The size of a population may increase, decrease, or stay the same over time. Trends in population size are a key focus of ecological research and its application to conservation biology. A decrease in size through time, for example, might signal declining health in a local population—a matter of practical concern when the population in question is a commercially harvested fish or an endangered species. In the 1990s, catastrophic declines in cod populations led to a ban on commercial cod fishing on the Grand Banks of Newfoundland. The decline affected both cod and cod fishers, and it also led to changes in Grand Banks ecosystems: Crab and lobster populations have increased as there are fewer cod to prey on them.