Chimpanzees’ mastery of technology has moved on to the next level, after researchers spotted groups of the primates using sticks to fish algae out of a river. Describing the behavior in the American Journal of Primatology, the study authors say the ingenious chimps use the technique to access a highly nutritious food source that is only available at certain times of the year.

Scientists in the Bakoun Classified Forest in Guinea became curious when they noticed long sticks lying close to the banks of rivers and streams, and decided to set up 11 camera traps in order to investigate how they got there. Much to their surprise, they found that local chimpanzees were using the tools to fish algae from the riverbeds.

Interestingly, the algae they were fishing for belonged to the genus Spirogyra, a variation of which is also consumed by people in Northern Thailand and referred to as tao. Containing 16.7 percent protein, 55.7 percent carbohydrates and a high concentration of antioxidants, the algae is an excellent source of nutrients, which explains why the chimps are so keen to get hold of it.

However, while other chimps such as those in the Bossou region of Guinea have previously been seen using small sticks to scoop Spirogyra off the surface of rivers and lakes, in Bakoun the algae is only found on the bottom of rivers, which means that accessing it requires a little more ingenuity.

The chimpanzees in this area therefore use long sticks – some measuring up to 4.3 meters (14 feet) in length – to obtain it.

“All age and sex classes of Bakoun chimpanzees were seen in the camera trap videos to successfully fish for algae in a river, stream or pond using woody branches or twigs as fishing rods,” explained study co-author Ammie Kalan in a statement.

However, while the animals were regularly seen fishing during the dry season, the researchers report that they were never observed doing so in the rainy season. This, they suspect, is because heavy rains tend to wash the algae away at this time of year.

Though chimps were seen dangling their rods into the water for up to an hour at a time, the average length of each fishing trip was just over 9 minutes. After testing out the technique themselves, the study authors calculate that 364 grams (12.8 ounces) of algae can be retrieved in this time.

Summing up, the team write that this never-before-seen tool use “permits a more efficient access to a rarely available but highly preferred resource, such as algae, that permits chimpanzees to flourish in an environment otherwise more limited in food and water.”