Basking sharks - maybe?

Divers love seeing the big stuff; from dolphins to turtles to sharks, we adore the big charismatic wild animals that we have a chance of encountering.  For some divers, the pursuit of 'big things' resembles the ghosts of the I-Spy guides from years ago. Tick each one off, add a date and a location, and then move on to try and fill the rest of the book.  I'm not sure that the Chief I-Spy still exists to countersign your completed book, but maybe the National Diving Officer would step in?
 
Of course there are significant challenges to be overcome in order to tackle a safari trip for some of the big stuff.  Planning a trip to coincide with the seasonal arrival of whale sharks off Christmas Island presents considerable logistical hurdles, but at least their arrival is linked to the predictable phases of the moon which triggers the crab spawning and the release of the whale sharks favourite high protein snack.  Other animals can be much harder to predict. 
 
Basking sharks arrive around the Isle of Man in around April/May each year, and the data suggest they are here for around 4 to 5 months.  Our old crusty divers, who have been observing these patterns for years, will repeat the mantra that the sea temperature needed to reach 11.5 degrees before the sharks appeared.  The precision of this prediction has always bemused me. Not 11 degrees but 11.5, the sharks obviously need the extra half a degree. Of course, the factors relating to basking shark sightings are more complex than that.  There is reasonable evidence to suggest that basking sharks come to Manx waters to give birth to their young and to mate.  Similar to some other sharks, basking shark young are born live, having been carried in the mother being fed on eggs that she releases while they grow to around 1.5m.  We regularly see very small basking sharks round these waters.
 
The arrival of the basking sharks each year is timed with the spring explosion of plankton, but that isn't purely driven by water temperature.  Hours of sunshine plays a huge role in the plankton bloom, driving a rapid explosion.  So an overcast spring can delay the sharks by a few days or even weeks.  This year's extensive snow fall took weeks to defrost, and the cold melt water helped to hold temperatures down.  All of which makes predicting the basking sharks' return even more difficult.  
 
One of the other aspects we need to consider is that there just simply isn’t enough information about these creatures.  A huge volunteer effort which encourages the public to report basking shark sightings reveals some interesting data, but using that information to find sharks can lead you on a wild goose chase.  Let me explain.  80% of reported sightings are in the SW of the Isle of Man, so you might think there is something special about these waters.  Yes, there is some deep upwelling of water carrying nutrients that feed the plankton, but we know that high tidal flows mix the water around anyway, so this isn’t the full picture.  The high level of sightings is directly correlated to the height of the cliffs and proximity of the coastal path.  Get up high and look down on to the sea and you have a good chance of spotting sharks.  Lower cliff heights and remote areas are less likely to have shark sightings reported nearby.

Then there’s the matter of timing.  Sightings start in April and end around September…or do they?  We have met sharks under water in October and early November.  We couldn’t see the dorsal fins on the surface, but they were still here, lurking.  You need calm surface conditions to spot shark fins, and traditionally the winds increase in September so calm days are hard to find.  The weather deteriorates so that less people are out looking anyway…leading to a statistic that indicates the sharks have left.  Experience tells us this is wrong.

So for some animals it’s not enough just to know the phases of the moon to be able to tick them off your wish list.  You’re going to need a whole lot of information and a bit of fuzzy logic to interpret it all.  Humans are generally good at fuzzy logic, although many of us would recognise it more as an ‘instinctive feeling’ which we can’t fully justify. And so we come to the last crucial factor in spotting the big stuff, a large helping of luck.  Hope you've been lucky this summer!