Biological and ecological effects of a Ligula intestinalis (L.) infestation of the gudgeon, Gobiogobio (L.), in Lough Neagh, Northern Ireland

A study was made of the effects of a Ligula intestinalis infestation on the biology and distribution of gudgeon in Lough Neagh, Northern Ireland. During the summer months, when the majority of the gudgeon population was restricted to the inshore areas of the lough, Ligula was present in all age (size) classes of the host although levels of infestation were highest in the oldest individuals. While there was no significant effect on the length‐weight relationship of the gudgeon, the presence of the parasite did adversely affect gonad size in both sexes. Ligula also influenced the distribution of the gudgeon population by decreasing the swimming depth of infested individuals and by delaying their autumnal migration to the deeper offshore parts of the lough.