Biodiversity encompasses all levels of biological organization, from individuals to communities and ecosystems. Fish populations can contain distinct groups displaying unique phenotypes, each of which contributes to the productivity of the population by exploiting a different suite of available resources (e.g., Kerr et al. 2010). Populations can also be composed of mixed stocks that share habitats but vary spatiotemporally in resource use and are genetically distinct, increasing productivity through biocomplexity (Hilborn et al. 2003). Failure to protect such distinct units can lead to reduced abundance and the productivity of a fishery and its supporting food web (Villasante 2012). At the community level,
many aquatic systems contain fish communities that are spatially distinct yet biologically linked (e.g., warm water – cold water; lentic–lotic), which combine to enhance productivity across habitats. For example, warm-water prey species can be an important food for cold-water predators (Vander Zanden et al. 1999). More is not always better though, and a distinction must be made between intact native faunas and diversity achieved through biomanipulation. Intentionally introduced species can have unpredicted negative effects, particularly in the absence of thorough risk assessments (Cambray 2003; Leprieur et al. 2009).