Students will be able to
- interpret and make inferences about fluctuations
in mussel populations from actual data
- analyze the effects of human use and habitat changes
on a mussel population
- analyze the effects of price fluctuations on mussel
Students graph and interpret actual Mississippi River
mussel harvest data in relation to historical river
events or price changes.
Data gathered about a wildlife population in a similar
manner over a period of time may be useful in detecting
trends in that population. The same data may be interpreted
by those analyzing it in a variety of ways. Because a
mussel population is influenced by many factors, it may
be difficult to measure the effect of a single factor.
Thus, assumptions must often be made that factors other
the ones being measured are not significantly affecting
When measuring populations of mussels, biologists are
seldom able to get a total count. Ideally, biologists
would like to have a total count of mussel populations
for the period of time they are interested in. However,
usually only a sample of the population can be obtained
and inferences about the total population must be made
from this sample. Errors or inconsistencies in gathering
the data over time may greatly influence the accuracy
of the data. Despite the influence of unknown factors
and possible inconsistencies in data gathering, regularly
conducted counts or inventories of a population may still
be the best information available and decisions must
be made from this data.
Unfortunately, there were very few surveys of mussel
populations in the past and sampling mussels is a very
expensive and time consuming endeavor. Therefore, it
is often necessary for biologists to rely on other types
of data to analyze trends in mussel populations. Because
of the economic importance of mussels, records of tons
harvested as well as the price paid have been kept.
- graph paper (or prepared graphs)
- mussel harvest data (Table
- price data (Table
|Note: Examples of graphs are provided.
However, you may choose to have students make their own graphs
- Provide students with the mussel harvest data only
Have them graph data from 1894 to 1986. Students
should put a legend on their graph. You may want
to make an overhead or copy of the graph provided
1) for students to check against and for class
discussion. A bar chart is more appropriate because
the data is not continuous.
- When they have completed graphing their mussel
harvest, have them divide into groups and give each
group a copy of the time line.
- Have each group look for correlations between historic
events and changes in harvest.
Comparison of historic harvest and price/ ton
(Students may use mussel harvest graph from previous
- Hand out price per ton data (Table
- Next they need to add a second Y axis for price/ton
or they may draw a second graph using data on this
page to compare the harvest data to (Figure
- Have them look for correlations between price and
- In what year were the greatest tons of mussels
- Look at the time line of historic events. What
were most of the shells used for during the year
of greatest mussel harvest? Pearl Buttons.
- What happened between 1900 and 1910 that may partially
explain the drastic decline of mussels harvested? Six-foot
channel project (dredging changed habitat and covered
up mussel beds; construction of additional closing
dams closed off flow into side channels).
- What two decades had the lowest harvest? 1940
- What explanations are there for these low harvests? The
main reasons were the invention
of plastic and its use in button making and construction
of the locks and dams that dramatically changed
habitats on the River. However, pollution and its
on mussels also contributed to lower mussel populations.
- Why did harvest of mussels increase in the 1960s? Development
of market for mussels shells to be used as nuclei
(seeds) for cultured pearls.
- What harvest technique resulted in a greater harvest
in 1966? Scuba diving.
The color and strength of the shell were important to
the button industry. Current clammers choose
species based on the thickness of the shells. Do some
background research to determine which species of mussels
have the thickest shells and see if they are harvested
for use in the cultured pearl industry.
Mussel Market Mystery
- Part 2
Students graph actual Mississippi River mussel harvest
data from 1986—1997. Students interpret relationships
between harvest levels and price per pound.
The market for freshwater mussel shells is the same
as it is for any business, supply and demand are interdependent.
For example, if the price for mussel shells (the product)
is high, then the demand for the shell is great
and many people are harvesting them. However, if the
number of shells harvested (supply) is great,
then the price of shell usually drops.
Because of the demand for freshwater mussels, a size
limit was placed on commercial freshwater mussels in
based on the management concept of sustainable yield. Sustainable
yield management of mussel populations is based on the
assumption that mussel size limits are sufficient to
protect enough adults to reproduce numbers equal to what is being harvested.
The data provided in this activity was looked at by
biologists as one method of monitoring the populations
of washboard and threeridge
mussels to determine if over-harvesting of their populations was
occurring. What they saw was decreasing harvest of washboards even
though the price per pound continued to rise. Additionally, when
they noticed that the tons harvested of threeridge continued to rise,
they theorized that this was due to clammers switching from the
more preferred washboard to the less sought-after threeridge because
a decline in washboard populations. This indicated to biologists
that the size limit set may have been too low to protect the washboard
A detailed biological survey was conducted in the mid-1990s
which verified the biologists' concern, the size limit
was inadequate to maintain sustainable yield. Over-harvest
was having a severe impact on the populations of washboard
mussels along Wisconsin's portion
of the Mississippi River (Figure
6). The decline was so severe in
some areas that surveys were documenting more endangered Higgins'
eye pearlymussels than the commercial washboard. The season for
washboard mussels in Wisconsin's portion of the Mississippi was closed
based on the market data presented in this activity and the biological
- graph paper (or prepared graphs)
- Table 3
|Note: Examples of graphs are provided.
However, you may choose to have students make their own graphs.
Using the information provided in Table
3, have the students
do a comparison of harvest and price for two species commercially
harvested from 1986-1997.
- In this activity
students can make several different types
- They may want to make two, two-line graphs,
one with a comparison of
the number of washboard vs.
threeridges harvested (Figure
2) and one comparing the price/pound
of threeridge vs. washboard (Figure
- Or they
may want to draw two graphs with
two Y axis each, comparing the
price/pound and the number of mussels harvested.
will end up with a graph showing the cost/pound
vs. the amount harvested for each species
4 and 5).
- Have students
compare how many of each species is harvested,
and then compare those numbers to the price/pound.
- Use the background
materials to discuss the concept of sustainable
yield, importance of size limits and the survey conducted
to document why current regulations were not adequate
- When the harvest of washboard mussels was at its
greatest, the price per pound was at its (lowest/highest)? Lowest.
- In general, price/pound increases when there is a
(lower/higher) demand for mussels than supply. Higher.
- Why do biologists believe the harvest of washboard
mussels was decreasing from the late 1980s to late
even though the price per pound was getting higher? Washboard
mussel populations were being over-harvested to the
point that there were fewer and fewer mussels of legal
size and fewer adults in the population.
- What may have caused a decline in the harvest of
both species beginning in 1996? By 1996 many areas
of the Mississippi River were heavily infested by zebra
which attached themselves to native mussels and
made clamming much more difficult. The clammers could
not easily identify
species underwater and had to spend a lot of time
cleaning off the zebra mussels before taking the shells