Managing Freshwater Crocodiles

The NT Government has released its new Draft Management Program for the Freshwater Crocodile which you can now download, read and - if you're so inclined - comment upon (full draft / summary). For those of you overseas, this concerns the Australian Freshwater Crocodile (Crocodylus johnstoni), Australia's other crocodile that is frequently overlooked and/or ignored by the general population. However, freshies are extremely cool critters. There's some concern about changes in their population density these days, but frankly I need a whole other blog post just to talk about it. So, don't go too far...

Are Freshwater Crocodiles Going Extinct?

Barely a week into 2010 and already the crazy crocodile stories are starting to appear. This one concerns the possible decline of freshwater crocodiles at the hands of expanding saltwater crocodile populations. The story itself is clearly prompted by the release of the revised management plan which contains a graph showing the apparent decline in freshwater crocodiles detected during surveys of the Daly River.

I'm not sure that I'd be leaping to such conclusions based on the findings of one river system's survey results just yet, especially considering the variability inherent in survey results. There's no information in the report, for example, to assess the impact that changing the survey methodology in 1998 (the first year of the apparent decline) had on the results. It's a good point to bear in mind when assessing croc survey data.

But I digress. Let's say that the decline is real (and I suspect it probably is). The news article implies that this is a real problem for freshwater crocodiles, with one noted expert even suggesting that freshwater crocodiles might be "on the way out". That sounds pretty dramatic! However, I think the point is being missed because there's a very reasonable explanation behind it. When saltwater crocodile populations were protected in 1971 (in the Northern Territory) there were only a few thousand left hiding in backwater swamps and creeks. Early crocodile surveys conducted by Harry Messel from the University of Sydney found that freshwater crocodiles had moved a long way downstream. There were even some freshwater crocs found in estuarine areas. This told us two things: first, that "freshwater" crocodiles are much more tolerant of saline water than their name implies, and secondly that freshwater crocodile populations are largely restricted to upstream areas by saltwater crocodiles. Take those saltwater crocodiles away, and freshies are free to move a lot further downstream.

You can probably see where this is going. Since saltwater crocodiles have recovered in the last few decades, they have progressively reclaimed territory that was formerly "theirs". Freshwater crocodiles have been pushed back into their more usual haunts: upstream freshwater and low escarpment habitat. So it's really not surprising to find that in the last decade, this trend has continued with densities declining in more downstream areas as saltwater crocodile populations mature and their size demographic becomes increasingly biased towards larger adults. Suddenly those Daly River results don't seem so surprising.

In straightforward terms, freshies have had it too good for too long! If there's one thing we know about crocodiles, it's that they are opportunists. We know they're happy to disregard notions of territory if there's plenty of food around, and we know that most species will range into less suitable habitat even if it means not growing so quickly or being able to breed. Being long-lived gives you a bit of flexibility with such things. Species are also frequently sympatric (sharing the same habitat) in areas where there's plenty of food and space. The Daly River is one example, but there are plenty of others where it would be very informative to see if similar trends exist. Unfortunately freshwater crocodiles haven't really been a management priority in the last decade and a lot of these trends have been lost. Perhaps the decline in the Daly River reflects a reduction in available food for both species? It certainly warrants further investigation.

These dynamic interactions between saltwater and freshwater crocodiles make even more sense when you consider recent fossil evidence showing that freshies very likely evolved from salties. Resources and habitat partitioning led to speciation in the ancestors of saltwater crocodiles, and freshwater crocodiles became perhaps better suited to the more extreme habitat conditions where food and nesting resources are less readily available. Populations are very dynamic concepts, and that includes their genetics.

So are freshies really on the way out? They're reverting to a familiar relationship that they've shared with saltwater crocodiles for thousands of years, where healthy saltie populations deny access to most downstream areas. That in itself wouldn't be a worry if other conditions remained as they were. However, since then cane toads have devastated freshwater crocodile populations in many areas. Could the combined onslaught of saltwater crocodiles and cane toads lead to the disappearance of freshwater crocodiles? Some people might think so, but I have more faith in this species' tenacity. They've already proven capable of thriving in some pretty unlikely habitats (dry, seasonally flooded, upstream escarpment) and they've largely survived the cane toad onslaught albeit in reduced numbers. Even now, they are adapting to new circumstances as they have done for millions of years. The question is, can they adapt quickly enough if anything else is thrown at them. What about climate change? What about the most vulnerable and possibly unique "pygmy" populations? Although I doubt we'll be seeing the last of freshwater crocodiles anytime soon, we're not making life easy for them.

Pygmy crocodiles of Bullo River

Back in early 2006, we were contacted by Marlee Ranacher from Bullo River Station about their freshwater crocodiles. Not any old freshwater crocodiles, however, these were pygmy freshwater crocodiles. These tiny bundles of attitude are essentially very similar to freshwater crocodiles (Crocodylus johnstoni) but they live in far upstream escarpment areas around the Northern Territory in particular. In these areas there is very little food, mostly various species of small fish no more than a couple of inches in length, small crustaceans and... well, that's about it. As a result of not having much food available, they don't grow very large. They are essentially stunted crocodiles, but they have adapted to this unique environment. And, more importantly, they are essentially geographically isolated from downstream, "normal" freshwater crocodile populations. That in itself makes them interesting, but they have the potential to be even more interesting.

If they are geographically isolated and there is little or no genetic input from more downstream populations, then they have the potential to become genetically distinct over many generations. It's evolution at work. One tantalising clue comes from a breeding pair of these tiny crocodiles, originally from the upstream Liverpool River, that were bred by Melbourne Zoo. The offspring were fed a normal captive diet, and therefore they should have grown into normal freshwater crocodiles. However, they did not - they were also stunted. Although that doesn't prove anything, it makes enquiring minds want to know whether they are indeed different.

There is another spanner in the works. Cane toads! These warty horrors can get into the most amazing places, and that includes upstream freshwater escarpments. We already know that freshwater crocodiles eat cane toads, and that they are susceptible to their toxin, and a recent study showed a 70% decline in crocodile populations before and after cane toad arrival. Pygmy crocodiles, being smaller, are more susceptible to the toxins from these toads, and therefore we are all extremely concerned about the long-term impact of toads on these pygmy populations.

So we are here at Bullo River conducting surveys and to collect tissue samples from a representative sample of crocodiles. The surveys involve walking along an upstream creek during the day, mainly to find pools and to figure out how to climb between them - some areas are quite treacherous to get around - and then walking back at night with a headtorch and spotlight to count the number of eyeshines in each pool. Crocodiles have a reflective layer in the retina of their eyes, and the reflected light is bright red so they are quite easy to see at night if they are on the surface. The problem is, many crocodiles are not on the surface - usually they're hiding amongst vegetation, under logs, and particularly in cracks between submerged rocks. So you really have to spend a lot of time looking around, double and triple checking each pool.

Catching them to take a tissue sample is another challenge. The best, and perhaps only effective way of doing it is to get into the water and catch them by hand. Most of the time this isn't a problem because they're fairly small, although trying to hand-catch a 1.5 metre crocodile (the biggest any pygmy croc appears to get) requires a bit of care! Many of the crocodiles are also very naive towards people and will happily swim over to check you out. Some of the larger, territorial males aren't best pleased by your presence either. Once captured, however, they become quite placid and can be measured and photographed with ease. We take a small sample of tissue from the tail, basically a piece of mostly dried skin, and then release the crocodile back into its pool where it glowers at us. If only we could tell them that it's all in their best interests.

We're here for a few more days yet and we've seen some pretty amazing creatures up here, so there'll be more to come.