According to Wkipedia,and the UK White Fish Authority,Pilchards are just larger Sardines.
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Sardines, or pilchards, are several types of small, oily fish related to herrings, family Clupeidae. Sardines were named after the Mediterranean island of Sardinia, where they once lived in abundance.
The terms sardine and pilchard are not precise, and the usual meanings vary by region. Britain's Sea Fish Industry Authority for example classifies sardines as young pilchards.One criterion suggests that fish shorter in length than 6 inches (15 cm) are sardines, and larger ones pilchards. The FAO/WHO Codex standard for canned sardines cites 21 species that may be classed as sardines; FishBase, a comprehensive database of information about fish, calls at least six species pilchard, over a dozen just sardine, and many more with the two basic names qualified by various adjectives