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Difficulty: HardResolve the Paradox

Wildlife biologists managing a temperate river basin installed extensive artificial gravel spawning beds to reverse a persistent multi-year decline in the river's native salmon population. Comprehensive aquatic surveys over the following three years revealed that the intervention was a clear biological success at the early developmental stage: egg deposition rates and juvenile hatch counts across the river basin more than tripled relative to pre-installation benchmarks. Nevertheless, annual counts of mature adult salmon returning from the ocean to spawn in the river system declined even more rapidly over those same three years than they had in the period preceding the intervention. Which of the following, if true, most helps to resolve the apparent discrepancy described above?

  1. A
    The installation of the artificial spawning beds displaced several non-salmonid benthic fish species that previously occupied those riverbed sectors.
  2. B
    Neighboring river systems that did not install artificial gravel spawning beds experienced an increase in the number of returning mature adult salmon over the same three-year period.
  3. Dense spatial clustering of newly hatched fry around the artificial gravel beds attracted unprecedented concentrations of predatory fish and birds, which consumed juvenile salmon in such high volumes that fewer total smolts survived to complete their seaward migration than when spawning was naturally dispersed.Answer
  4. D
    Water quality monitoring stations situated downstream from the artificial gravel beds recorded dissolved oxygen and temperature readings well within optimal survival parameters for adult salmon throughout the three-year period.
  5. E
    Adult salmon returning to the river system showed an overwhelming behavioral preference for laying their eggs in the artificial gravel beds over surviving natural gravel patches.

Answer

The statement explaining that dense spatial clustering of hatchlings around the artificial beds led to catastrophic predation losses before seaward migration resolves the discrepancy by demonstrating how higher hatch counts yielded fewer total surviving mature adults.
The correct response introduces an intermediate ecological bottleneck: while the artificial beds successfully boosted initial hatching, the extreme spatial concentration of fry attracted heavy predator aggregations that decimated the population before ocean migration. Consequently, fewer net juveniles survived to become returning adults, allowing both the tripling of hatch rates and the accelerated decline of returning adults to be simultaneously true.

Step-by-Step Solution

1
Identify the two seemingly contradictory facts presented in the stimulus.
Fact 1: Egg deposition and juvenile hatch counts more than tripled following the installation of artificial spawning beds. Fact 2: The count of mature adult salmon returning to spawn declined at an accelerated rate over the same period.
Resolving a paradox requires clearly isolating the two conflicting premises that must both remain true.
2
Determine what intermediate causal mechanism is required to bridge the two facts.
An event or condition occurring between the juvenile hatching stage and the adult return stage that causes an overall reduction in adult recruitment despite the initial surge in hatchlings.
Because salmon mature in the ocean before returning to rivers, a bottleneck during out-migration reconciles high initial hatch numbers with lower adult returns.
3
Evaluate the answer choices to find the one providing a bridge without denying either premise.
The option noting that dense clustering of fry attracted intense predator aggregations, resulting in fewer total juveniles reaching the sea than under dispersed natural conditions, perfectly reconciles both premises.
It explains how a 300% increase in hatching paradoxically produces a net deficit in surviving migratory cohorts due to a density-dependent predation bottleneck.

Key Concept

Resolving a paradox involving developmental life stages through an intermediate density-dependent bottleneck mechanism
Estimated Time:1m 45s
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Topics

Resolve the Paradox

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