Reef accessibility impairs the protection of sharks (2018) Juhel et al., Journal of Applied Ecology 55

The Crux

The importance of sharks goes well beyond what Jaws did to Hollywood, or one week in the USA each July. In any reef ecosystem, sharks perform a key functional role, exerting top-down pressure, stabilising food webs, and improving general ecosystem functioning. They’re also ‘charismatic’ species, meaning they’re easier to raise funding for, and bring money in through tourism. Yet pressure from fishing suggests that reef shark populations may be under threat, and with high body sizes and long lifespans, their populations are more sensitive than most to overfishing, making extinction risks higher.

Yet the lack of data on shark populations means that the effectiveness of the few existing management programs is largely untested. This paper looks at Marine Protected Areas (MPAs), areas in which national or international bodies prevent fishing or even entry, to see whether or not they are an effective conservation method for shark populations.

How it Works

The study took place off the coast of New Caledonia, an island nation in the Pacific Ocean. It’s surrounded by coral reefs, which include 15 MPAs. Both in and outside of these areas, the researchers used two methods to survey shark populations. The first was underwater videos placed near bait which were subsequently analysed by video software, and visual censuses taken by two divers along 50m transects. Sharks were surveyed both for abundance and diversity of species.

The numbers of sharks found were then modelled against a number of human variables, which included whether or not the area was inside an MPA, remoteness from population centres, and human population density. The researchers used a boosted regression tree model, which allowed them to estimate the importance of each variable in determining shark numbers.

What They Found Out

Whilst one would hope that the presence of MPAs would contribute to higher shark abundance and diversity, it had very little effect. Both remoteness and human population density were more significant indicators than whether or not the area was protected, with remoteness explaining between 40% and 60% of all variation. Shark diversity and abundance was higher in remote areas than in protected areas, even areas where all entry was barred, including the oldest MPA, which was classified as such almost half a century ago.

Did You Know: Overfishing In many reefs worldwide, sharks and other large fish species have been overfished, to the point where local fishermen have had to focus on other, smaller species. As fishermen move further down the food chain in a reef ecosystem, they begin to eliminate herbivores from the reef. We’ve touched on eutrophication in a previous breakdown. Eutrophication also occurs on reefs, and leads to the growth of algae. If the herbivore population is sufficient, the herbivores can control the algal population, which allows the coral to dominate. However if enough coral is removed, algae dominates and the reef experiences a phase shift, from a coral to an algal reef.

Problems?

Though the figures above are convincing, they vary somewhat based on which survey technique was used. Since it was not possible to use both techniques in every area, this could provide a little imbalance in results. However this is doubtless due to limited resources, a common issue that all ecologists run into at some point, and the amount of variation explained by the models is still high enough to look past this.

So What?

The fact that the presence of MPAs does not seem to matter if the area is close to human activity is worrying, with sharks all but absent less than one hour from human activity. The reefs around New Caledonia have been well protected for longer than many others worldwide, which suggests that the estimates here are conservative, and that shark populations close to human centers in other reefs may be even lower.

However it does provide effective management suggestions. Setting up larger MPAs in remote areas to ensure future stability in shark populations is one obvious target. And MPAs could still play a role in sustaining herbivore populations, especially with release from constant shark predation.