Mysis diluviana Demographics in Lake Champlain, Vermont, between 1975 and 2012

Changes in Opossum Shrimp (Mysis diluviana) Population
Demographics in Lake Champlain, Vermont, between 1975 and 2012
Suzanne C.
1
Ball ,
Jason D.
2
Stockwell ,
Timothy B.
3
Mihuc
Densities of the mid-trophic level omnivore have decreased nearly ten-fold since the 1970s
• The omnivorous Mysis exhibits strong diel
vertical migration and is an important prey for
benthic and pelagic fishes.
• Mean density (± 95% CI) in 2012 (82 ± 60
mysids/m2) was significantly lower than in 1975
(712 ± 168 mysids/m2) (p < 0.0001, Fig. 3).
• Cohort trends do not appear to differ between
1975 and 2012, with at least two cohorts present
throughout the sampling seasons (Fig. 4).
• We hypothesized that Lake Champlain Mysis
densities decreased since 1975 because of
negative relationships between invasive species
and Mysis in other systems1,2,3.
• Sampling methods and
location were identical
to the 1975 study5. Five
replicate, vertical tows
were collected during
the day at a 100-m
deep station with a 0.5m diameter plankton
net with 250-μm mesh
(Fig. 2a)
a
b
1200
18
23
600
1000
1000
May
Mysids (Summer)
3
8
13
18
23
June
20
800
800
600
600
400
400
200
200
0
25 3
22-May
100%
Mature
Male
Mature
Female
Immature
Male
Immature
Female
Juvenile
50%
0%
J J A S
Months (1975)
O A M
• Length-weight data from both years suggest
Mysis from 1975 were in slightly poorer
condition than those from 2012 (Fig. 6).
8
13
18
23
July
10
0
23
78
13
12
20
1718
2223
August
15
10
2012
60
0
3
8
20
13
18
23
September
15
10
5
0
40 3
8
13
18
23
Early
October
30
20
0
2012
20 3
1975
15
40
8
13
18
23
Late
October
10
5
20
0
3
0
5
10
15
Total Length (mm)
20
25
Fig. 6. Length-weight relationships for 1975 (W = 0.0074 * L3) and
2012 (W = 0.0206*L2.7). Only the regression was available for 1975
so no statistical comparison could be made.
1996
1998
2000
2002
2004
2006
2008
2010
• The reason(s) for the order of magnitude decline in Mysis
densities is not yet apparent, but the decline appears to
have occurred abruptly in 1995.
• Mysis condition appears slightly better in 2012 than in
1975, consistent with density-dependent processes.
5
10
80
1994
Discussion
5
25
0
Year 1992 to 2008 from daytime vertical
Fig. 7. Mean Mysis density estimates from
tows at three reference stations sampled in June, July, and August each year.
15
25
0
1992
20
1-Jul
0
Fig. 2. (a) Picking Mysis out of samples on the R/V Melosira, (b) Mysis stage-4
embryo, and (c) fecund female..
13
40
0
12-Apr
Percent Composition
a
8
40
60
J J A S O
Months (2012)
Fig. 5. Percent composition of Mysis life stages in 1975 (left) and 2012 (right).
c
3
2012
A M
Fig. 1. Map of Lake Champlain.
Study site is marked by star. Map
courtesy of Lake Champlain Basin
Program.
60
1975
1800
Weight (mg)
• A subset of individuals
were weighed to
develop a lengthweight regression.
0
0
• Juveniles become less prevalent starting in
September in both years. Mature males are
more prevalent in 2012 than 1975 (Fig. 5).
• Individuals were
measured, counted,
and sexed (Fig. 2b,c).
April
20
10-Aug
19-Sep
29-Oct
Date
Fig. 3. Mean densities (#/m2) with 95% confidence intervals on all
sampling dates in 1975 and 2012.
Methods
• Recently compiled data from the Lake Champlain LongTerm Monitoring Program indicate summer Mysis density
at three reference stations in the lake declined sharply in
1995 (Fig. 7) . Lake Champlain Mysid long-term trends
20
Percent composition
• In 2012, we repeated a Mysis study5 from 1975
to test for differences in Mysis demographics.
40
2400
Mean densities (# / m2)
• In Lake Champlain, invasive zebra mussels
(Dreissena polymorpha) were first detected in
1993 and alewives (Alosa pseudoharengus) in
2003.
60
Mysid abundance
(#/m2)
Results
2012
Mean Mysis density (#/m2)
Introduction
1975
8
13
18
23
Length (mm)
Fig. 4. Length-frequency distributions
for every other sampling date in 1975
(blue) and 2012 (red).
• Alewife did not invade Lake Champlain until 2003 and
can be ruled out as a cause. The coincidence of the
abrupt decline (1995) with zebra mussel invasion (1993)
warrants further investigation.
• We hypothesize that predation by the invasive alewife
will prevent Mysis from recovering to pre-1995 densities.
References
1Johannsson et al. (2011) Can. J. Fish. Aquat. Sci. 68:795-811; 2Pothoven et al. (2004) Trans.
Am. Fish. Soc. 133:1068-1077; 3Gal et al. (2006) Can. J. Fish. Aquat. Sci. 63:2734-2747;
4Mihuc et al. (2012) J. Great Lakes Res. 38:49-57; 5Gutowski, T.B. (1978) The ecology of
Mysis relicta (Lovén) in Lake Champlain. Master thesis, University of Vermont.
Acknowledgements
Funding was provided by Vermont EPSCoR, Lake Champlain
Research Consortium, and UVM Mini Grant. We thank Steve
Cluett and Patrick Gorman of the R/V Melosira, and Chelsea
Mitchell, Peter Euclide, and Mitchell Jones for helping with
data collection and analyses.
About the Authors
1SCB ([email protected]) is an undergraduate student in the Rubenstein School of
Environment and Natural Resources at the University of Vermont; 2JDS ([email protected])
is an Associate Professor of Aquatic Ecology and Director of the Rubenstein Ecosystem
Science Laboratory at the University of Vermont; 3TBM ([email protected]) is a
Professor of Environmental Science at SUNY Plattsburgh and Director of the Lake Champlain
Research Institute.