Deciding What a Correlation Between Ice Cover and Fish Mass Means
A lake ecologist records the date of spring ice breakup and the average mass of adult perch caught each year for 15 years. Earlier breakup dates are associated with larger average perch mass (r = 0.68). The ecologist also has data showing that earlier breakup years tend to have warmer water in May and that a key perch prey species (a small crustacean) peaks earlier in warm years. Perch spawn shortly after ice breakup. The data are observational; no lake was manipulated. One unusual year had very early breakup but small perch, and that year had an unusually cold June.
1. Explain how the May water temperature and prey timing data affect the interpretation of the ice-breakup/perch-mass correlation.
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They provide a plausible mechanism: early breakup → warmer May water → earlier prey peak → better timing between perch spawning and food availability → larger adult mass. This strengthens the biological plausibility but does not prove causation, because all variables are observational and could be driven by a common factor (e.g., regional climate patterns). The correlation remains correlational. The unusual year (early breakup but cold June and small perch) suggests the relationship depends on conditions after breakup, not breakup alone, and that a single variable does not fully explain mass.
2. A policymaker says, 'If we warm the lake, perch will get bigger.' Based on this study, what is wrong with that inference?
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The study did not manipulate temperature, so it cannot establish that warming causes larger perch. The observed pattern could depend on the timing match between spawning and prey, and the cold-June year shows that later conditions matter. Warming could also have nonlinear or negative effects not captured here. The correct statement is that earlier breakup is associated with larger average mass in these 15 years, with a proposed mechanism involving May temperature and prey timing; a causal claim would require experimental or more comprehensive evidence.