Fish poop: an underappreciated food source for coral reef fishes?

Coral reefs are one of the most diverse ecosystems on the planet, but they are also limited in nutrients. So, nutrient recycling is a vital part of supporting such reef organisms and their biodiversity. Organisms can’t process all the nutrients from the food they eat, so some of these nutrients come out in their poop. A new study documented a unique upcycling technique, previously unknown within the Caribbean, of herbivorous fish feeding on fish feces.

Parrotfishes and surgeonfishes are often praised as the great caretakers of coral reefs, feeding on reef algae and keeping overgrowth in check, which indirectly promotes healthy coral recruitment and growth.  Although it was previously known that Caribbean parrotfishes and surgeonfishes also fed off other food sources, such as cyanobacteria, sponges, and even corals themselves, a recent study added fish feces to this list.

The Study

This collaborative effort was co-led by Hannah Rempel, a PhD student from the University of Texas Marine Science Institute and Abigail Siebert, a former undergraduate student from California Polytechnic State University. They studied the foraging rates of parrotfishes and surgeonfishes on fish fecal matter. Because they found that over 99 percent of feces they consumed were from the Brown Chromis (Chromis multilineata), a plankton eating fish, they also observed Brown Chromis feces to see what other reef fish ate them and studied the nutritional value of these feces. The study was conducted in 2019 between June and September, across six dive sites along the western shores of Bonaire. This research is the first of its kind within the Caribbean and paves the way for continued exploration into the topic.

The Results

Throughout this study, researchers documented that almost 85 percent of the observed fecal pellets were ingested by fish with over 90 percent consumed by parrotfish and surgeonfishes alone. “Compared to algae, these fecal pellets are rich in a number of important micronutrients. Our findings suggest they may be an important nutritional supplement in the diets of these fishes” stated Rempel. Taking a closer look at the fecal matter itself, researchers found that these pellets had higher values of proteins, carbohydrates, total calories, and important micronutrients when compared to most algae. Therefore, consuming fecal matter may play an important role in nutrient transfer within the marine environment.

Future research

Understanding the intricate dynamics within coral reefs provides information management authorities need to safeguard these environments more effectively. These results highlight the importance of the consumption of fecal matter in upcycling micronutrients, although there is still much to be learned about the nutritional content of other food sources, such as algae mats, cyanobacteria, sponges and corals. Fish feces may play a vital role in nutrient supply within the reef environment, emphasizing the need for further insight into this topic moving forward.

More information

• Report Feces consumption by nominally herbivorous fishes in the Caribbean: an underappreciated source of nutrients?

Text: DCNA, University of Texas and California Polytechnic State University
Photos: Hans Leijnes (lead photo: school of ocean surgeonfish); Marion Haarsma; Hannah Rempel