Explaining advection: do larval bay anchovy (Anchoa mitchilli) show selective tidal-stream transport?

Bay anchovy (Anchoa mitchilli) have been reported in several systems to dis play net up-estuary larval movements, against the mean flow. However, the m eans by which such transport occurs is poorly understood. We address how es tuarine circulation and larval behaviors serve to transport larvae of the b ay anchovy. In two successive summer seasons, we conducted multiple near-sy noptic samples of larval distributions and water column structure along a 4 5-km section of the middle Hudson River estuary. The analysis focuses on pa tterns in the vertical distribution of larvae that may help explain transpo rt, and the along-river distribution of different ontogenetic stages. The p rediction that post-flexion larvae induce selective tidal-stream transport (STST) by vertically migrating in conjunction with tidal or diel cycles was tested via harmonic regression. Larval concentrations and average larval d epths often varied with tidal stage. Maximum concentrations tended to occur at times of slack water, and larvae were often closer to the surface durin g slack tides as well. These patterns may be the result of tidal movements of horizontal abundance gradients. rather than vertical migrations. The pre diction that larval transport is facilitated by a preference for deep water was addressed via analysis of variance, testing for depth effects on time- averaged concentration estimates. At some sites, larvae were most concentra ted at intermediate depths, which would promote retention (no net horizonta l movement) or slow up-river transport. However, in 1996, larvae were found most concentrated at the surface at two sites, suggesting down-river advec tion. With respect to along-river distribution, we tested the prediction th at ontogenetic stages differed in their distribution in a manner consistent with up-river transport. In 1995, pre-flexion larvae were distributed furt her up river than eggs, and post-flexion larvae were slightly up river of p re-flexion larvae. Along-river distributions were perturbed in 1996 by a st orm that caused high run-off and forced larvae down river. Following this e vent, along-river position did not vary with ontogenetic stage. The study d esign that combined analysis of larval depth distribution and along-river d istribution enabled us to make and test predictions regarding transport pro cesses. A portion of our depth distribution data implied stasis or weak up- river advection, and we found evidence that this was the case. (C) 2000 Int ernational Council for the Exploration of the Sea.