We used a large data set of African, Neotropical, and North American fishes to examine the frequency with which fishes have empty stomachs (n species ϭ 254; n individuals ϭ 36 875). Mean percentage of empty stomachs was low across all fishes (16.2 Ϯ 1.2%) but varied from 0% to 79.4% among individual species. Nocturnal fishes had empty stomachs more frequently than diurnal fishes. Trophic classification was strongly associated with the percentage of empty stomachs, a pattern also revealed from an intraspecific analysis. Fishes appear to adjust their feeding intervals relative to the energy density, conversion efficiency, and particle size of their food. Piscivorous fishes seem to be the only trophic group that regularly experience long periods of empty stomachs, with species that consume prey whole and those that provide extended parental care having the highest proportions of empty stomachs. Activity patterns and life histories of some piscivorous species probably have evolved in partial response to energetic benefits of large, energy-rich food resources.
Food web structure and major sources of primary production consumed by metafauna of Mad Island Marsh, a coastal saltmarsh on the NW coast of the Gulf of Mexico, were compared using stable isotopes and dietary analysis. Carbon and nitrogen isotope data were entered into a mixing model containing 5 potential production sources. Results were inconclusive due to overlapping isotopic signatures of certain sources, but nonetheless indicated that most fishes and macroinvertebrates assimilated material derived mostly from variable mixtures of macrophytes and filamentous algae. Highest estimates of percentage of material assimilated directly or indirectly from C 4 marsh grasses (ranging from 30 to 82%) were for spot Leiostomus xanthurus and Gulf killifish Fundulus grandis. Isotopic analysis could not reveal the detailed structure of predator-prey interactions at the species level; greater detail of trophic pathways was revealed by the dietary analysis. Estimates of vertical web structure (species trophic levels) by the 2 methods were largely concordant. The exceptions were 2 zooplanktivorous and detritivorous fish species and grass shrimp Palaemonetes pugio that had higher trophic levels according to nitrogen isotope ratios. For these taxa, the isotopic method more accurately indexed the number of trophic transfers than the dietary method, which depends on accurate dietary estimation for all food chain components leading to a consumer, and which assumes equal assimilation efficiencies for items found in stomach contents. The isotopic method underestimated trophic levels of several invertebrates, possibly due to inaccurate estimation of mean δ 15 N for production sources supporting these taxa and/or differential trophic fractionation. Together, stable isotope and dietary analyses provide a more accurate assessment of food web structure and dynamics of coastal marsh ecosystems than either method alone.
We used a large data set of African, Neotropical, and North American fishes to examine the frequency with which fishes have empty stomachs (n species ϭ 254; n individuals ϭ 36 875). Mean percentage of empty stomachs was low across all fishes (16.2 Ϯ 1.2%) but varied from 0% to 79.4% among individual species. Nocturnal fishes had empty stomachs more frequently than diurnal fishes. Trophic classification was strongly associated with the percentage of empty stomachs, a pattern also revealed from an intraspecific analysis. Fishes appear to adjust their feeding intervals relative to the energy density, conversion efficiency, and particle size of their food. Piscivorous fishes seem to be the only trophic group that regularly experience long periods of empty stomachs, with species that consume prey whole and those that provide extended parental care having the highest proportions of empty stomachs. Activity patterns and life histories of some piscivorous species probably have evolved in partial response to energetic benefits of large, energy-rich food resources.
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