OVERFISHING: Although fish and shellfish are renewable resources, each population's productivity is limited, and at some point excessive catches can depress a population's ability to sustain its historic productivity. Abundant fish and shellfish populations entice fishers with the promise of large and lucrative landings. As increasing numbers of fishers seek to catch any particular population, managers find it increasingly difficult to keep fishing efforts in balance with a fishery's ability to sustain its production. If fishing efforts are not controlled, overfishing often results. If overfishing continues for years, a population's reproductive capacity may be diminished, or destroyed. Overfishing also alters the natural balance among species in an ecosystem, sometimes leading to a dramatic shift in species dominance to yield more abundant lower-valued species.