Unexpected prey of juvenile spotted scat (Scatophagus argus) near a wharf: The prevalence of fouling organisms in stomach contents

A knowledge of fish diets can contribute to revealing the trophic role and ecological function of species in aquatic ecosystems. At present, however, there are no efficient or comprehensive methods for analyzing fish diets. In this study, we investigated the diets of juvenile Scatophagus argus collected near a wharf in Daya Bay, China, by dissection and high-throughput sequencing (HTS) using the 18S rDNA V4 region. Microscopy disclosed large amounts of bryozoans and unrecognizable detritus. In contrast, HTS analysis indicated that the fish diets were considerably more diverse than visual inspection suggested. After eliminating fish sequences, approximately 17,000 sequences from taxa in nine phyla (Ciliophora, Bryozoa, Annelida, Bacillariophyta, Chlorophyta, Arthropoda, Dinoflagellata, Tunicata, and Phaeophyta) were identified from the analysis of stomach contents. Twenty-one food categories were identified, most of which (95.2%) were benthic fouling organisms that could easily be collected around wharfs. These consisted of bryozoans (31.9%), ciliates (45.7%), polychaetes (14.6%), and green algae (3.0%). Therefore, to adapt to anthropogenic habitat modification, the fish had probably shifted from planktonic to benthic feeding. The prevalence of fouling organisms in the stomachs of juvenile S. argus indicates that the fish have responded to habitat changes by widening their food spectrum. This adaptation may have increased their chances of survival. The fouling organisms that inhabit highly perturbed coastal ecosystems could represent a food source for animals at higher trophic levels. Our results accordingly suggest that human activity might significantly influence fish feeding behavior and material transfer along the food chain.

Effects of juvenile fish predation (Cyprinus carpio L.) on the composition and diversity of free-living freshwater nematode assemblages

Free-living nematodes are well recognised as an abundant and ubiquitous component of meiobenthic communities, where they serve as a link between microbial production and higher trophic levels. However, the effect of fish predation on nematode assemblages is almost unknown. In this study, the predation effects of the benthivorous juvenile carp (Cyprinus carpio) on nematode abundance, biomass, diversity and species composition in the littoral zone of a natural freshwater pond were examined over 310 days using field enclosures and exclosures. Fish predation altered the abundance and biomass of nematodes, and especially of the dominant species Tobrilus gracilis, Eumonhystera filiformis and Monhystera paludicola/stagnalis. Species richness and species composition, but not the diversity and feeding type of nematode assemblages, were affected by fish predation. Our study provides insights into the food-web ecology of lakes and the first evidence of freshwater fish predation effects on nematode assemblages in a natural habitat.