Insights on the origin and drift trajectories of Portuguese man of war (Physalia physalis) over the Celtic Sea shelf area

Many marine animals are difficult to study because they are widely dispersed across oceans and are not captured by traditional sampling methodologies such as fishery surveys. A case in point is the Portuguese man of war (Physalia physalis) (Linnæus, 1758), which despite being pleustonic and remarkably conspicuous, is one of the least studied and understood gelatinous zooplankton species, especially in terms of its ecology. During August to October 2016, the Irish coastline experienced the largest mass stranding of P. physalis in over 150 years. At the same time, P. physalis were recorded offshore in the Porcupine Seabight. Here we used these stranded and offshore observations of P. physalis to inform a Lagrangian particle-tracking model forced by wind to 1) hindcast the backwards drift of this species for three months to determine their likely origin and provide some insights on likely pathways to Irish shores and, 2) forecast the drift of this species towards the Irish coastline. Hindcasting stranded P. physalis from the Irish coastline suggested that they most likely originated from an extensive source area located over the European basin but ultimately from the North Atlantic Current. Our forecast model indicated that particles released from the Porcupine Seabight stranded on Irish shores, in fact, stranding patterns were 82% similar to actual strandings. Both models combined suggested that the Porcupine Seabight was an important source area, but that many P. physalis likely originated from further south and took a more tortuous trajectory towards Ireland determined by wind. This study also highlights the value of collecting routine beach strandings data and opportunistic offshore visual observations to inform future coastal and shelf modelling studies.