Zooplankton Community Response to the Introduction of Cisco in Tiber Reservoir, Montana

Abstract

AbstractPrey fish introductions, some planned and some unplanned, can have major influences on ecosystem dynamics. In 1997, Cisco Coregonus artedi were introduced in Tiber Reservoir, Montana. The introduction of the planktivorous fish was intended to provide an alternative prey resource to predators. After the introduction of Cisco, average zooplankton densities increased 184% from 9.95 organisms/L (1990–1997 [preintroduction period]) to 28.25 organisms/L (1998–2014). Diet analysis indicated targeted feeding by Cisco on larger‐sized zooplankton relative to the average size of zooplankton within the water column. Within 2 years, Daphnia spp. mean length decreased from 1.45 mm in 1997 to 0.85 mm in 1999. From the preintroduction period to the establishment of Cisco, mean length of Daphnia spp. decreased by 29% (from 1.33 to 0.94 mm). Daphnia spp. over 1.5 mm became rare, and those over 2.0 mm were absent from all samples. Prior to Cisco introduction, 0.4–0.7‐mm individuals comprised 4–8% of the Daphnia spp. population; after the introduction, this size‐group increased to 26–46% of the Daphnia spp. population. The zooplankton community composition shifted toward a smaller, more diverse community with decreased calanoid copepods and increased cladocerans Diaphanosoma birgei and Bosmina spp. Prior to the Cisco introduction, D. birgei were not present in any samples. After 17 years, significant changes to the zooplankton community resulting from the introduction of Cisco persisted in Tiber Reservoir. This study provides information that fish managers should consider when contemplating the consequences of adding planktivorous prey to a forage base or when managing this species after an illegal introduction.