Over the years shark fishing – and particularly the practice of finning* – has garnered increasing attention from the public. It seems that these predatory fish that were once feared and even hated are slowly being seen in a better light, the impetus for their conservation beginning to make ground. Many shark populations have – and still continue to – experience high levels of overfishing. The problem with overfishing top predators is that they tend to be slow-growing, long-lived, and don’t produce lots of offspring to allow populations to rebuild quickly. Their removal can also cause what is known as a tropic cascade – when the removal of predators results in their prey becoming more abundant, which means that the critters they eat become less abundant….and so forth. However, removal of top predators isn’t the only possible cause for changes in marine ecosystems. There are a whole host of human stressors that can impact on the marine environment, and there are also natural stressors. Take coral reefs for example we may add too many nutrients to the water, causing an increase in algae which can dominate corals, we may overfish a whole host of other coral reef species to name a few. But these reefs may also be impacted by storm damage, flooding, and even ‘natural bleaching’, causing alterations in the benthic habitat (the reef itself). If we want to understand how overfishing of sharks on reefs has altered the coral ecosystem, we need to do our best to take these other stressors into account.
About 300 km off the northwest coast of Australia lay two groups of uninhabited atoll-like coral reefs – the Rowley Shoals and Scott Reefs. Scott reef has been, and continues to be, a top location for Indonesian fishers for centuries – including reef sharks. Rowley Shoals on the other hand lies in what is now a marine protected area, and compliance is thought to be pretty good there, giving us a now unfished reef system. Using long-term datasets, Jonathan Ruppert from the University of Toronto in Canada and fellow researchers took the opportunity to understand the effects of shark fishing, bleaching, and cyclones on changes in fish and habitat on these two reef systems. As with many field-based studies, they could not pin down any definitive ‘cause and effect’, but they did uncover a number of interesting correlations.
The abundance of corallivores (coral eating) and planktivores (plankton eating) fish species appeared to decline in line with loss of live coral (this is called negatively correlated). For these critters, their abundance didn’t significantly differ between the fished and unfished reefs. It seemed that they are more affected by changes in habitat as a result of the natural disturbances then the abundance of sharks. This makes a sense – they have just lost their food source and shelter. Herbivorous fish – like parrotfish – on the other hand seemed to fair much better after a these natural disturbance. When a cyclone or bleaching destroys coral, algae can (but not always) increase in its place. This is because algae grow much quicker than the corals, resulting in an increase in food for the herbivores. This is likely the reason why herbivores showed a positive correlation (an increasing trend) with increasing algal cover. But there were also less herbivores on fished reefs compared to unfished reefs. The loss of these guys is a particular problem because herbivores are vital for keeping algae in check to stop it completely dominating a coral reef. If you lose herbivores, you can lose much if not all of a coral reef. Given that bleaching and cyclones are thought to increase in frequency and intensity as our climate is changes, the role of herbivores may become even more important to reef survival.
As we move up the food chain, we reach the mesopredators – the mid-level predators, which on coral reefs means families like snapper and emperors. The abundance of these critters didn’t show any significant relationship with benthic habitat, but were much more abundant on fished reefs than unfished reefs. In a nutshell, the less sharks the more mesopredators, and the less herbivores. Sounds like a trophic cascade? Perhaps, but the authors are also quick to note that these differences on fished and unfished reefs could be down to other reasons – remember, their study found relationships (correlations) – not cause and effect. For example, the authors found some differences in the habitat between the Rowley Shoals and Scott Reefs that they could cause the differences in the fish communities between the two systems. They also note that variation in larval supply may be a significant factor in the differences seen.
Despite these issues, and the lack of definitive ‘the loss of sharks has directly caused x’, the authors suggest we don’t sit back on our laurels. We may not be able to make any appreciable difference to the frequency and intensity of bleaching or cyclone events in the short-term, we can reduce the pressure on sharks. For a marine ecosystem that is widely regarded as at risk on a global scale, it may be that every little bit helps.
The paper is published in the open access journal PLoS ONE – you can have a read of it here dx.doi.org/10.1371/journal.pone.0074648
Image: Black tip reef shark (Carcharhinus melanopterus). Credit Jim H./Flickr (CC BY-SA 2.0)