Stable isotopes reveal trophic ontogeny in Cisco (Coregonus artedi)

Cisco ( Coregonus artedi ) are a widespread, cold‐water zooplanktivore native to North America. Although Cisco are generally referred to as an “obligate zooplanktivore,” there is some evidence that the species exhibits considerable variability in trophic niche. Here, we assessed how Cisco body size relates to trophic position, that is, trophic ontogeny. We analysed 13 C and 15 N isotopes from Cisco ranging from 127 to 271 mm in body length ( n = 66) from Trout Lake, Vilas County, Wisconsin, USA. 15 N isotopes showed smaller Cisco had a trophic position of ~3, which steadily increased to ~3.5 for larger Cisco. Further, 13 C isotope signatures showed Cisco transitioned to be more pelagically reliant (lower 13 C signatures). Using gillnet catch data, we found that larger Cisco were using deeper habitats than smaller Cisco. Our results support that Cisco have significant variability in trophic niche even though they are traditionally thought of as an obligate planktivore. Overall, we emphasize that researchers should be cautious when generalizing Cisco trophic function, particularly when considering the broader food web.