There is growing evidence that patterns of marine fisheries on some Pacific islands underwent significant changes over the period of human occupation. One such island is Aitutaki in the Southern Cook Islands. Over the millennia of human occupation, there were shifts in habitat use, changes in targeted prey, and the abandonment of some fishing technologies. However, the most striking trend was an apparent decline in fishing altogether. This paper brings together several lines of evidence in an effort to understand why fishing became less important on this small Polynesian "almost-atoll." The possibility of over-harvesting or resource depression is considered. Resource depression could have been a factor at one mainland locality, where occupations were at least semipermanent, but was apparently not involved in declines at an offshore islet site where occupations were short term but intensive. However, fishing on the offshore islet, and deeper water fishing in general, may have been adversely affected by the loss of a key raw material traditionally used for fishhooks, namely pearlshell (Pinctada margaritifera). Further consideration of the offshore islet assemblages is assisted by mtDNA analyses, which have allowed for species level determinations within a key family, the Serranidae. Considering the suite of changes as a whole, the costs of fishing apparently increased significantly over the 1000-year period of occupation. What is less certain is the potential role that terrestrial components (i.e., agriculture and animal husbandry) of subsistence played in fishing declines. Stable isotope studies, now underway, may further elucidate the relationships between marine and terrestrial components of subsistence.