  Biological diversity is known to play an important role in generating and
maintaining ecosystem productivity and other functions, and has consequently
become a central focus of many efforts to preserve ecosystem services.
Theoretical parallels suggest the diversity of fishing fleets may have a
similarly important role in determining the productivity and ecological impacts
of fisheries, but this possibility has rarely been explored. Here I present
theoretical analyses showing that the diversity of m\'etiers - combinations of
technology, target species, and fishing grounds - and technical efficiencies in
a fishing fleet have important impacts on the productivity, profitability, and
ecological impacts of fisheries, particularly mixed-stock or multispecies
fisheries. Diversification of m\'etiers can increase yields and reduce threats
to weak stocks in both managed and unmanaged multispecies fisheries.
Diversification of technical efficiencies creates opportunities for larger
profits in managed fisheries, but often decreases yields and worsens impacts on
weak stocks in unmanaged fisheries. These results suggest that the potential
impact of management may be highest in fisheries with diverse fleets.
