Feeding ecology of herring (Clupea harengus) larvae in the turbid Blackwater Estuary

Most studies on feeding by herring larvae (Clupea harengus) have taken plac e in clear, open waters, but several herring stocks around the world spawn in inshore and estuarine regions. An example is the spring-spawning Blackwa ter Estuary (Essex, England) stock. Samples were collected in this estuary to examine prey selectivity and feeding levels in relation to biological an d environmental conditions. Herring larvae negatively selected copepod naup lii, but positively selected the copepodite and adult stages of Acartia spp . Gastropod larvae were also positively selected. Particles >150 mu m width were preferred, whilst particles smaller than this value were preferential ly rejected. Concentrations of potential prey items in the water were in th e range of 6.0 to 49.7 organisms l(-1) with a median concentration of 15.0 organisms l(-1) (n = 26). These values are towards the low end of prey conc entrations quoted in the literature as being required to sustain herring la rval growth and survival. However, theoretical considerations suggest that, in this environment, levels of tidally-induced turbulence enhance encounte r rates between larval herring and their prey. On the other hand, turbidity is also related to tidal current speed and might reduce feeding success by decreasing underwater light levels. Measurements at two sites in the estua ry confirmed that tidally-induced turbidity reduced the effective water dep th in which herring larvae could visually feed by up to 50% at times of pea k current speed. However, with the gut-content data available in the presen t study, it was not possible to discern any clear relationships between fee ding success and the state of the tide. Feeding success appeared to be more strongly influenced by surface light-levels.