Estuaries often comprise a habitat mosaic, the connectivity of which depends in part on mobile organisms that move in between habitats for feeding and breeding. We assessed resource utilization by 2 co-existing puffer fish species, Sphoeroides testudineus and Sphoeroides greeleyi, in mangroves, saltmarshes and shallow subtidal channels of a subtropical bay. We hypothesized that puffer fish migrate into mangroves mainly to feed and that the coexistence of the 2 species may be explained by differences in resource utilization and/or by a differential preference for different foraging grounds. We combined a stable isotope approach with an Ecopath model that contrasted detritus-driven and herbivory-driven foodweb scenarios. The most parsimonious foodweb scenario involved the feeding of puffer fish on benthic invertebrates associated with Spartina marshes. This emphasizes the importance of saltmarshes as feeding grounds for both puffer fish species, independently of where they were sampled. Small differences in isotopic signatures between S. greeleyi and S. testudineus indicated major resource overlap, but also some degree of food partitioning probably through the selection of differently sized prey. The smaller S. greeleyi consistently had a slightly higher trophic level than the larger S. testudineus. S. testudineus had larger isotopic niche sizes as a consequence of greater inter-individual variation in resource use. Our results emphasize the importance of considering multiple habitats and foodweb scenarios when investigating resource use and species interactions in estuarine ecosystems. |
