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When only one neighboring data point is available, scientists use a forward or backward secant line to approximate the derivative. At the first data point, a forward secant line connects the first and second points. At the final point, a backward secant line connects the last two points. This method calculates the slope between the two available points to estimate the rate of change.

The central difference method uses the slope of a secant line connecting points before and after the target point, providing a more balanced estimate of the instantaneous rate of change. This approach averages the rates of change on both sides, yielding a better approximation than using only one adjacent point. For interior data points in the temperature-growth dataset, this method captures the derivative more accurately.

A negative derivative indicates that as water temperature increases, the rate of weight gain becomes increasingly negative, meaning trout lose weight rather than gain it. The derivative, expressed in grams per degree Celsius, shows how much weight changes for each temperature increase. The consistent decline in these values demonstrates that higher temperatures are increasingly unfavorable for trout growth and survival.

The derivative quantifies how rapidly the growth response changes with temperature, revealing the rate of change in weight per degree Celsius. By calculating derivatives at multiple temperature points, scientists can plot these rates to visualize trends. The resulting graph shows a consistent decline, confirming that brook trout growth efficiency decreases as water temperature rises, with metabolic stress occurring at warmer conditions.

Derivative analysis reveals that brook trout achieve significant weight gain at lower temperatures, such as 15.5 degrees Celsius, where the derivative is positive and relatively large. As temperature increases toward 24.4 degrees Celsius, the derivative becomes increasingly negative, indicating weight loss. This pattern demonstrates that brook trout thrive in cooler water environments and experience reduced growth efficiency or metabolic stress in warmer conditions.

The slope of a secant line connecting two points approximates the derivative by calculating the change in weight divided by the change in temperature. Forward secant lines estimate derivatives at the first point, central secant lines provide more accurate estimates at interior points, and backward secant lines approximate derivatives at the final point. These slope calculations form the basis for understanding how growth rates respond to temperature changes.

The table of derivative estimates compiled from secant line slopes reveals that the derivative function is consistently negative and declining across the temperature range studied. This shows that the rate of weight change becomes progressively more negative as temperature increases. The derivative function demonstrates a strong negative correlation between water temperature and trout growth, indicating that the derivative as a function of temperature is monotonically decreasing.