The current accepted migratory theory is of a west to east (deep-water to coastal) migration in April and a gradual northward trend following increases in sea surface temperature and copepod density until late August. This is when the sharks are believed to return to deeper waters to the west. Opposite directions apply in the western Atlantic zone. This gradual trend northwards is speculated to attract or ‘catch’ the sharks in tidal bays and inlets where copepods remain on the surface due to tidal up-welling and mixed water temperatures (Bloomfield 2006, Sims et al 2005, Nicholson 2000). Of note is the fact that global climate variability has been linked to zooplankton and basking shark relative abundance in the past and also in more recent years. This link has been promoted as a potential detector of trends in abundance of species that are influenced by climatic fluctuations in the North Atlantic Oscillation (Cotton et al 2005).
A single female basking shark was tracked by satellite tagging from the Isle of Man to Newfoundland waters during 2007 (Figure 2), generating considerable debate on the possibility of a single genetic population in the North Atlantic (Gore et al, 2008). This would have significant implications for conservation bodies and the scale on which protective measures are implemented.