The importance of small-scale turbulence in the feeding of herring larvae

Herring larvae form a large, persistent and well-defined aggregateoff southwest Nova Scotia coincident with a region of strongtidally generated turbulence. Our objective is to test the hypothesisthat turbulent, tidally well-mixed regions constitute a preferentialfeeding environment for herring larvae, and are thus the proximatecause for the apparent retention of larvae in these areas. Dimensionalanalysis, coupled with relevant biological parameters and fielddata, are used to derive two non-dimensional numbers from existingpredator-prey encounter models, and to propose a third numberwhich incorporates the minimum encounter rate required for larvalmaintenance. The results of this analysis show that when root-mean-squareturbulent velocities are of the same order of magnitude as theprey velocities, they start to influence encounter rates significantly;and that when turbulent velocities are an order of magnitudegreater than the predator velocities, they dominate the encounterprocess. At reported natural food concentrations, food itemsare on average one order of magnitude away from the larvae'sreach. The importance of turbulence as a mechanism to bringfish larvae closer to their prey is revealed by the findingthat in tidally well-mixed regions herring larvae would requireone order of magnitude less food than their counterparts inthe stratified regions. The model shows that the feeding environmenton the adjacent stratified waters off southwest Nova Scotiais detrimental for herring larvae, and that high mortality wouldbe expected if larvae had to overwinter in this region. Ouranalysis also shows that the growth pattern observed for autumn-spawnedlarvae is explained by the combination of the relevant physicaland biological scales. The overall results of this analysislend support to the hypothesis that turbulent, tidally well-mixedareas provide ideal feeding conditions for herring and may bethe proximate cause for an apparent retention.