Objectives

Students will be able to

1. interpret and make inferences about fluctuations in mussel populations from actual data
2. analyze the effects of human use and habitat changes on a mussel population
3. analyze the effects of price fluctuations on mussel harvests

Method

Students graph and interpret actual Mississippi River mussel harvest data in relation to historical river events or price changes.

Background

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 than the ones being measured are not significantly affecting the population.

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.

Materials

• graph paper (or prepared graphs)
• mussel harvest data (Table 1)
• price data (Table 2)
• timeline

Procedure

1. Provide students with the mussel harvest data only (Table 1). 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 (Figure 1) for students to check against and for class discussion. A bar chart is more appropriate because the data is not continuous.
2. When they have completed graphing their mussel harvest, have them divide into groups and give each group a copy of the time line.
3. 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 activity.)

1. Hand out price per ton data (Table 2).
2. 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 2).
3. Have them look for correlations between price and tons harvested.

Assessment

1. In what year were the greatest tons of mussels harvested? 1914.
2. 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.
3. 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).
4. What two decades had the lowest harvest? 1940 and 1950.
5. 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 effects on mussels also contributed to lower mussel populations.
6. Why did harvest of mussels increase in the 1960s? Development of market for mussels shells to be used as nuclei (seeds) for cultured pearls.
7. What harvest technique resulted in a greater harvest in 1966? Scuba diving.

Extension

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

Method

Students graph actual Mississippi River mussel harvest data from 1986—1997. Students interpret relationships between harvest levels and price per pound.

Background

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 the 1980s 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 of a decline in washboard populations. This indicated to biologists that the size limit set may have been too low to protect the washboard population.

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 surveys.

Materials

• graph paper (or prepared graphs)
• Table 3

Procedure

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.

1. In this activity students can make several different types of graphs.
   • 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 3).
   • Or they may want to draw two graphs with two Y axis each, comparing the price/pound and the number of mussels harvested.
     They will end up with a graph showing the cost/pound vs. the amount harvested for each species (Figures 4 and 5).
2. Have students compare how many of each species is harvested, and then compare those numbers to the price/pound.
3. 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 to protect the washboard population.

Assessment

1. When the harvest of washboard mussels was at its greatest, the price per pound was at its (lowest/highest)? Lowest.
2. In general, price/pound increases when there is a (lower/higher) demand for mussels than supply. Higher.
3. Why do biologists believe the harvest of washboard mussels was decreasing from the late 1980s to late 1990s 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 harvest size and fewer adults in the population.
4. 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 mussels, 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 to market.