The aquaculture industry is suffering from losses associated with bacterial infections by
opportunistic pathogens. Vibrio anguillarum is one of the most important pathogens, causing
vibriosis in fish and shellfish cultures leading to high mortalities and economic losses. Bacterial
resistance to antibiotics and inefficient vaccination at the larval stage of fish emphasizes the need
for novel approaches, and phage therapy for controlling Vibrio pathogens has gained interest in the
past few years. In this study, we examined the potential of the broad‐host‐range phage KVP40 to
control four different V. anguillarum strains in Atlantic cod (Gadus morhua L.) and turbot
(Scophthalmus maximus L.) larvae. We examined larval mortality and abundance of bacteria and
phages. Phage KVP40 was able to reduce and/or delay the mortality of the cod and turbot larvae
challenged with V. anguillarum. However, growth of other pathogenic bacteria naturally occurring
on the fish eggs prior to our experiment caused mortality of the larvae in the unchallenged control
groups. Interestingly, the broad‐spectrum phage KVP40 was able to reduce mortality in these
groups, compared to the nonchallenge control groups not treated with phage KVP40,
demonstrating that the phage could also reduce mortality imposed by the background population
of pathogens. Overall, phage‐mediated reduction in mortality of cod and turbot larvae in
experimental challenge assays with V. anguillarum pathogens suggested that application of
broad‐host‐range phages can reduce Vibrio‐induced mortality in turbot and cod larvae,
emphasizing that phage therapy is a promising alternative to traditional treatment of vibriosis in
marine aquaculture.