What is one of the ecological impacts of leeches on fish populations?

Leeches play a significant role in aquatic ecosystems as a food source for a variety of organisms, including fish. Many species of fish will consume leeches, particularly when they are small and available. This relationship contributes to the food web dynamics within aquatic environments, as leeches can help support the diet of predatory fish and foster a balanced ecosystem. Furthermore, their presence indicates a certain level of ecological health, as healthy fish populations often inhabit waters with a variety of food sources, including leeches.

In contrast, the other options focus on competitive interactions or harmful effects. Competing for nesting grounds or consuming fish eggs would typically be detrimental interactions, while transmitting diseases represents a negative impact on fish health rather than a positive ecological role. The identification of leeches as a food source highlights their importance in the food web, showcasing a more beneficial aspect within the ecosystem.