Swarms of the Ponto-Caspian mysid Hemimysis anomala in Montreal Harbour, St. Lawrence River, Canada

Abstract

Following the recent invasion of the mysid Hemimysis anomala (HA) in the Great Lakes basin and given that the species is often reported in harbors and navigation waterways, this study was designed to determine the abundance and spatial distribution of HA in Montreal Harbour, in the St. Lawrence River and to compare the sampling efficiency of two plankton nets for monitoring the species. Samples were collected in September and October 2009 at 24 stations including harbor and non-harbour sites using 50- and 75-cm diameter plankton nets. High density swarms (>1000/m2) of HA were found along the harbor piers at sites of slow water exchange, while open water sites in the central part of harbor basins and along the river mainstream were characterized by very low densities (<5/m2). HA was not found at non-harbor sites. The spatially contagious distribution of HA was not directly related to variability in water quality parameters. Sampling gear comparisons showed that the larger aperture net yielded higher abundance estimates and larger size range of specimens. The high abundance of HA in a relatively confined area of Montreal Harbour and the absence of the species at upstream sites suggest that the species might have been present in the Montreal area for a number of years as a result of local discharge of ship's ballast waters. Harbour zones in the Great Lakes-St.Lawrence River basin are expected to offer adequate habitats to HA and should be selected as priority sites for monitoring of the species.