Abstract: Chaos fascinates ecologists because it generates wildly fluctuating population numbers from deterministic biological mechanisms. Further, such chaotic mechanisms are highly sensitive to randomness, producing extremely rapid divergence of population numbers, from even the smallest perturbations. Here, the biological relevance of chaos is considered by examining fluctuations in Dungeness crab populations. We show that large amplitude fluctuations in crab numbers may orignate from extremely small environmental perturbations. Moreover, even though the underlying biological mechanisms are density-dependent, they are not, in fact, chaotic, nor is the combination of biological and environmental mechanisms chaotic.