Trophic ecology of marine mammals in the Mexican Pacific Ocean: Prey diversity, network structure, and overlap with fisheries

Abstract

During the 20th century, interaction between marine mammals and fisheries pervaded all Mexican seas and fishing activities, with severe impacts for both fishermen and mammals, generating the need to investigate these animals’ trophic ecology to better manage the country’s fisheries. Aimed to characterize the trophic ecology of marine mammal fauna in the Mexican Pacific Ocean and their interaction with fisheries, here we built a network for the diet similarity of marine mammals and examined its modularity to identify trophic guilds and analyze its relationship with trophic level, prey diversity, and trophic overlap with fisheries. We reviewed literature and data in our group to identify and validate for comparison, 380 prey species of 40 marine mammal species from the Mexican Pacific Ocean. We determined a similarity matrix between marine mammal diets that depended on the diversity and ingested amount of 8 prey types. From this matrix, we built a non-directional and weighted network, with mammal species being the nodes, and diet similarities the edges. We examined modularity and other network traits in relation to mammals’ trophic level, prey richness, and overlap with fisheries. We identified 5 network modules of marine mammals that we defined as trophic guilds, being I) planktophagic, II) ichthyophagic, III) teuthophagic of low trophic level, IV) teuthophagic of high trophic level, and V) sarcophagic. We observed a wide variation among mammals for their weighted degrees (added pairwise similarities), prey richness, and trophic levels that combine differentially known diets and diets with different prey diversities. Inverse relationships between prey richness and weighted degree at the species level, and between trophic level and weighted degree at the guild level, indicate that Mexican Pacific marine mammals belong to two trophic systems −surface and deep waters− mainly structured by competitive exclusion, which is stronger at higher trophic levels. Marine mammals with greater trophic overlap with fisheries in the Mexican Pacific Ocean occur in guilds I and II, principally Phocoena sinus, Zalophus californianus, Tursiops truncatus, and Delphinus bairdii.