2.2 | Fish tissue collection
To assess changes to the trophic structure of the San Juan River’s fish community, we obtained fish tissue for isotopic analysis from a historical period that was just prior to closure of Navajo Dam (1959 to 1961) and a contemporary period, which comprised the years 2003 – 2014. We attained historical samples (~0.5 g of white muscle) from fish accessioned to MSB (supplemental material) and used raft-mounted electrofishing and seines to collect contemporary samples (see Franssen et al ., 2015 for details). Contemporary tissue samples consisted of white muscle, except for Colorado Pikeminnow and Channel Catfish. For these species, we sampled approximately 5 mm2 of tissue from the caudal fin. In addition to contemporary fishes, we sampled riffles and backwaters for invertebrates from the Order Ephemeroptera (~2.5 mg dry mass; Franssenet al ., 2019). All contemporary tissue samples were preserved in table salt in the field and rinsed before processing (Arrington and Winemiller, 2002). Apart from historical Colorado Pikeminnow, we only included species with a sample size of 10 or more. To help control for ontogenetic trophic changes within species and between periods, we restricted the minimum size of fishes analyzed to 50 mm total length (TL) and the maximum to that available from MSB (Table 1). All samples were dried at 60ºC for at least 24 hours before being homogenized into powder via a mortar and pestle.