Ecology of the emerald shiner Notropis atherinoides Rafinesque in Dauphin Lake, Manitoba

Abstract

The emerald shiner, Notropis atherinoides Rafinesque, is an important forage and bait fish in many lakes and rivers of North America, yet there is little information about this species. The emerald shiner dominated the forage fish assemblage of Dauphin Lake, Manitoba in 1984 and 1985 with peak offshore abundance estimates of >50,000 ha -1, 12,000 ha- 1 and 5,000 ha -1 for young-of-the-year, yearling and adults, respectively. Maximum inshore densities for the three same age groups were 17,000 ha- 1, 2,000 ha -1 and 9,000 ha -1. Average whole-lake abundance of yearling and adult shiners during the summer was only 200-500 ha -l in both years. Comparison of the abundance of 0+ age fish in one year with yearling abundance in the next year suggests significant overwinter mortality. The ages of Dauphin Lake emerald shiners (0+ to 4+ years) were determined using otoliths and scales. Overlapping length-frequency distributions necessitated the use of cumulative probability "Cassie" curves to separate cohorts for growth analysis. Shiner growth in Dauphin Lake was only 40-80% of growth reported for other shiner populations. Spawning in Dauphin Lake began in early July and continued through early to mid-August. Fish ranging from 56 to 81 mm in total length contained 384 to 3365 eggs, respectively. Egg diameters ranged widely within individual females. The prolonged spawning season of this species, together with the wide range in egg diameter observed within individual females suggested that emerald shiners in Dauphin Lake are multiple spawners. Feeding habits examined during periods of peak walleye fry abundance indicate that emerald shiners may compete with larvae by substantially reducing standing stocks of zooplankton preferred by walleye fry. Emerald shiners were opportunistic feeders, ingesting zooplankton, invertebrates, aquatic insects and fish eggs, depending on their habitat. Preference was demonstrated for daphnids and copepodids. Emerald shiner distribution varied according to habitat substrate, depth and the presence or absence of vegetation. Water clarity affected the emerald shiners ability to avoid capture by the sampling nets. Emerald shiners fill an important ecological niche in Dauphin Lake as an abundant and prolific source of forage, inhabiting almost every available habitat. Growth and age-at-sexual-maturity are retarded in the lake but are compensated by multiple spawning and increased longevity.