Movement patterns of winter flounder (Pseudopleuronectes americanus) in the southern Gulf of Maine: observations with the use of passive acoustic telemetry

Despite its recreational and commercial importance, the movement patterns and spawning habitats of winter flounder (Pseudopleuronectes americanus) in the Gulf of Maine are poorly understood. To address these uncertainties, 72 adult winter flounder (27–48 cm) were fitted with acoustic transmitters and tracked by passive telemetry in the southern Gulf of Maine between 2007 and 2009. Two sympatric contingents of adult winter flounder were observed, which exhibited divergent spawning migrations. One contingent remained in coastal waters during the spawning season, while a smaller contingent of winter flounder was observed migrating to estuarine habitats. Estuarine residence times were highly variable, and ranged from 2 to 91 days (mean=28 days). Flounder were nearly absent from the estuary during the fall and winter months and were most abundant in the estuary from late spring to early summer. The observed seasonal movements appeared to be strongly related to water temperature. This is the first study to investigate the seasonal distribution, migration, and spawning behavior of adult winter flounder in the Gulf of Maine by using passive acoustic telemetry. This approach offered valuable insight into the life history of this species in nearshore and estuarine habitats and improved the information available for the conservation and management of this species

An Interdisciplinary Assessment of Winter Flounder Stock Structure

Stock structure and management of winter flounder (Pseudopleuronectes americanus) was evaluated throughout its geographic range in the northwest Atlantic. Information on genetics, morphology, meristics, larval dispersal, life history traits, applied mark experiments and environmental signals was reviewed. Winter flounder in U.S. waters are currently managed as three separate units; Georges Bank, Gulf of Maine and Southern New England/Mid-Atlantic. Estuarine spawning, which likely plays an important role in reproductive isolation and population structure, appears to be obligate in southern New England, non-existent on Georges Bank and variable in the Gulf of Maine. Despite evidence for reproductively isolated estuarine groups, information from tagging, meristic analysis, and life history studies suggest extensive mixing, thereby supporting the current U.S. management regimen. In Canadian waters, winter flounder are managed as three units: western Scotian Shelf (NAFO Div. 4X), eastern Scotian Shelf (NAFO Div. 4VW), and the southern Gulf of St. Lawrence (NAFO Div. 4T). Genetic analysis and parasite markers indicate that these Canadian management units are distinct. However, examination of inshore and offshore winter flounder within division 4X suggests little interchange occurs between these groups. Several separate stocks probably exist within the 4T management area as well. Stock assessment and fishery management would likely benefit from stock composition analysis of mixed-stock fisheries of both U.S. and Canadian fishery resources

Rapid incision of the Mekong River in the middle Miocene linked to monsoonal precipitation

The uplift of orogenic plateaus has been assumed to be coincident with the fluvial incision of the gorges that commonly cut plateau margins. The Mekong River, which drains the eastern Qiangtang Terrane and southeastern Tibetan Plateau, is one of the ten largest rivers in the world by water and sediment discharge. When the Mekong River was established remains highly debated—with estimates that range from more than 55 to less than 5 million years ago—despite being a key constraint on the elevation history of the Tibetan Plateau. Here we report low-temperature thermochronology data from river bedrock samples that reveal a phase of rapid downward incision (>700 m) of the Mekong River during the middle Miocene about 17 million years ago, long after the uplift of the central and southeastern Tibetan Plateau. However, this coincides with a period of enhanced East Asian summer monsoon precipitation over the region compared with the early Miocene. Using stream profile modelling, we demonstrate that such an increase in precipitation could have produced the observed incision in the Mekong River. In the absence of an obvious tectonic contribution, we suggest that the rapid incision of the Tibetan Plateau and the establishment of the Mekong River in the middle Miocene may be attributed to increased erosion during a period of high monsoon precipitation