A study published in the journal Nature Communications suggests, that urine from big fish is essential in maintaining healthy coral reefs. Researchers found that reefs with more large predatory fish had healthier levels of phosphorus, from fish pee, compared to reefs depleted of large fish.
Researchers from the University of Washington departments of Aquatic and Fisheries Science, surveyed 143 fish species at 110 sites over 43 Caribbean coral reef and found that reefs with fewer large fish had nearly 50 percents fewer nutrients. The number of fish at each site varied from no-take zones to areas completely devoid of larger fish.
They found that reef sites were less affected by the total number of fish removed, and instead that changes in community size and the number of large predatory fish were the primary cause of shifts in ecosystem function. “Part of the reason coral reefs work is because animals play a big role in moving nutrients around,” said lead author Jacob Allgeier.
Phosphorus in fish pee and nitrogen excreted through their gills are important nutrients for coral reefs to grow. In many reef communities, fish will take shelter in and around coral during the day, peeing out valuable nutrients, then forage for prey in and around the reef by night.
“Fish hold a large proportion, if not most of the nutrients in a coral reef in their tissue, and they’re also in charge of recycling them. If you take the big fish out, you’re removing all of those nutrients from the ecosystem.”
“Simply stated, fish biomass in coral reefs is being reduced by fishing pressure. If biomass is shrinking, there are fewer fish to pee,” Allgeier said.
Allgeier spent four years measuring a number of nutrients in fish pee and fish tissue to eventually build a massive dataset that tracks fish size and nutrient output and the amount they store in their tissue.
Allgeier’s inspiration for taking on this research was a 1980s paper by now emeritus research professor Judith Meyer at the University of Georgia, which showed that coral reefs with fish grew at over double the speed of reefs where fish were absent.
Coral reefs are the very definition of a delicate ecosystem. They are highly productive in terms of the biodiversity they support, but there aren’t a lot of nutrients to spare. Reefs operate on what scientists call a “tight” nutrient cycle, meaning there must be an efficient transfer of nutrients for coral to grow. This cycle is largely controlled by fish, which hold nutrients in their tissue and then excrete them through their gills and urine. [Phys.org]