Grazing by sprat schools upon zooplankton within an enclosed marine lake

Abstract

We have investigated grazing by sprat schools upon zooplankton within Lough Hyne (Ireland), a marine lake with only a narrow and shallow connection to the sea. Acoustic surveys showed the presence of large numbers of sprat (Sprattus sprattus), preyed upon by mackerel Scomber scombrus. The sprat formed dense schools during the day and dispersed at night. Zooplankton were widely distributed within the lough, although absent below the oxythermocline. However, echo traces showed clear volumes, indicating an absence of zooplankton, surrounding the daytime sprat schools. Pumped sampling of the zooplankton at different depths, close to and away from the sprat schools, confirmed that those volumes of water that appeared acoustically clear were largely devoid of macro zooplankton (92.7% reduction overall). Comparison of clear areas with adjacent areas showed that almost all decapod larvae and calanoid copepods and most bivalve larvae were absent. The differences for cladocerans were also significant but for gastropod larvae they were not significant. Large calanoid copepods were much less abundant in the clear areas and their size distribution had changed. We conclude that the sprat schools had rapidly depleted their surroundings of zooplankton, suggesting that fish within the schools may be substantially food-limited. Dispersal at night, when visual predators are less efficient, may enable the sprat to feed more effectively. Thus, although schooling may confer benefits to individual fish there are concomitant disadvantages in terms of food depletion. The reduction in zooplankton as a result of heavy grazing by sprat within the enclosed lough may affect the phytoplankton, with significant effects upon the ecology of Lough Hyne.