Food, Feeding Selectivity and Ecological Efficiencies of Fundulus notatus (Cyprinodontidae)

Abstract

Terrestrial arthropods formed 41% of the diet of field- collected blackstripe top minnow, with a consistent Electivity Index (E) value of positive-one; snails, aquatic insects and microcrustaceans, re- spectively, occupied 19%, 16% and 10% of the diet. Positive E values were maintained for aquatic insects, except in stagnant pools where inter- specific fish competition probably occurred. Crustaceans assumed negative E values except in a single habitat, while snails took positive E values only in stagnant pools. Top minnows smaller than 38 mm, S.L., supple- mented terrestrial arthropods with microcrustaceans for 28% of their ration; crustaceans decreased and aquatic insects increased in relative importance as fish size increased. Algae were ingested with 41% fre- quency, but apparently not digested. Fishes in feeding experiments showed a preference for chironomid larvae but an electivity for floating, terrestrial insects, which were readily available. Physid snails, with 42% frequency of occurrence in field- collected fishes, were avoided in the presence of abundant alternative prey. Assimilation efficiency was 97% for all size classes of top minnows. Top minnows of 48 to 65 mm S. L. had a lower ecological growth effi- ciency, (5.9%) than smaller fishes (8.5%). The average daily energy expenditure for 48 to 65 mm fishes was lower, 20 cal x g of initial dry wt-1 (gWo-1), than the 55 cal x gWo-1 for fishes smaller than 48 mm, S. L..; but the net efficiency of food utilization for growth in the latter (21.6%) was not significantly higher than that in 48 to 65 mm fishes (15.0%). Fundulus notatus facilitates the entrance of allochthonous organic material into the ecosystem by digesting and defecating the remains of terrestrial arthropods, and it indirectly taps the flow of niatter and energy from allochthonous plant sources by preying upon aquatic insect detritus-feeders, getting more than one-half of its ration directly or in- directly from basically allochthonous sources. It also functions as a third- or fourth-level predator.