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Q31 - Smallmouth Bass Removal and Population Structure

Theoretical A Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license
Ecology

Ecology

The smallmouth bass (Micropterus dolomieu) is an introduced species in the area examined in the following experiment. To protect native fish species, a smallmouth bass removal program was initiated in Little Moose Lake in 2000 (indicated by the vertical dashed line). It continued until 2007. A research group examined the effect of the removal on the smallmouth bass population. They found that while the total biomass of the population decreased during the program, the population size increased. To determine the cause, they subdivided the population into three groups based on individual size: yearlings (<100mm, panel a), juveniles (100-200mm, panel b) and adults (>200mm, panel c). They collected two samples each year (spring and fall).

Three stacked line-graph panels (a: yearlings, b: juveniles, c: adults) sharing the x-axis (year, 1998-2007) with a vertical dashed line at 2000 marking the start of the removal program. Solid lines = spring samplings, dashed lines = fall samplings, y-axis = catch per unit effort (CPUE). Panel (a) yearlings: relatively low and flat before 2000, becomes markedly higher and more variable (oscillating) after 2000, generally increasing over the program. Panel (b) juveniles: modest, roughly flat/slightly increasing throughout, with fall values pulling higher than spring toward the end. Panel (c) adults: starts moderate, peaks around 1999, then declines sharply and steadily throughout the removal program to a low, flat level by 2007. Fig.1. The change in abundance of the subpopulations (y-axis) over time (x-axis). Solid lines = spring samplings, dashed lines = fall samplings. Axis 1 = Year. Axis 2 = Catch per unit effort (CPUE).

Which of the following models can possibly describe how the removal of individuals affected the population? Indicate with an X if each of the following models can (T) or cannot (F) describe the changes.

Q31.1. A smallmouth bass removal program (2000-2007) decreased total population biomass but increased population size. The population was split into yearlings (<100mm), juveniles (100-200mm), and adults (>200mm), sampled each spring and fall. Model: the percentage of individuals removed was highest among adult fish.
Q31.2. The decrease in adult abundance leads to a higher per capita birth rate.
Q31.3. The decrease in adult abundance lowered intraspecific competition, so more yearlings and juveniles could survive.
Q31.4. The removal program was less effective at removing small fish than large fish.
Q31.5. The size of breeding adults decreased over time due to this removal program.
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Question reproduced from IBO 2019, Theoretical Exam A, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Official answer key