Day 2 at Mafia Island… and our regular boat wasn’t available this morning. Office day instead. Not much to report.
The shark that both of us saw yesterday, and another that Larize saw afterward, were both fairly skinny. It’s difficult to quantify their health – the stereo-video system we’re using this year may help – but establishing a practical “condition index” (can we call this “jiggle factor?”) for whale sharks is certainly an interesting research topic moving forward.
Anyway, skinny sharks. What might be going on?
I’m going to randomly speculate, because that’s my jam.
Whale sharks at Mafia Island are highly resident. We’ve done scientific work on this previously, and Chris has a new manuscript that’s almost ready for submission. Around November each year, we normally start seeing lots of surface-feeding activity in the sharks as they target sergestid shrimp, which accumulate in dense patches within Kilindoni Bay.
Some unpublished, back-of-the-envelope calculations Chris did with an MSc student, Jens Paulson, a couple of years ago suggest that the Mafia sharks are getting a huge calorie boost from the shrimp. About 10 mins of feeding might cover a shark’s energy needs for a day, and they’re sometimes feeding for four hours straight.
What we don’t know is why the sharks still hang around Mafia in the “off-season”. Is there a food source we’re not aware of? It’s entirely possible. We’ve certainly seen them feeding on small fish, and they could also feed on emergent zooplankton that hide in the substrate in the daytime. Whale sharks aren’t fussy eaters.
What I’m wondering, though, is if the seasonal shrimp buffet available at Mafia might be enough to tide the sharks over year-round. In that case, why leave? They know the shrimp will probably be available again in a few months. A long-distance swim to another feeding area might simply not be worthwhile.
Dr Clare Prebble’s PhD research looked at, among other things, the connectivity of whale shark populations in the Western Indian Ocean. She found that Mafia sharks probably don’t stray far from ‘home‘ based on their biochemical profiles.
If that is indeed the case, these sharks could literally be fasting. This has been documented to (voluntarily) occur in whale sharks living in aquaria, so it’s probably something they do naturally.
In that case, these skinny whale sharks should be fine. Probably rather hungry though!
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