Competing with Crocodiles

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 But either way, when we arrived to pull our nets, any fish had been replaced by a gaping gash in the mesh.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.

Setting nets in Guma Lagoon
Setting nets in Guma Lagoon

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Another field site near Guma.
Another field site near Guma.
Delta field site near Guma.
Delta field site near Guma.
Setting nets in the main channel near Guma
Setting nets in the main channel near Guma

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.

Redbreast tilapia
Redbreast tilapia.
Removing gonads from a male tilapia.
Removing gonads from a male tilapia.
Gonad, spleen, liver, and kidney samples from a tilapia, placed in a cassette for fixation (preservation) in formalin
Gonad, spleen, liver, and kidney samples from a tilapia, placed in a cassette for fixation (preservation) in formalin

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.

Field dissections. Me and Mr. Mosie.
Field dissections. Me and Mr. Mosie.
Field dissections of tilapia.
Field dissections of tilapia.
Removing the otoliths requires a bit of muscle. Otoliths are small bones in the fish's head. Like trees, otoliths have rings, one for each year the fish has lived. So otoliths can be used to age a fish.
Removing otoliths requires a bit of muscle. Otoliths are small bones in the fish’s head. Like trees, otoliths have rings, one for each year the fish has lived. So otoliths can be used to age a fish.

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.

It is always good to study a species you can eat. Fried delta tilapia is delicious. And butternut squash is in season.
It is always good to study a species you can eat. Fried delta tilapia is delicious. And butternut squash is in season.
The diningroom at Guma Camp is frequented by a semi-tame owl, who likes butternut squash and mice (not necessarily together)
The dining room at Guma Camp is frequented by a semi-tame owl, who likes butternut squash and mice (not necessarily together)
Evening at Guma Lagoon Camp.
Evening at Guma Lagoon Camp.

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.

Rain storm, viewed from the margins of papyrus.
Rain storm, viewed from the margins of papyrus.
Waiting out a storm in the papyrus.
Waiting out a storm in the papyrus.
Rain drops on papyrus.
Rain drops on papyrus.
Close up of vegetation at the margins of the main channel near Guma lagoon, Okavango Delta.
Close up of vegetation at the margins of the main channel near Guma lagoon, Okavango Delta.

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).

Mosquito bites on my ankle.  Mosquitoes are large, abundant, and hungry this time of year in the Delta.
Mosquito bites on my ankle. Mosquitoes are large, abundant, and hungry this time of year in the Delta.