Reduced recruitment of Chinook salmon in a leveed bar-built estuary

Estuaries are commonly touted as nurseries for salmonids, providing numerous advantages for smolts prior to ocean entry. In bar-built estuaries, sandbars form at the mouth of rivers during periods of low stream flow, closing access to the ocean and preventing outmigration. We evaluated how summer residency in a leveed bar-built estuary affects the growth, survival, and recruitment of a Chinook salmon (Oncorhynchus tshawytscha) population. We performed a mark-recapture study on outmigrants to determine juvenile estuary abundance, growth, and survival. We used returning adult scales and otoliths to determine the relative proportion of summer estuary residents in spawning adults. Juveniles in the estuary grew less after mouth closure, and ultimately summer estuary residents had lower smolt-to-adult survival and contributed disproportionately less to the spawning population than juveniles that reared in the ocean their first summer. Mouth closure may lower food availability and deteriorate estuary conditions by reducing marine prey influx and estuary circulation. This research demonstrates the complexity of estuary dynamics and function as salmonid nurseries, particularly when considering the extensive modification of estuaries.

Baroclinic Effects on Wind-Driven Lateral Circulation in Chesapeake Bay

The 2-month-long mooring data were collected in a straight midsection of Chesapeake Bay to document the lateral circulation driven by along-channel winds. Under upestuary winds, the lateral circulation featured a clockwise (looking into estuary) circulation in the surface layer, with lateral Ekman forcing as the dominant generation mechanism. Under downestuary winds, however, the lateral circulation displayed a structure dependent on the Wedderburn number W: a counterclockwise circulation at small W and two counterrotating vortices at large W. The surface lateral velocity was phase locked to the along-channel wind speed. Analysis of the streamwise vorticity equation showed that the strength and structure of the lateral circulation in this stratified estuary were largely determined by the competition between the tilting of planetary vorticity by along-channel currents and lateral baroclinic forcing due to sloping isopycnals. Under strong, downestuary winds, the lateral baroclinic forcing offset or reversed the tilting of planetary vorticity on the western half of the estuarine channel, resulting in two counterrotating lateral circulation cells. A bottom lateral flow was observed in the deep channel and appeared to be generated by lateral Ekman forcing on the along-channel currents.

Estuary circulation: assessment and application

SynopsisThe Hansen and Rattray (1966) procedure for classifying estuary circulation pattern is applied to the estuaries of the Dornoch and Cromarty Firths, and the Tay, Forth, and Clyde. All are found to be partlymixed, with the Cromarty Firth and Clyde approaching the stratified condition. The ability of the method to forecast stratification and circulation is examined by comparing predictions and field measurements in the five estuaries. Acceptable results are obtained in cases of better mixing, but some significant departures occur with low range tides and with irregular depth.One dimensional dispersion coefficients are calculated for the five systems and vary from 70 to 1285 m2s−1. Values increase both as mixing diminishes and with distance seaward along the estuary. Comparison with the table relating values to contributing mechanisms given by Cox and Macola (1967) shows in these cases density-driven circulation to produce higher coefficients than storage basin effect. The results also suggest that transverse circulation induced by Coriolis force produces a further order of increase in dispersion coefficient, values then exceeding 500m2s−1.