Removing top predators doesn’t help smaller reef fish

Removal of large predators through fishing has been suggested to help the populations of smaller fish.

An international team of scientists tested this assumption and found that no-take Marine Protected Areas (MPAs) benefited all major groups – higher carnivores like sharks and groupers, benthic carnivores that live near the bottom like butterflyfishes, planktivores and herbivores. Their biomass was 40-200% greater where large predators were left alone compared to open-access areas where fishing took place.

Masked Butterflyfish by Tim Nicholson

The effect was most pronounced for large fish. However no size class of any group showed signs of depressed biomass, as had been predicted from higher predator abundance. This implies that fishing for large fish on shallow rocky and coral reefs reduces the numbers of all fish on the reef.

While predatory fishes are most heavily exploited during fishing, fishers also target species in lower trophic groups such as herbivores. This may lead to reduced grazing of algae with negative effects on live coral cover. In the Caribbean, for example, the abundance of large parrotfishes increased in MPAs, which resulted in a doubling of grazing pressure on algae which is a major competitor of coral. Such examples highlight the complex trophic changes that can follow MPA establishment.

The researchers collected marine ecological survey data worldwide from 1844 rocky and coral reefs through the Reef Life Survey program from 2006 until 2012 in countires including Australia, Costa Rica, the Galapagos, New Zealand, Panama, UK, Colombia, South Africa and the United States.

Global map showing sites investigated. The density of fill color applied to each marine ecoregion relates to the number of sites surveyed within it.

Fish species, abundance and size classes were surveyed by scuba divers. The divers laid a 50 m transect line and surveyed fishes within 5 m strips either side of the line (so the total area surveyed is 500 m²). They identified all fish species present in each survey and estimated and their abundances and sizes. Fish lengths were allocated into 2.5 cm bins to 15 cm, 5 cm bins between 15 and 40 cm, and to 12.5 cm bins for fish larger than 50 cm. Using the abundance and sizes of fishes on transects and species-specific length-weight relationships provided in Fishbase the scientists could estimate Fish biomass.