Comparative ecology of two sympatric species of dace, Rhinichthys cataractae and Rhinichthys atratulus, in the Mink River, Manitoba
Gibbons, John R. H.
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The Great Lakes longnose dace (Rhinichthys cataractae cataractae) and the western blacknose dace (Rhinichthys atratulus meleagris) occur sympatrically in the middle portion of the Mink River, Manitoba. Their abundance is associated with high gradient (7 m./km.), which results in rapid water velocities and gravel or rock substrates, and with plant detritus, which supports the insect fauna used as food. Fry of both species were found in July, but longnose hatch earlier and grow faster than blacknose dace. For a short period both species are found together in shallow, silted margins of the stream, of little or no current. Longnose move out into fast water in July and August when they are between 25 and 30 mm in length, while the majority of blacknose remain in the margins for up to one year, until they reach a fork length of about 45 mm. Thereafter, blacknose are found mainly in channels (15-45 cm/sec) and longnose in riffles (> 45 cm/sec). Blacknose males are territorial over pea-sized gravel, longnose males over small rocks in riffles. There is a marked habitat difference between the sexes of both species during spawning, females entering territories only when completely ripe. In the late fall, blacknose adults and juveniles were found only in beaver ponds, while longnose were found under large flat stones in riffles. The diet of the two species is strikingly similar. Longnose and blacknose in their first twelve months fed almost exclusively on the families Baetidae, Tendipedidae and Hydropsychidae, but their proportions differed. Hydropsychiadae was always the major food item by weight for older longnose. Older blacknose were similar, but they switched to Tipulidae and Ephemeridae in May and October. Surber samples of benthic fauna were taken in a riffle and a channel. Baetidae and Tendipedidae were highly foraged by both species. Despite their similar diet, the degree of spatial and temporal isolation between the two species is thought sufficient to allow for their coexistence.