Selenium in Scaup: A Disturbing Trend in the Great Lakes

Selenium in Scaup:
A Disturbing Trend in
the Great Lakes
Feature Article by Dr. Scott Petrie
Photo: Ethan Meleg
E
ach year, hundreds of thousands of
Greater and Lesser scaup stop to
fuel up in the lower Great Lakes as
they head north after wintering along the
eastern seaboard and Gulf Coast. Low on
reserves, they feast on abundant zebra
mussels – filter feeding exotic mussels that
have proliferated throughout the lower
Great Lakes. While this relatively new food
source likely helps restore their reserves for
the long trip north to their breeding
grounds, many scaup are, unwittingly,
ingesting selenium concentrations in
possibly dangerous amounts.
The continental scaup population has
declined substantially since the 1970s.
“Scaup” is actually comprised of two
closely related diving ducks, Greater and
Lesser scaup. Numbers of the two species
are usually combined during aerial
waterfowl surveys and continental
population estimates because of their
similar size, coloration, and habitat needs.
(About 85% of the continental population is
made up of Lessers). Breeding ground
surveys indicate that the continental scaup
population declined from 8 million birds in
1972 to 3.7 million in 2001. However, most
of this decline has occurred since the mid
1980s, which coincides with when zebra
mussels began colonizing the Great Lakes.
While it is not unusual for wildlife
populations to fluctuate over time, the
decline in numbers of scaup is of particular
concern because it occurred over a
relatively short period of time, and when
populations of most other waterfowl
species in North America were increasing.
Scaup Decline and
the Great Lakes
Several theories as to why scaup have
declined are currently being investigated
by waterfowl scientists. One theory is that
environmental contaminants acquired on
staging and wintering areas may be having
adverse effects on their reproduction or
survival.
The Great Lakes have
historically been a recipient of
environmental contaminants. While
contaminant levels have generally been
decreasing, persistent toxins are present
and will likely remain in the system at low
levels for a very long time.
Waterfowl biologists are concerned
that Greater and Lesser scaup may be
ingesting and bioaccumulating
contaminants while feeding on the lower
Great Lakes. One reason for concern
relates to the substantial increase in
numbers of scaup staging and wintering
there since zebra and quagga mussels
Lesser Scaup/Petit Fuligule Photo: Wayne Lynch
SUMMER 2004, NUMBER 28
9
(hereafter collectively “zebra mussels”)
colonized the lakes in the mid 1980s. For
example, the peak number of scaup
counted during fall at Long Point on Lake
Erie increased from an average of about
7500 birds during the 1970s and 1980s to
nearly 50,000 in the 1990s. Further,
numbers of scaup counted on the Canadian
side of Lake Ontario during the annual
Mid-winter Waterfowl Inventory also have
increased, from an average of about 4800
prior to zebra mussel colonization (19801990) to 24,000 after colonization (19912000).
The remarkably efficient filter-feeding
habits of zebra mussels give them a high
capacity for storing environmental
contaminants. Rather than feeding on
algae and other food items that adhere to
rocks and plants on the lake bottom (as do
native gastropods), zebra mussels filter
suspended food items out of the water
column. A single zebra mussel can filter
about one litre of water per day. Because
they filter suspended particles
indiscriminately, zebra mussels
incorporate and accumulate waterassociated contaminants into their tissues
more readily than do organisms that were
traditional food items of scaup (i.e., native
gastropods). Subsequently, contaminants
can be passed up the food chain and
accumulate in waterfowl that consume
zebra mussels.
Bioaccumulation of contaminants can
eventually impair reproductive output
and/or survival of waterfowl. For example,
at least one study has shown that Tufted
Ducks (a European species closely related
to scaup) had reduced reproductive success
and low survival after captive birds were
fed contaminated zebra mussels. Thus, it is
plausible that consumption of large
quantities of zebra mussels on the lower
Great Lakes may be contributing to the
continental scaup decline. Based on this, in
1999 the Long Point Waterfowl and
Wetlands Research Fund (LPWWRF)
began to study contaminant burdens,
dietary intake, and nutrient reserve (fat and
protein) dynamics of Lesser and Greater
scaup staging on lakes Ontario, Erie, and
St. Clair during fall and spring.
Results indicate that neither PCBs nor
DDE are being acquired by scaup on the
lower Great Lakes at levels that could
impact health or reproduction. In fact, all
birds analyzed had PCB and DDE
concentrations well below levels where
reproductive impairment would be
expected. Because previous studies have
suggested that zebra mussels may be
responsible for unusually high selenium
uptake by diving ducks in the Great Lakes,
levels of this element were also of interest.
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BIRDWATCH CANADA
Zebra mussels may be lowering their high spring selenium levels by
transferring quantities to their larvae. Les moules zébrées réussissent
peut-être à réduire le taux de sélénium présent dans leur corps en
transférant une partie de cet élément dans leurs larves.
Photo: Mike Schummer
Selenium Levels vary
with Season
Zebra mussel tissues taken directly from
collected birds were analyzed to determine
if, in fact, these organisms acquired large
quantities of selenium as a result of filter
feeding. Results revealed sizeable seasonal
differences in zebra mussel selenium
burdens. During spring, zebra mussels had
a relatively high mean selenium burden of
10.8 µg/g, whereas the fall zebra mussel
samples had selenium burdens below the
3.1 µg/g detection limit of the laboratory
equipment. Zebra mussels produce about 1
million larvae throughout summer, which
may provide a way for zebra mussels to
lower selenium burdens within their tissues.
Similarly, waterfowl may be able to
dramatically reduce their selenium burdens
by depositing this element into their eggs.
This process may explain why selenium
levels were relatively low in zebra mussels
during fall. In contrast, since zebra mussels
do not reproduce during winter, they likely
have a limited capacity for purging
themselves of selenium at that time. The
spring selenium burdens of 10.8 µg/g found
in zebra mussels are cause for concern
because it has been suggested that 3 µg/g is
the toxic threshold for consumption of
aquatic organisms by fish and wildlife.
In Mallards, liver concentrations of 10
µg/g selenium cause reproductive
impairment, whereas concentrations above
33 µg/g impact health and survival. Hence,
reproduction appears to be adversely
impacted at lower selenium burdens than is
health or survival. No similar studies have
been performed on scaup, but the results for
Mallards do provide a useful context.
Selenium burdens analyzed in scaup
were also found to be substantially higher in
spring than fall on the lower Great Lakes.
And, in almost all spring Lesser Scaup
(77% of birds, mean burden = 19.8 µg/g)
and Greater Scaup (94% of birds, mean
What is Selenium?
S
elenium is a semi-metallic trace
element occurring naturally in
some soils (“selenate”). It is also
a byproduct of fossil fuel burning and
other industrial activities (“selenite”).
High selenium burdens in zebra
mussels can be attributed to increased
anthropogenic inputs of selenium to
the Great Lakes (fossil fuel burning,
etc.), as well as the high capacity of
zebra mussels to concentrate this
contaminant through their filter- Selenium levels in scaup were found to be much higher in spring than
feeding activities. Although selenium fall. Le taux de sélénium présent dans les tissus des fuligules au
is nutritionally required by birds in printemps est beaucoup plus élevé qu'à l'automne. Lesser Scaup/ Petit
small amounts, it is highly toxic in Fuligule Photo: Wayne Lynch
slightly greater quantities.
Selenium concentrations quickly accumulate in tissues when birds are introduced
to a selenium-contaminated diet. Selenium is also quickly excreted from the body
when birds are removed from a selenium source. Females use the egg as a route of
selenium excretion, and, as such, high selenium burdens can impair reproduction.
“… the continental scaup
population declined from 8
million birds in 1972 to 3.7
million in 2001.”
spring, the remaining question is whether
scaup accumulate selenium prior to their
arrival on the lower Great Lakes. The
answer is, not likely. Our results suggest
that Lesser Scaup have substantially lower
burdens in the early spring than birds that
are sampled later in the season. Therefore,
birds appear to be acquiring selenium
throughout the time they spend on the lower
Great Lakes. For Greater Scaup, some of
the samples collected early in the spring had
high burdens. However, these birds had
likely overwintered in the area, which is
rarely the case with Lesser Scaup. Longer
periods of exposure may explain why
Greater Scaup (24.8µg/g) had higher
selenium burdens than Lesser Scaup (19.8
µg/g).
Future Directions for
Selenium Research
Greater Scaup/Fuligule
Scaup/Fuligule milouinan
milouinan Photo:
Photo: Wayne
Wayne Lynch
Lynch
Greater
burden = 24.8 µg/g) selenium levels were
well above the 10 µg/g threshold identified
as causing reproductive impairment in
Mallards. In contrast, substantially fewer
Lesser (17% of birds, mean burden = 7.6
µg/g) and Greater (40% of birds, 9.9 µg/g)
scaup in fall had selenium burdens above
10 µg/g. These seasonal differences can be
directly related to the life cycle of scaup and
the mussels they feed on. As previously
mentioned, zebra mussels contain higher
selenium burdens in spring than they do in
fall. Both species of scaup consume
substantially more zebra mussels in spring
than they do in fall (Lesser Scaup, spring =
64% of diet, fall = 38% of diet; Greater
Scaup, spring = 40% of diet, fall = 8% of
diet). During spring, scaup arrive on the
lower Great Lakes in relatively poor
condition, and likely spend a lot more time
foraging to accumulate body fat than they
do during fall. As well, our research
suggests that both scaup species tend to
consume larger zebra mussels during spring
(Lesser = 6.8 mm, Greater = 9.9 mm) than
during fall (Lesser = 5.3, Greater = 6.6).
Larger mussels generally have a higher
capacity for contaminant intake than
smaller ones.
Since selenium levels are highest in the
We now know that the introduction of
the exotic, filter-feeding zebra mussel has
provided an efficient avenue for selenium
uptake by Lesser and Greater scaup, and
quite possibly other waterfowl that eat these
mussels. Whether selenium uptake on the
lower Great Lakes impacts reproduction
and/or survival of scaup requires further
investigation. The fact that United States
waterfowl harvest data indicate that the
proportion of juveniles and adult females in
the Lesser Scaup population has declined
over the last two decades suggests that it
might. However, to conclusively answer
this question, we need to determine how
much selenium scaup can eliminate from
their tissues prior to nesting, and how
susceptible they are to body burdens of this
trace element.
Recently, LPWWRF obtained tissue
samples from about 100 adult female
Greater Scaup that were collected during
the nesting season by a colleague in Alaska.
Preliminary analysis of these tissues
indicates that the mean selenium burden is
9.6 µg/g, which is below levels observed on
the Great Lakes but very near the level at
which reproduction is compromised in
Mallards. Therefore, at least some scaup are
arriving on the breeding grounds with
selenium burdens that could compromise
reproductive output. Selenium levels
identified in this study will enable scientists
to set a baseline for future captive studies to
determine the actual impacts of high
selenium burdens on reproduction and
health in scaup.
SUMMER 2004, NUMBER 28
11
Taux inquiétant de sélénium chez les fuligules
Petit Fuligule/Lesser Scaup Photo: Jim Flynn
C
haque printemps, des centaines
de milliers de Fuligules
milouinans et de Petits Fuligules
quittent leur territoire d'hivernage situé
sur la côte est et se dirigent vers le nord
dans le but de rejoindre leur territoire de
nidification. En chemin, ils s'arrêtent
dans le bassin inférieur des Grands Lacs
où ils se nourrissent abondamment de
moules zébrées, un mollusque exotique
qui a rapidement proliféré dans les
Grands Lacs après son introduction dans
les années 80. Bien que cette nouvelle
source de nourriture permettent aux
oiseaux de refaire le plein d'énergie et
d'emmagasiner des réserves qui leur
serviront à effectuer le reste du trajet,
elle recèle un danger. En se nourrissant
de moules zébrées, ces espèces ingèrent
également une importante concentration
de sélénium qui pourrait nuire à leur
santé.
Les inventaires réalisés sur les
territoires de nidification des Fuligules
milouinans et des Petits Fuligules
indiquent que la population de ces deux
espèces combinées a diminué entre 1972
et 2001, passant de 8 à 3,7 millions
d'oiseaux, et que cette baisse a
commencé à être plus marquée à partir
des années 80, ce qui correspond au
moment où les moules zébrées ont été
introduites dans les Grands Lacs. Bien
qu'il soit normal qu'une population
d'oiseaux fluctue, le déclin des
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BIRDWATCH CANADA
populations de fuligules soulève des
inquiétudes, car celui-ci s'est déroulé sur
une courte période et, qu'au même moment,
la majorité des autres espèces de sauvagine
en Amérique du Nord voyaient leur
population augmenter.
Une des théories proposées pour
expliquer ce déclin est que les produits
toxiques ingérés par les oiseaux, que ce soit
sur les haltes migratoires ou sur leurs sites
d'hivernage, auraient des conséquences sur
leur comportement, leur reproduction et
leur survie. On sait depuis longtemps que
les Grands Lacs sont contaminés, mais ce
n'est que depuis peu que les fuligules
utilisent aussi intensément cette région, que
ce soit comme halte migratoire ou site
d'hivernage. Selon les inventaires aériens,
le nombre de fuligules qui fréquentent les
Grands Lacs serait de cinq à six fois plus
élevé aujourd'hui qu'avant les années 80.
Les moules zébrées filtrent l'eau pour
se nourrir et accumulent dans leur corps les
produits toxiques qui s'y trouvent.
Contrairement aux moules originaires de la
région qui se nourrissent d'algues et
d'autres organismes qui vivent au fond des
lacs, les moules zébrées se nourrissent des
éléments qui se trouvent en suspension
dans l'eau. Dans le but d'étudier le
phénomène de bio-accumulation chez les
fuligules, le Fonds pour la recherche sur la
sauvagine et les marais de Long Point a
permis de mettre sur pied, en 1999, un
projet visant à mesurer les taux de ient les
contaminants présents dans les tissus
d'individus qui fréquentaient les lacs
Érié, Ontario et Sainte-Claire au cours
des migrations printanière et automnale
et à déterminer les habitudes
alimentaires et les réserves énergétiques
(gras et protéines) de ces derniers.
Comme plusieurs études
semblaient suggérer un lien entre la
présence de moules zébrées et le taux
anormalement élevé de sélénium présent
dans les tissus des canards plongeurs qui
fréquentaient les Grands Lacs, nous
avons décidé de mesurer ce taux chez les
fuligules et les moules. Les oiseaux ont
besoin d'une petite quantité de sélénium
dans leur alimentation, mais lorsqu'ils en
ingèrent plus (la proportion n'a pas
besoin d'être beaucoup plus élevée), le
sélénium devient hautement toxique.
Les résultats de l'étude ont
démontré que les taux de BPC et de DDE
présents dans les tissus des oiseaux
n'étaient pas assez élevés pour affecter
leur santé ou leur reproduction. Les
données font également ressortir que le
taux de sélénium présent dans les tissus
des fuligules et des moules variait de
façon importante selon les saisons. Au
printemps, le taux de sélénium mesuré
chez les moules était relativement élevé
(10,8 :g/g) alors qu'il était inférieur à la
capacité de détection de nos appareils à
l'automne (3,1 :g/g). Chez les fuligules,
on a constaté que le taux de sélénium
était également plus élevé au printemps
qu'à l'automne. Au printemps, le taux de
presque tous les individus étudiés
dépassait 10 :g/g, un seuil qui a été
identifié, chez le Canard colvert, comme
étant le niveau où le sélénium
commençait à avoir un impact sur la
capacité de reproduction. À l'automne, ce
taux était beaucoup moins élevé chez la
plupart des individus étudiés.
Au cours de l'été, les moules zébrées
produisent chacune près d'un million de
larves, ce qui leur permet peut-être de
réduire le taux de sélénium présent dans
leurs tissus. De la même façon, les
fuligules évacuent peut-être une partie du
sélénium dans leurs oeufs. La
concentration plus élevée de sélénium
observée chez les fuligules au printemps
s'explique aussi peut-être en partie par le
fait que ces derniers mangent davantage
de moules, et de plus grosses, au
printemps qu'à l'automne.
Nous savons aujourd'hui que
l'augmentation du taux de sélénium
observé chez les fuligules et possiblement
chez d'autres espèces d'oiseaux qui se
nourrissent de moules zébrées et quaggas
est une conséquence de l'apparition de ces
dernières dans les Grands Lacs. Des
recherches supplémentaires devront que
Depuis que les moules zébrées ont été introduites dans les Grands Lacs, le nombre de fuligules migrateurs présents dans certains secteurs du
bassin inférieur des Grands Lacs a plus que quadruplé. Since the arrival of zebra mussels, the number of scaup staging in some areas of the lower
Great Lakes has more than quadrupled. Photo: Mike Schummer
toutefois être entreprises dans le but de
déterminer si les niveaux de sélénium
observés chez les fuligules affectent leurs
chances de survie et leur reproduction. La
baisse du nombre de jeunes et de femelles à
l'intérieur des populations d'oiseaux
hivernants aux États-Unis au cours des
vingt dernières années semble indiquer que
c'est peut-être le cas. Grâce à une récente
étude, nous savons que des individus se
présentent sur leur territoire de
nidification avec un taux de sélénium
susceptible de nuire à leur reproduction
(9,6 :g/g). Il reste à savoir quelle
quantité de sélénium les oiseaux peuvent
éliminer de leur organisme avant la
reproduction et quel est leur niveau de
susceptibilité à cet élément.
SUMMER 2004, NUMBER 28
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