Fish as Fertilizer by Mark L. Kuhlmann
Part 2C—Do Marine-Derived Nutrients Affect Stream Organisms?
Instructions—First Jigsaw Group
Read the background information for your figure carefully, then work with your group to understand the figure or figures in the section you are assigned. Use the Step 1–Step 2 approach described below.
Step 1: Describe the graph and what it shows. Make sure you understand how the figure is set up, what the axes show, and what information is depicted. Carefully describe the overall patterns in the data.
Step 2: Try to interpret the data.
When all of the members of your group understand the figure, work together to answer the accompanying questions; they will help guide you as you interpret the graphs and make conclusions. Write down anything your group still doesn’t understand (ask for help from your instructor if needed).
Next, prepare to help the rest of the class understand what you just learned. Think about how you can best explain the graph (its elements and what it shows) and your conclusions to other students who are seeing it for the first time. Be sure everyone in your group is ready to explain your figure(s).
Figure 5—Background
From: Wipfli, M.S., J. Hudson, and J.P. Caouette. 2003. Marine subsidies in freshwater ecosystems: Salmon carcasses increase the growth rates of stream-resident salmonids. Transactions of the American Fisheries Society 132: 37–381. Used with permission.
This study measured the effects of salmon carcasses on the growth rates of fish using both laboratory and field experiments. Growth rates of young coho salmon were measured in 36 artificial stream channels stocked with different numbers of salmon carcasses. Artificial streams contained stream gravel and received stream water flow for 26 days prior to the experiment to ensure colonization by macroinvertebrates. (Macroinvertebrates are animals without backbones that are large enough to see without magnification; larval insects are the most common in fresh water.) Growth rates of cutthroat trout from sections of natural streams with or without experimentally-added salmon carcasses were measured for individuals that were marked and recaptured on two dates. In the “addition” stream sections, salmon carcasses were added on 5 dates to simulate the natural pattern of salmon spawning (and dying).
Figure 5—(A) Mean (+ 1 standard error) growth (percent change in wet mass over 66 days) of young coho salmon in artificial streams exposed to 5 salmon carcass treatments. (B) Mean (± 1 standard error) wet mass of cutthroat trout recaptured on 2 dates from stream reaches with (circle, n = 9) or without (triangle: control, n = 5) experimentally added salmon carcasses. Arrows along the horizontal axis indicate dates when salmon carcasses were added. (C) Mean (+ 1 standard error) growth rates (% change in wet mass/day) of the cutthroat trout in part (B). Growth rates are calculated between successive capture intervals. Asterisks indicate significant differences in growth rate within a season (t-test): *P , 0.05; **P < 0.01. (Redrawn from data in Wipfli et al. (2003), Table 4.)
Questions
Does the presence of salmon carcasses affect the growth of fish? Support your answer by referring to specific data in the figures.
What are some likely ways that salmon carcasses could affect fish growth—i.e., how could the nutrients from salmon carcasses get to or affect fish growth?
Speculate as to why, in the winter, the growth rates of trout in the enriched stream reach were almost the same (actually, a little less than) the growth rates of trout in the unenriched stream section (Figure 5B and 5C).
Originally published at http://www.sciencecases.org/salmon_forest/case2c.asp
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