Sampling fish in Amazon oxbow lakes: a way to understand giant otter habitat quality

Giant otters are large, active, aquatic carnivores that spend hours foraging and socializing every day. Their diet is almost exclusively based on fish. 

When we started the Peru Giant Otter Conservation project, we realized that sampling fish, an important component of oxbow lake ecosystems, could be crucial to understanding the ecology of giant otter populations and determining the best tools for their conservation.

Past work on giant otters in Manu National Park, showed that the size of an oxbow lake, which is considered a proxy of fish availability, has important demographic consequences for giant otters. Groups inhabiting larger oxbow lakes had higher reproductive rates and were more likely to produce successful dispersers (producing cubs of their own at least once).

In collaboration with Peruvian foundation CINCIA and Wake Forest University, we set out to examine the abundance and diversity of fish in these lakes. Some of the lakes we study are protected, while others are subject to different human activities, such as gold mining.

To sample the quantity and diversity of fish, we use custom-made nylon fishing nets. These nets are deployed during the afternoon and kept open into the night. This helps us understand the amount of food resources available to giant otters. 

The fish sampling aspect of our project has another goal: to quantify the risk of fish and giant otter exposure to mercury, a toxic metal released into air and water through gold mining activities. 

Fishing in oxbow lakes is not without its problems. Part of the work is done at night, when some fish species are more active. Carnivorous fish like piranhas have razor-sharp teeth. Once caught, they wait for an opportunity to bite our fingers as we remove them from the nets. 

Also, black caiman are abundant in Amazonian oxbow lakes and attempt to approach the nets to gain an easy meal, especially at night when they feel safer. 

These large, bold reptiles frequently need to be driven away from the nets, forcing the project’s dedicated Peruvian assistants to go on regular caiman-chasing patrols. This year, we faced these challenges without suffering injuries, except for a few bitten fingers...

During 2018, we captured fish in ten seasonal lakes both in the protected area and in the mining zone. As we expected, preliminary results suggest that the biomass of fish is much higher (by a factor of four) in protected areas. 

Fish tissue samples, as well as air and soil samples, are still awaiting laboratory mercury analysis. We expect the results to help us estimate the possible effects of mining on the oxbow lake ecological system. We also hope these results can assist us in delineating the best strategies for the conservation of giant otters and the aquatic ecosystems they are a part of.