Incorporation of wastes by native species during and after an experimental aquaculture operation

Abstract

Freshwater aquaculture increases dissolved and particulate nutrients near fish cages, but the degree to which they are incorporated into tissues of native animals is uncertain. At the Experimental Lakes Area in northwestern Ontario, Canada, a cage culture of Rainbow Trout was operated seasonally for 5 y, and invertebrates and fishes were collected before, during, and after culturing from the experimental lake and a reference lake to assess changes in stable isotopes of C and N in their tissues. The feed contained marine fishmeal and was higher in C and N isotope values (δ13C and δ15N) than lake biota by ≥4‰ (all taxa) and 3‰ (all invertebrates), respectively. During the aquaculture operation, δ15N of littoral and pelagic invertebrates, profundal chironomids, minnows, and Lake Trout (Salvelinus namaycush) increased by 2 to 5‰ relative to before aquaculture values. In the 1st and 2nd years after aquaculture, δ15N of several invertebrate taxa and all fishes continued to increase 1 to 2‰/y. In contrast, during aquaculture, only minnows and trout had significant increases (up to 3‰) in δ13C. In the period after aquaculture, the δ13C of fishes, plankton, and profundal chironomids declined to below values measured before or during aquaculture. Isotopic analysis of native biota can be used to monitor assimilation of cage culture wastes in freshwater ecosystems.