On when herrings feed, their favourite foods at different points in their life cycle and the effect of what they eat on their own oiliness
Following the zooplankton, herring make daily vertical migrations, usually feeding on or near the sea bed during the day and approaching the surface at night.
Echo-sounding of the shoals, first carried out in 1960, in Passamaquoddy Bay, New Brunswick, presented a slightly more complex picture of the night time feeding, with a rise to the surface at sunset and a midnight sinking to intermediate depths, another rise to the surface at dawn, followed by a return to the bottom.
The pattern correlated with Muzinic’s 1931 study of herring on the Fladen Ground and at Brucey’s Garden, off Hartlepool, which show that most food was taken between 3 and 6am and between 5 and 9pm. Herrings are not vegetarians and avoid areas loaded with phytoplankton.
The larvae feed on diatoms, copepod eggs, flagellates, metazoan and copepod larvae and small copepods: they start off on the smaller items and work their way up as they grow. On the nursery grounds, they feed on small crustacea and the copepods of the estuaries.
Among other species, the planktonic palate of adults runs to Calanus, Temora, Pseudocalanus, Oikopleura. They also eat the small sand eels known as Ammodytes. In the North Sea, Calanus is particularly important.
The oiliness of the fish depends of the fat content levels of the zooplankton, which, in turn, is determined by the phytoplankton, which, of course, depends upon the climate.
As a general rule, warm, sunny summers produce rich plankton and the healthiest, best tasting herrings, there being a discernible, but not exclusively reliable correlation with good vintages in European wine.
Herrings don’t feed during spawning and, as spents (fish that have spawned), they take a few months to recover their body weight.