Quick identification guide for siphonophores and hydromedusae (Cnidaria: Hydrozoa) reported as harmful for finfish aquaculture in the Northeast Atlantic.

This guide covers all pelagic hydrozoans reported as harmful to aquaculture in Northeast Atlantic waters. It is intended as a resource to facilitate identification of the species included in this guide, with practical notes on how to distinguish them from potentially alike species. However, over 100 other non-harmful species occur in the region, and definitive identification of specimens may require expert advice and broader ID resources.