Transport Barriers and the Retention of Calanus finmarchicus on the Lofoten Shelf in Early Spring 

<p>Large aggregations of the copepod <em>Calanus finmarchicus</em> occur each spring in the shelf-slope-oceanic regions off the Lofoten-Vester&#229;len Islands where productive fisheries have traditionally supported local and global economies. The retention and off-shelf transport of<em> C. finmarchicus</em> populations were studied by analyzing ocean color remote sensing and satellite altimetry data between 2010 and 2019 and employing a Lagrangian Coherent Structures (LCS) model. Results revealed the existence of a transport barrier reoccurring at the shelf break that retains<em> C. finmarchicus</em> on the shelf for 30-70 days in the spring when <em>C. finmarchicus</em> were seasonally ascending to the surface layer. The analysis of baroclinic and barotropic energy conversions indicated that the topographically steered Norwegian Atlantic Current (NwAC) is the primary mechanism in the formation of the transport barrier, which restricts exchanges of <em>C. finmarchicus</em> populations between shelf and oceanic waters. In the mid- or late April, an increase in baroclinicity leads to an increase in mesoscale eddies generated on the shelf break near Lofoten-Vester&#229;len Islands, breaking down transport barriers and causing off-shelf transport of <em>C. finmarchicus</em>. The transport barrier predictably reoccurs in early spring which supports the entrapment of <em>C. finmarchicus</em> in the shelf region.</p>