Laboratory-reared herring larvae adjusted several characteristics of their swimming behaviour to the presence of food in the experimental tank. When food density was low, as in the outer limits of a food patch in the sea, the larvae increased their turning frequency, an exploratory behaviour to locate the centre of the patch. At a higher prey concentration, such as in the centre of a food patch, the larvae crossed their previous swimming path more frequently, indicating a local search strategy aimed at foraging in the centre of the patch.
The prey of actively foraging planktonic predators, such as fish larvae, is patchily distributed. The main factors governing the ability of the predators to encounter prey are the relative velocities of predator and prey, and the prey detection radius by the predator relative to the distance between prey items (Gerritsen & Strickler, 1977). The enhancement of encounter rate by turbulence through its effect on relative velocity has been widely discussed in the literature since Rothschild & Osborn's (1988) publication. However, an effective forager should be able to adjust its behaviour to optimize prey encounter by, for example, altering its position in the water column in relation to conditions of turbulence, or modifying its searching behaviour in response to spatial aggregation of prey. This latter aspect has received comparatively little attention. Different swimming modes have been described in herring (Clupea harengus L.) larvae and in larvae of many other fish species. Foraging larvae swim in a more linear path between food patches and follow a more complex track when a patch of prey is located.