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Migration Conundrum

Monday, 10 June 2013 19:31
Malin head a possible shark migratory cue point?
Malin head a possible shark migratory cue point?<br>
(c) E. Johnston 2013

As basking sharks poured into the waters off Malin head last week researchers hit the waves with high hopes of an extended shark season for 2013. With 50 sharks seen from one spot on Monday and a stable high pressure with associated sunny conditions forecast for the week, few would forgive the enthusiasts of thinking their time had indeed come. But alas the Basking shark once again invoked its elusive powers and disappeared into watery depths, leaving little trace and only two satellite trackers deployed.

Traditionally persons would have associated shark presence on the surface with calm sea conditions and hot sunny weather. Food or plankton availability has been a factor in areas such as Malin head but more in the seasonal sense rather than day to day. ‘once sharks had arrived’ a good calm day always produced good numbers of animals on the surface, the trick was getting the good calm days. This is not so in the early run of 2013, sharks moved through the area rapidly, and chose not to remain either on the surface or close to the Malin head frontal system even with extensive favourable weather conditions.

So what does this mean? Shark numbers have not been seen off Malin head in the 50’s at one time since 2010, yet we know from visual tagging that individual animals have always moved rapidly through ‘Hotspot’ areas, even when zooplankton densities remained comparable. Are these Salmon like run’s indicators of an over arching migratory drive by the species? Is Malin head a possible migratory cue point or had the sharks simply eaten their fill or exhausted the local food supply? These questions are key to our understanding of what drives the sharks movements throughout their time in coastal waters and why they aggregate in areas such as Malin head on a regular seasonal basis.

The second run of sharks should start in late July early August and our research teams will be on hand to sample and tag these magnificent animals, aiming in part to answer some of these basic questions about the life cycle of our most iconic marine species. Special thanks to all who have reported sightings of late your effort is valuable and the national sightings database is a valuable asset in identifying patterns of distribution.