Interactions of Zooplankton, Mysis relicta, and Kokanees in Lake Granby, Colorado

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Interactions of Zooplankton, Mysis relicta, and Kokanees in Lake Granby, Colorado


Abstract. In studies of zooplankton and kokanees Oncorhynchus nerka in Lake Granby, Colorado, conducted from 1981 to 1983, we investigated the suspected role of introduced Mysis relicta in the decline of the kokanee sport fishery and egg take. Mysis relicta entered surface waters
at night and preyed on zooplankton, except when summer temperatures above 14°C excluded it from the epilimnion and created a temporary refuge for cladocerans. We attributed the disappearance of hypolimnetic Daphnia longiremis to predation by mysids, and the virtual elimination of Daphnia pulex (once the preferred item in the kokanee diet) to the effects of intense selective predation by abundant M. relicta and to kokanee overstocking. Daphnia galeata mendotae, historically the most abundant daphnid, has replaced D. pulex as the principal item in the kokanee diet. Premysid populations of Daphnia spp. appeared by late May and peaked by late July, whereas postmysid populations appeared in late June and peaked in late August or early September. Mysis relicta appeared more frequently in stomachs of large  kokanees ( 21)0 mm in total length) and sometimes contributed substantially to the biomass of the kokanee diet. However, actual numbers of mysids and their frequency of occurrence in individual kokanee stomachs remained low. The disappearance or persistence of Daphnia spp. in other Colorado waters containing mysids appears to be explained by thermal conditions. It is clear that the introduced M. relicta has not adequately substituted for the diminished daphnid populations that were used heavily by planktivorous fishes.


Martinez, Patrick J.
Bergersen, Eric P.


Mysis relicta
Habitat management


16 pages

Date Created










Martinez, Patrick J. and Bergersen, Eric P., “Interactions of Zooplankton, Mysis relicta, and Kokanees in Lake Granby, Colorado,” CPW Digital Collections, accessed June 17, 2024,