Semelparity is adaptive when:
The supplied key links semelparity with uncertain offspring survival, but that statement is not generally reliable. When reproductive success varies unpredictably among years and adults survive well, iteroparity often spreads risk across seasons. Semelparity is instead favored when survival to another breeding opportunity is low or when concentrating resources produces a disproportionately large fecundity gain. Thus the keyed condition alone does not establish the claimed strategy. The evolutionary currency is lifetime reproductive success, not the number of offspring in one event. Present reproduction can reduce adult survival, future fecundity, or offspring quality, while delayed investment risks death before reproduction. Adult survival, environmental variability, and the shape of allocation costs determine whether repeated or terminal breeding is favored. The decisive distinction is therefore between a descriptive label and the demographic mechanism that generates it. Interpreting the example at the appropriate population scale keeps the causal mechanism distinct from a simple correlation or an absolute rule.
Ref: Ecology: From Individuals to Ecosystems, Begon et al., 5th Ed., Ch. 5