Preliminary analysis of nearshore food web changes in Lake Michigan inferred from isotopic analysis of yellow perch scales
Craig A. Stricker1, Brent A. Murry2, David F. Clapp3, Donald G. Uzarski2
1. Background
4. Study Area
6. Results and Discussion
Disturbance in aquatic ecosystems can manifest from natural
or anthropogenic changes in ambient chemical-physical
conditions and species introductions leading to top-down or
bottom-up impacts on food webs. The Great Lakes have
undergone a variety of perturbations during the 20th century.
Recent
invaders,
such
as
dreissenid
mussels,
waterfleas,
and round
goby likely
have had
profound
effects on
food web
energetics. Concurrent abiotic perturbations such as climate
change, nutrient alterations, and changing water levels may
interact synergistically with species invasions to alter food
web relations, decrease system stability, and affect the
productivity of desirable fish species. For example, reductions
in
phosphorus
loading
combined
with
dreissenid
establishment have resulted in a nearshore energy and
nutrient shunt (redirection from pelagic-profundal to nearshore
benthos). The nearshore shunt has likely negatively influenced
offshore fisheries, but effects on nearshore fish species
remain less clear.
Yellow perch used in this
study (n = 182; average
age = 5) were collected
from the southern basin
of Lake Michigan using
bottom gill nets (19701980, 1996-2005) and creel
surveys
(1985-1987).
Most were caught within
15-20 m water depth.
A. d13C ranged over 4 ‰, with highest values observed
2. Statement of Problem
Species introductions pose a significant challenge to the
health and viability of fish populations endemic to the Great
Lakes. Long-term records are desperately needed to better
understand past and present fluctuations in species abundance
and condition.
Identification of factors influencing these
populations will greatly aid resource managers in adaptively
managing the Great Lakes to meet future challenges.
We investigated the long-term trophic dynamics of Lake
Michigan nearshore environments through the analysis of
archived (1972-2005) yellow perch (Perca flavescens) scales.
Yellow perch are among the most popular
nearshore sport fishes in the Great Lakes.
The abundance of Lake Michigan yellow
perch has undergone large fluctuations
since at least the early 1970s, but for
reasons largely unknown have remained at
low numbers since establishment of
dreissenids and round goby.
3. Objectives
A. Describe historical changes in the trophic ecology of Lake
Michigan yellow perch.
B. Identify potential dietary shifts related to invasive species
introductions.
C. Estimate changes in growth related to nearshore food web
perturbations.
5. Methods
A. Study Organism
We chose to focus our initial efforts on
yellow perch since it is a common nearshore species of the
Great
Lakes.
Additionally,
Lake
Michigan yellow
perch abundance has undergone major
fluctuations, are hypothesized to be
influenced by invasive species, and
therefore represent an excellent sentinel
species for the study of Great Lakes
trophic dynamics. Adult perch consume
aquatic invertebrates, such as amphipods
isopods, chironomids, Mysis relicta, and
fish, such as sculpin and round goby.
B. Scales
Scales are ideal structures for the study of
trophic ecology owing to the record of age
and growth and the ease of chemical and
isotopic analyses. Scales
record annual information
throughout the life cycle of
the
fish,
but
grow
allometrically and therefore
largely integrate dietary
information from the last season of growth.
The isotopic composition of scale collagen
is highly related to muscle.
C. Isotopic analyses
Scales were cleaned in DI water, residual
skin excised, and the lower half trimmed.
Approximately 1 mg of scale was weighed
into tin capsules for d13C and d15N analysis
measured by continuous flow techniques.
D. Age and growth
Scales from the same
fish were used to age
and back-calculate the
last
increment
of
growth
using
the
Fraser-Lee method with
a fixed intercept of 30
mm.
following 1996 and coincident with the establishment of
zebra mussels and round goby. ANOVA indicated
significant differences (F4,176 = 54.343, p = 0.000) across
time periods; samples from 1972-73 also had higher on
average values.
B. Marked changes in d15N were observed over the 33year period, with a significant (F4,176 = 73.539, p = 0.000)
decline following the early 1970s; a more subtle decline
was observed in the 2003-05 samples (not shown).
C. Elevated d13C and d15N values during the early 1970’s
are consistent with elevated P loading; subsequent
decreases are likely related in part to abatement
practices, but also concomitant changes to the fish
communities of Lake Michigan.
D. The range in isotope values within individual time
periods was fairly similar throughout the study,
suggesting Lake Michigan yellow perch have a relatively
static trophic niche breadth, but can adapt to changes
in the nearshore forage base.
E. ANOVA indicated significant differences in growth
across time periods (F4,176 = 14.506, p = 0.000); age was
used as a covariate and was also significant (F1,176 =
117.013, p = 0.000) owing largely to the older aged fish
used in the late 1990s. Growth was highest in the early
1970s, probably owing to reduced abundance and
competitive interactions; however, growth was lowest
following the invasion and establishment of dreissenids
and round goby.
F. Abundance peaked in the late 1980’s and has since
remained relatively low, with a few prominent year classes in the mid-2000s (not shown).
G. Yellow perch in the southern basin of Lake Michigan have been known to shift diet in
response to environmental and biotic perturbations. The most recent shift appears to be a
significant increase in consumption of round goby. Round goby feed on zebra mussels and
hence provide an energetic link between yellow perch and zebra mussels, with concomitant
coupling to pelagic (nearshore and offshore) nutrients.
7. Conclusions
A. Yellow perch in the southern basin of Lake Michigan appear to have undergone a recent
diet shift, as evidenced by significant changes in d13C. The shift is most likely toward a higher
dependence on the non-native round goby, which provides a direct energetic link to zebra
mussels as well as a probable indirect benefit from the nearshore shunt.
B. The trophic level of yellow perch has fluctuated relatively little since the late 1970’s, despite
the recent diet shift. This may be due to the mere substitution of secondary consumers in the
diet, i.e. traditional prey fish to round goby. Diet likely remains subsidized by common
invertebrates, though sphaerids and oligochaetes have declined.
C. Growth and abundance are lower compared to pre-invasion of Dreissena and round goby,
suggesting factors other than species invasions (beneficial or not) influence yellow perch in
the southern basin of Lake Michigan.
D. Fish scales represent excellent records of aquatic trophic dynamics and offer an efficient
and inexpensive means of evaluating changes through time.
8. Future Research:
Expand analysis of covariates influencing trophic ecology, fill in
additional years, and include other important species from different habitats/trophic levels.
9. Acknowledgments: C. Gulbransen, M. Dreier
(1) US Geological Survey, Denver, CO, cstricker@usgs.gov; (2) Department of Biology, Central Michigan University, Mt. Pleasant, MI; (3) Charlevoix Fisheries Research Station, MI Department of
Natural Resources, Charlevoix, MI