Unlocking the secrets of coral reef fisheries

In a recently published study, we suggest a phenomenon that helps explain how coral reef fisheries can still exist, even when fish populations drop due to overexploitation.

While coral reef fisheries often occur alongside extensive biomass depletion, they still provide food for millions of people worldwide. Our latest study out of the Bellwood Lab explores how, by looking at the effects of overfishing on the relationship between reef fish standing biomass—the conventional way of assessing fisheries resources on coral reefs—and productivity.

Although standing biomass is an intuitive representation of how many fish are available today, it provides only limited, if any, information on how fast new fish is produced. It cannot predict how much biomass will be available tomorrow, or next week, or next year. This is because standing biomass represents fish that has accumulated over an unknown period of time. Maybe months, maybe years.

We can liken this to judging a family’s food habits based solely on the size of their pantry, without accounting for how often they go to the grocery store.

Similarly, banks could use your account balance today to judge if they would offer you a home loan. Instead, they evaluate statements of previous income and indicators of future income. This is because your account balance might not be a fair representation of your purchase power or earning potential.

On a coral reef, what indicates the future ‘fish income’ is the system’s productivity.

Using a large dataset of Indo-Pacific parrotfishes as a case study, we showed that standing biomass cannot be used to predict actual reef fish biomass production. But what could have caused this pattern?

Although both parrotfish biomass and average size strongly declined as human population increased, productivity did not respond to human population. This is important as human population density is widely used as a surrogate for fishing pressure. Thus, in other words, fish communities that are heavily fished do not necessarily show the expected declines in fish productivity.

Why?

To answer that, we simulated fishing on virtual fish communities following strictly natural rules. Because fishers tend to target larger fish and leave smaller ones out, fishing normally reduces the average fish size. However, what we found is that size-selective fishing triggers increased production per unit biomass in low biomass. Since large, old fish have little left to grow compared to small, young fish, the collective of remaining fishes will grow and produce more. This results in a compensatory productivity response to overfishing that avoids immediate productivity collapse. We term this, the ‘buffering productivity’.

However, this does not mean business can always continue as usual. While we offer a complementary explanation as to why some coral reef fisheries, with a human preference for larger fishes, are more resistant to overexploitation, sustained fisheries still rely on sustained reproduction and recruitment.

If overexploitation alters fish sizes to an extent in which reproduction and recruitment are impaired, then compensatory buffering productivity alone will not rescue coral reef fisheries. The fish that are left behind could be too young to reproduce, or too small to produce enough eggs.

Fish that are removed by fishing still need to be replaced, regardless of how much faster the fish that are left can grow.

Our study cautions against assessing the potential of coral reef fisheries and food production exclusively through the lens of available fish standing biomass.

At the same time, it points to the more valuable tool of biomass productivity. The study offers hope that even biomass depleted reef systems may still be able to partially deliver the goods and services needed by people during this new transitional period we are living.

Morais R, Connolly S, Bellwood D. (2019). Global Change Biology. ‘Human exploitation shapes productivity-biomass relationships on coral reefs’. DOI: 10.1111/gcb.14941