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Read the following background information and prepare an outline as described in the last paragraph. Bring the outline with you to lab. |
The sockeye salmon of Chobe, British Columbia have been harvested continually since the 1860's. Estimates of the number of fish caught (the “catch”) have been collected since 1890. These are presented in Table 1. The number of fish allowed to travel upstream to spawn (the “spawners”) is presented in table 2. At this time a weir (a fence with a narrow opening through which fish can pass and be counted) was constructed across the mouth of the Chobe River to count the spawners. The weir was moved to the outlet of the lake in 1945. The total catch of fish plus the number of spawners released is the total returning population in a single year (Figure 1).
The spawning runs occur in the summer each year. The number of fish passing through the weir during 1910-1919 and 1940-49 are graphed in Figure 2. The spawning fish migrate up the Chobe River into Chobe Lake. Twenty-five percent of the sockeyes spawn in the beaches of the lake while 75% spawn in the small streams above the lake. Both males and females return to the stream in which they were originally hatched to spawn. Female salmon scoop out a bed in the gravel and deposit their eggs in it. The male salmon fertilize the eggs and then the eggs are covered with gravel. After spawning the fish die and their bodies decompose in the streams and the lake.
The eggs hatch during the late winter or early spring and the fry emerge from the gravel in the spring or summer. Fry hatched in streams descend into the lake. The young live from 2 to 3 years in the lake. Then in May to July of their third or fourth year they leave the lake as smolts and go down the river into the ocean. In the ocean, the young salmon travel long distances away from the Chobe River and only return to this area when they are ready to spawn. Smolts entering the sea return to spawn after 3 or 3 years, i.e., fish return to spawn at 5, 6, or 7 years of age. These details are summarized in Figure 3.
Two other species of salmon inhabit the Chobe River drainage. These are the pink salmon and the coho salmon. The pink and the coho salmon lay their eggs in the same breeding sites as the sockeye salmon and their young grow in the same lake before leaving for the sea. Nothing is known about the fish which the sockeye encounter in the ocean. Eggs and small fish living in the lakes and streams of this region are eaten by many species of fish, such as coho salmon, steelhead, sculpins, Dolly Varden trout, the three-spined stickleback, and alpine char. Birds such as the common merganser, terns, ducks, gulls, and loons also commonly eat small fish. Bears catch adult salmon as they migrate up the streams on their way to spawning areas. Nothing is known about the sockeye's sojourn in the ocean except that they are caught by fishermen at the mouth of the river when they return on their spawning run.
You are a population ecologist appointed to study this situation and determine the reason for changes in the population of sockeye salmon in the Chobe River drainage. Determine what factors might influence the size of the salmon population and decide what specific information you need to evaluate the importance of each factor. Present this information at the start of the next lab in outline form as a study proposal not to exceed 2 pages in length. The proposal (outline) will be worth 10 points and will not be accepted after the start of the lab period. During lab, we will examine the information gathered by fisheries biologists studying this situation for 70 years.
Problem Set (part 2)
During the lab period we will provide you all the data and information gathered by fisheries biologists studying the sockeye salmon in Chobe River, Kodiak Island, Alaska for 70 years. Use these data to write up 1-2 pages to explain:
1. Strong downward trend in catch and number of spawners as seen in Table 1.
2. Cyclic behavior of populations (about every 10 years).
3. Synchronous changes in populations in other river systems
4. Decline in late runs after 1940 as seen in Figure 2.
You have a specific amount of time in lab to work on this (about an hour so that we have 30 minutes left at end). Your write up is worth 10 points. After everyone has handed in their write-up, we will discuss potential explanations together as a class.
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Revised: 25 August 2011