The Case Study

The district of Fairfield conducted an environmental study on freshwater reservoirsin its region. These include lakes, creeks, and public ponds. The study was instigated by recent concerns voiced by a local environmental protection group that fish in these reservoirs may have been contaminated by mercury that they are no longer safe for human consumption.

Mercury is a toxic metal that occurs naturally in the environment. At times, however, human activities may result in unnatural releases of mercury into water bodies, which could in turn enter fish. Consuming mercury-contaminated fish can lead to severe neurological and physiological disorders in humans.

Fairfield's officials identified 943 water reservoirs (including natural lakes) that have significant fisheries and are relatively accessible, based on information found in a previous survey carried out a decade ago. Of these, using the simple random sampling technique, 142 reservoirs were selected for the current study. Then, samples of fish were collected from only 122 reservoirs that contained a targeted group of predator fish species that the researchers are interested in. There are certain criteria that the researchers used for deciding the targeted fish species.

Fish were collected by angling, gill nets, trap nets, dip nets or beach seines. Up to 5 fish from the hierarchical order of preferred predator species were obtained. Care was taken to keep fish clean and free of contamination. In the laboratory, the fish fillet (muscle) of each fish was extracted and the fillets from each reservoir were ground up, combined and homogenised. Then, the tissue was subsampled to analyse the mercury levels.

In addition to collecting fish samples, the officials examined other possible factors that could contribute to elevated mercury levels in fish. They reckoned that this information could be useful for policymaking by members of the Fairfield legislature.

Following completion of the field study, you were handed a dataset containing 122 records of the studied reservoirs. Each record is described by the following variables:

• Reservoir:name of the reservoir
• Fish:number of fish sampled
• Mercury:mercury level from sampled fish in parts per million (ppm)
• Elevation:reservoir's elevation (in feet)
• Drainage:drainage area (in square miles). A drainage area is the area of land which collects and drains the rainwater which falls on it, such as the area around a reservoir.
• Surface Area:surface area of a reservoir (in acres)
• Max. Depth:maximum depth of a reservoir (in feet)
• RF:Runoff Factor. Runoff is the amount of rainwater or melted snow that flows into rivers and streams. Higher runoff factors may lead to more surface waters from the reservoir watershed reaching reservoirs, influencing mercury concentration in fish.
• FR:Flushing Rate. The flushing rate is the number of times all water in a reservoir is theoretically exchanged during a year.
• Dam:Impoundment class (1 = no functional dam present; all natural flowage. 0 = at some man-made flowage in the drainage area)
• RT:Reservoir Type. Three types of reservoirs are identified (1 = oligotrophic. 2 = eutrophic. 3 = mesotrophic)
• RS:Reservoir Stratification. Two indicators are used (1 = reservoir is stratified. 0 = reservoir is not stratified). A reservoir is considered 'stratified' if a temperature decrease of 1 degree per meter exists with depth

Question: Describe what constitutes the populations and samples in the case study and give a critical commentary on the sampling technique used by Fairfield officials.