Thursday, September 18, 2008

Saltwater Crocodiles: a major predator of hatchlings on Crab Island

The one noticeable factor during this trip has been the number of crocodiles on the island. Most trips along the beach we count at least 25 crocodiles. During low tide they tend to sit out wider on the sand bars. They range in size from around 1.5m (5ft) up to 4.5m (15ft). Some days the water is crystal clear and it is amazing watching them as they sit on the bottom ‘hiding’ or swimming away underwater and popping up out wider keeping a watchful eye. Either way, they are still very scared of us and will be in the water long before we are close to them.

The predation of sea turtles by crocodiles has been little recorded in the world. Crocodiles feeding on hatchlings has rarely, if ever, been documented. While we suspected that it was happening on Crab from our first visits, we still needed the evidence. So in the last few weeks we have been collecting some good data which shows that saltwater crocodiles are a major predator of hatchlings on Crab Island. Each morning we record hatchling emergences and we also record crocodile tracks which have been actively chasing hatchlings on the beach. Each night we will see crocodiles on the beach, and each morning we will find tracks showing crocodiles moving across the beach. They tend to occur where there have been a large number of hatchling emergences. How the crocodiles know where these dense areas are each night we do not know – it could be that they rely on the rufous night herons and pelicans to give away locations, or it could be that they can sense the march of the hatchlings down the beach.

When we are on the beaches at night we tend to know when crocodiles are feeding on a clutch of hatchlings. From the distance with our spotlight we can see crocodiles spread across the beach getting the hatchlings as they emerge, and others waiting at the waters edge.

It has been difficult to get any good footage, however we have been able to witness crocodiles actually eating hatchlings on a couple of occasions. To get good photos or video we really need night vision – the spotlight tends to scare the crocodiles, whilst also attracting the hatchlings back towards the light. Even so, we have been able to see a crocodile grab a hatchling on the waters edge and another actually feeding on one – the crocodile will either wander after or sit and wait for the hatchling, and then turn its head on its side and pick it up with it’s teeth, before tilting it’s head in the air and manoeuvring the hatchling down its throat before swallowing.

The photo in this post shows a crocodile about to eat a hatchling at the waters edge. While the photo is very grainy (we took it from a distance), it is probably the first ever photo of such an event.

We are still being very crocodile savvy and still never get complacent. We enjoy seeing the crocs during the day and at night – we now recognise individuals and have names for a couple, including ‘Charlie’ and ‘Obey’, which we will write more about later….

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