smallmouth bass ages 5 to 13
are from McCullough’s (1988) population esti-
mate (11.8 per ha) and were partitioned among
ages using a survivorship of 0.75 (Chrisman and
Eckert 1999). Numbers of young smallmouth bass
age 1 to 4 were calculated using a survivorship of
0.38 (Carlander 1977). Biomass of smallmouth
bass was calculated from mean weight-at-age data
collected during the annual NYSDEC warmwater
fish community assessment (Chrisman and Eckert
1999, Eckert 1999b).
Standing
Crop
Weight Number / ha (kg /ha)
Age
1
2
3
4
5
6
7
8
9
10
11
12
13
93
108
160
265
351
488
632
799
941
153.0
58.1
22.1
8.4
14.23
6.28
3.53
2.22
1.12
1.17
1.13
1.08
0.95
0.75
0.62
0.49
0.38
3.2
2.4
1.8
1.4
1.0
0.8
0.6
0.4
FIG. 2. Harvest rate (smallmouth bass/hr) from
creel surveys versus the catch-per-unit-effort
(CPUE) data from index gill nets. Harvest rates
are from the eastern basin creel surveys (panel a;
Panek 1981, Anonymous 1989, McCullough and
Einhouse 1999) and the Henderson site from the
Lake Ontario Fishing Boat Census (panel b; Eck-
ert 1999a, 1999b). CPUE data are from small-
mouth bass captured in NYSDEC index gill nets
set in the eastern basin of Lake Ontario, August
1976 to 1998 (data from: Chrisman and Eckert
1999).
995
1,092
1,155
1,190
0.3
Total age 2–4
Total age 5–13
Total age 1–13
88.6
11.8
253.4
12.04
7.69
33.96
rate in the fishery was positively related to small-
mouth bass abundance (Fig. 2a). Simple linear re-
gression analysis of the harvest rate from the
fishing boat census (Eckert 1999a) and gill net data
indicated a direct relationship between the fishery
harvest and abundance in the local bass population
(r2 = 0.58 and p = 0.005; Fig. 2b).
age-3 to -5 bass were not fully recruited to the gear,
relative mortalities between successive ages were
variable and trends were difficult to detect. Relative
mortalities of pre-recruits and fully recruited bass
did, however, show clear trends. Single-factor
ANOVAs of the relative mortalities between age 3
and 6 and those between age 4 and 6 detected dif-
ferences between the time periods 1976 to 1988 and
Changes in Mortality of
Young Smallmouth Bass
The relative mortalities of smallmouth bass were
compared at successive ages from age 3 to 6, and
between pre-recruits (ages 3 to 5) and bass fully re-
cruited to the sport fishery (age-6 and older). Be-
cause of low CPUEs from weak year classes,
year-to-year variability in catch rates, and because
1989 to 1994 (F1,17 = 25.712, p < 0.001 and F1,18
=
5.352, p = 0.033, respectively). While relative mor-
talities of bass between ages 3 and 6 from 1976 to
1988 did not appear related to cormorant abun-
dance, relative mortality began increasing in 1989

198
Lantry et al.
FIG. 3. The relative mortality of smallmouth
bass from the eastern basin of Lake Ontario
between ages-3 and 6 (triangles: 1976 to 1988 and
squares: 1988 to 1995) and the number of cor-
morant nests on Little Galloo Island (circles) ver-
sus year. Relative mortality is equivalent to the
CPUE of age-3 smallmouth bass divided by the
CPUE of age-6 bass within the same cohort 3
years later. Relative mortalities are plotted for the
year in which the age-3 smallmouth bass were
sampled. Dotted lines represent estimates of the
means of relative mortality data for years 1976 to
1988 and 1989 to 1994 and are extended to cover
each time interval.
FIG. 4. Simple linear regressions of the relative
moralities for bass between ages 3 and 6 versus 3
year averages of nest counts (diamonds and dotted
line: 1976 to 1988, squares and solid line: 1988 to
1994). Relative morality values integrated mortal-
ity of age-3, -4, and -5 bass, therefore, 3-year aver-
ages of nest counts were calculated for the years
in which each bass cohort would be sampled at
ages 3, 4, and 5 (e.g., for the 1987 year class of
bass the 3-year average would be calculated with
nest counts from 1990, 1991, and 1992). The slope
for the 1976 to 1988 regression was not signifi-
cantly different from zero (r2 = .038, p = 0.520, n =
13). For 1988 to 1994, the relative mortality (RM)
of bass from age 3 to 6 was significantly related to
the 3-year averages of cormorant nest counts on
Little Galloo Island (CN): RM = 0.000111 * CN—
3.532 (r2 = 0.91, p < 0.0002, n = 7).
when cormorant nest counts on Little Galloo Island
exceeded 3,500 (Fig. 3). The relative moralities for
bass between ages 3 and 6 integrated mortality of
age-3, -4 and -5 bass. Therefore, a 3-year average
of nest counts in simple linear regressions (Fig. 4)
was used. The average was calculated for the 3
years of nest counts in which the bass cohort would
be sampled at ages 3, 4, and 5 (for the 1987 year
class of bass the 3-year average would be calculated
with nest counts from 1990, 1991, and 1992). For
the time period 1976 to 1988, there was no relation-
ship between the relative mortality of bass from age
3 to 6 and the 3-year average number of cormorant
nests on Little Galloo Island (r2 = .038, p = 0.520, n
= 13). For the time period 1988 to 1994, the relative
mortality of bass from age 3 to 6 was related to the
3-year average number of cormorant nests on Little
Galloo Island (CN):
The relative mortality of the 1992 year class of bass
(age 3 in 1995) from age 3 to age 6 dropped to lev-
els unaffected by cormorant predation, but this was
the third weakest year class recorded with CPUE
near zero (0.11). Because age-3 CPUE was divided
by age-6 CPUE of the same cohort, the very low
numbers of this year class captured at age 3 would
cause even a relatively small catch at age 6 to give
a low index value.
Abundance of Smallmouth Bass and
Losses Due to Cormorant Predation
Using a mean cormorant predation level for 1993
to 1994 of 394,000 bass distributed between ages 1
to 7 according to the 1993 to 1994 cormorant diet
data yielded a 23% reduction in bass ages 3 to 5.
The 1993 to 1994 level of consumption of age-5
RM = 0.000111 * CN – 3.532
(2)
(r2 = 0.91, p < 0.0002, n = 7).

Smallmouth Bass and Cormorants in Lake Ontario
199
and -6 bass by cormorants exceeded population es-
timates for those ages. The 1998 predation level of
608,000 bass distributed between ages 1 to 5 ac-
cording to the 1998 cormorant diet data yielded a
36% reduction in age-3 to -5 fish and a reduction in
age-3 numbers alone by 40%. Projection of cohort
abundance from age 3 to 6 with conditional mortal-
ity rates from the bass population estimate and
losses from the 1998 cormorant predation data re-
sulted in a 78 % decline in recruitment to the adult
fishable stock. While it is not likely that cormorant
predation would remove entire cohorts from the
bass population, the results indicate that potential
loss rates induced by this new source of mortality
could severely limit recruitment to the adult stock.
The effects of successive years of cormorant preda-
tion on a single cohort could be a recruitment bot-
tleneck similar to that suggested by the shift in
modal age in catch curves (Chrisman and Eckert
1999) and increased mortality of young fish sug-
gested by the analysis of relative mortality.
dance, complete analyses of those interactions
were beyond the scope of the current study. Evi-
dence exists, however, that indicates that these fac-
tors were not the primary force behind bass stock
reductions.
Declines in total phosphorous loadings since the
1970s have reduced current open lake concentra-
tions below the target level of 10 µg/L set by the
Great Lakes Fishery Commission (Johannsson et al.
1998, Williams et al. 1998). These changes were
expected to reduce primary productivity and could
have ultimately lead to reduced carrying capacity
for fishes. Declines in bass abundance in the eastern
basin of Lake Ontario were not believed to be
linked to phosphorous declines for two reasons.
First, fall length-at-age of smallmouth bass age 4 to
6 showed no clear trends from 1976 to 1998, but
growth of age-2, -3 and -7+ bass improved since the
mid-1980s (Chrisman and Eckert 1999). While in-
creasing growth rates during the 1990s may have
been a compensatory response to reduced numbers,
growth rates from 1988 to 1992 were during a pe-
riod of high population abundance. Second, the en-
tire lake has experienced phosphorous declines, but
bass abundance has not responded similarly
throughout the lake. Eckert (1999a) observed that
declines in bass abundance and angler harvest-rates
during the 1990s at the Henderson site were not ac-
companied by lake-wide fishery declines which
would have been expected if lake-wide nutrient de-
clines were driving observed bass population dy-
namics. Comparisons within the present study
indicated that trends in Lake Ontario bass census
data (Panek 1981, Anonymous 1989, Eckert 1999a,
McCullough and Einhouse 1999) agreed with index
gill netting CPUEs from the eastern basin (Chris-
man and Eckert 1999, Eckert 1999b), which sug-
gests bass abundance does not appear to have
declined at sites west of the eastern basin.
Introductions of exotic organisms have the poten-
tial to change fish community dynamics and
ecosystem structure and function. The most dra-
matic of these in recent times has been the introduc-
tion of dreisenid mussels The zebra mussel
(Dreissena polymorpha) appeared in the Lake On-
tario drainage in the late 1980s, but was not abun-
dant in eastern Lake Ontario until 1993 to 1994
(Schaner and Stewart 1995, Mills et al. 1999). Pro-
liferation of zebra mussels, however, occurred after
increases in age-3 to -5 bass mortality rates in 1989.
About 31 species of fish are commonly caught in
NYSDEC index gill nets set each August since
1976 (Eckert 1999b). Of those caught commonly,
DISCUSSION
The analyses indicated that mortality of bass be-
tween age 3 and 6 increased dramatically after 1988
when cormorant nest counts on LGI exceeded
3,500. Further, relative mortality of bass was di-
rectly related to 3-year average nest counts from
LGI between 1988 and 1994 (r2 = 0.91). The results
agree with previous studies which found that the
greatest proportion of smallmouth bass consumed
by cormorants during the 1990s were age 3 to 5
(Adams et al. 1999, Schneider and Adams 1999).
Finally, the bass population projections that in-
cluded observed age-specific predation by cor-
morants, in the absence of other mortality sources,
demonstrated the potential to remove large propor-
tions of bass year classes before they recruited to
the adult population.
The Lake Ontario ecosystem has changed since
the NYSDEC warm-water fisheries survey first
began monitoring smallmouth bass population dy-
namics in 1976. Some obvious changes with the
potential to alter adult bass abundance and young
bass mortality included: decreases in total phos-
phorous concentrations since 1972 (Johannsson et
al. 1998, Williams et al. 1998); the introduction
and proliferation of dreissenid mussels (Mills et al.
1999) and subsequent increases in water clarity;
and increases in the abundance of piscivorous
walleyes (Eckert 1999b). While any of these fac-
tors may have contributed to declines in bass abun-

200
Lantry et al.
only walleyes have increased substantially over that
time period. Walleyes are piscivorous and capable
of consuming YOY and yearling bass. Piscivory by
walleye had the potential to reduce young bass
numbers, however, out of 3,604 walleye captured in
the U.S. and Canada in agency sampling programs
since 1958, no bass were observed in stomachs
(Schneider et al. 1999).
sion) and the many authors of the of the other 10 re-
ports prepared for the: “NYSDEC Special Report—
February 1, 1999, Final Report: To Assess the Im-
pact of Double-Crested Cormorant Predation on
Smallmouth Bass and Other Fishes of the Eastern
Basin of Lake Ontario” for the use of their data and
contributions to the production and editing of this
manuscript.
Implications for Management
REFERENCES
The recent increase in mortality could be ex-
plained from declining recruitment of age-3, -4
and -5 smallmouth bass to the adult population
and the fishery. The information presented in this
study furnishes support for the existence of a
cause/effect relationship between increased abun-
dance of cormorants and declines in the bass pop-
ulation and the quality of the fishery in the eastern
basin of Lake Ontario. The main results from this
study were that mortality of age 3 to 5 bass in-
creased substantially after 1988 and cormorant
predation on these age-classes appeared to be sub-
stantial enough to cause the observed declines in
the bass population.
Adams, C.M., Schneider, C.P., and Johnson, J.H. 1999.
Predicting the size and age of smallmouth bass
(Micropterus dolomieui) consumed by Double-crested
Cormorants (Phalacrocorax auritus) in eastern Lake
Ontario, 1993–1994. In Final Report To Assess the
Impact of Double-Crested Cormorant Predation on
Smallmouth Bass and Other Fishes of the Eastern
Basin of Lake Ontario, section 6: pp.1–8. New York
State Department of Environmental Conservation
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Production of strong year classes of bass in the
eastern basin of Lake Ontario has occurred at inter-
vals from 4 to 10 y and is thought to be related to
environmental conditions during spawning. Be-
cause of the relatively long periods between these
year classes, the bass population in the basin is de-
pendent upon high survivorship of young bass age
2 to 5. The potential for cormorant predation to
effectively suppress bass cohort abundance at suc-
cessive ages between ages 2 to 5, may compromise
the ability of the bass population to compensate
for this relatively new form of predation. The 1995
year class of bass, sampled as age-3 fish in 1998,
was the fifth strongest on record. Cormorant diet
studies for 1998, however, indicated that predation
was particularly intense on the 1995 bass cohort,
removing an estimated 340,480 individuals.
Results of this study indicate that cormorant preda-
tion at levels similar to those observed in 1998, on
the 1995 and future cohorts of bass, could continue
to suppress adult abundance and fishery success
rates.
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servation Special Report, February 1, 1999.
. 1999b. Summary of 1976–98 Warm Water
———
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Double-Crested Cormorant Predation on Smallmouth
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ACKNOWLEDGMENTS
We acknowledge the agencies (New York State
Department of Environmental Conservation and
U.S. Geological Survey, Biological Resources Divi-

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Submitted: 2 August 2001
Accepted: 10 February 2002