Survival planet

For the thirteen years since the Evenson space yacht crash-landed on the richly vegetated planet of Iduna, Soren Evenson and his four children have lived among the native Thrulls—gentle, nonviolent, fishlike mammals. But now the peaceful way of life on Iduna is being threatend. A scouting party of corrupt government leaders has come from Evensons' native planet, Earth, on pretext of finding a habitable planet for the starving billions they have left behind them. It soon becomes apparent, though, that they do not intend to report back to the dying planet. They have decided to abandon Earth and rule Iduna instead.