It turns out that crocodiles are attracted to nets full of fish. At least I imagine my nets were full of fish. Maybe there was just one. But either way, when we arrived to pull our nets, any fish had been replaced by a gaping gash in the mesh. Somewhere there is a crocodile picking monofilament out of his teeth.
And this happened not once, but twice, effectively putting an end to our fishing expedition. The second time, the guilty croc was still lurking by the net. She/he was a big one, with a head maybe 20 inches long. The guys I work with drove the boat right up to the net as the crocodile sank below the surface. They seemed unperturbed by the croc’s presence. I personally would have stayed back a moment.
Anyway, nothing happened. We just pulled in our net, examined the damage, lamented our lack of fish, ate granola bars, and started home. The croc was not seen again.
That is not to say that we didn’t catch fish at Guma, despite the low oxygen and competing crocodiles. I resolved to stay until we succeeded, which is why we were there four days. In that time, we sampled 23 fish. About 2/3 were tilapia of various kinds – 3 spot, redbreast, green head. We also got small tigerfish and a few catfish.
At Guma camp, Guy has a large aquarium full of local fish that he catches as babies from the lagoon. In the tank are a pair of breeding green head tilapia. These fish are so interesting. The female broods her babies in her mouth while they develop. This gives her a slightly inflated, puffed cheek appearance. The male green head was also around, chasing other fish and generally making space for his lady. I never saw the babies out, but perhaps he watches the kids during dinner time.
Catfish are entirely not my thing. They ooze slime when you handle them, are as slippery as a bar of soap in the tub, and have a skull made from celestial bronze. That said, they are remarkable as fish out of water. I had one leap out of a bucket and make it half way across our campsite before we could get it back in water. I let that one go back in the river.
The tilapia, being my study species are not so fortunate, but will hopefully tell us a few things about the condition of the Okavango. I can take out their organs and preserve them in formalin. Later, back in Tennessee, we’ll slice the organs very thinly, place the tissues on slides, and look at the cellular structure. We’ll be able to determine a lot about the health of the fish, which in turn tells us about water quality in the Delta.
So far, I’ve observed that the Delta is remarkably clean. Conductivity measurements range from 30-100 uS/cm. They are low in the north and higher as you move down river. Conductivity measures dissolved salts. Pure water theoretically has conductivity of 0 uS/cm. Our tap water in Maun measures around 400 uS/cm. The higher the conductivity, the more “stuff” in the water.
We’ll also know more when my collaborator from the University of North Carolina has a chance to analyze his samples. He is coming here in June and we’ll take water samples all over the Delta. He will measure various pesticides and industrial chemicals in the water. Not all of these contribute to conductivity, so they might still be there, despite the low uS.
I can’t wait to go back to the panhandle of the Delta in June. In March, we got caught in the soaking rain twice. One of these times resulted in a long boat trip back to camp in the cold, driving rain. You know things are desperate when the life preserver seems like a good alternative to the rain proof jacket you didn’t bring. I was pleased to find my water-proof pants clean and dry, waiting in my tent when I got back.
In March the mosquitoes are also large, abundant, and hungry. By June, the rainy season will be done, so no soggy tents. And mosquitoes might be fewer since June is winter here in the southern hemisphere. With more oxygen in the water by then, we should get lots of fish as well. Whoopee! (Music to a researcher’s ears).