Written for BiOME by Margaux d’Ambly.

Over-exploitation of marine resources is known to have a devastating impact on the marine environment leading many species on the brink to extinction, but the effect of fisheries goes well beyond the depletion of fish stocks. Unsustainable fishing is disrupting a precious and fragile balance in our oceans, enhancing irreversible chain reactions, which affects us all. Indeed, recent scientific discoveries have shown that overfishing of large predators is enhancing cascading effects and a shift in the foodweb structure of coastal waters, causing loss of vital ecosystems.

Coral reefs, seagrass meadows and kelp forests are vital ecosystems as they provide services that are fundamental for ecological, economical and societal reasons. They harbor a huge concentration of the marine biodiversity and supply people with seafood, recreational activities and coastal protection. A great diversity of marine organisms, including many species of high commercial value, depend on seagrasses, coral reefs and kelp forests for breeding, feeding and sheltering. Besides, corals, seagrasses and kelps perform photosynthesis, providing oxygen and sequestering considerable amounts of carbon from the atmosphere, making them powerful allies in our fight against climate change. Additionally, they have an influence on the hydrodynamics, reducing currents and waves, which prevents sediment re-suspension and guarantees high water quality. Unfortunately, these precious habitats are undergoing a dramatic decline as they face many threats such as global warming, ocean acidification, pollution and eutrophication. Besides that, researchers have made a frightening discovery about the indirect effect that overfishing has on these ecosystems and all the life they support.

Seagrasses are plants adapted to the marine environment. They are very different from algae as they possess roots and leaves and produce flowers and seeds.

Kelps are large seaweeds growing along rocky coastlines. They do not possess roots but a holdfast that keep them anchored to the seafloor.

Corals are animals whose survival depends on the presence of a microalgae living inside their tissue.

A recent study conducted in Sweden showed evidence that overfishing of big predatory fish (the Atlantic Cod) has resulted in a considerable reduction of their population size, which led to an increase in the population of their preys, the intermediate predatory fish. These smaller predatory fish feed on herbivores, which themselves feed on filamentous macroalgae. These changes in the foodweb composition resulted in a drastic reduction of the herbivores’ numbers due to the overpopulation of their predators, which caused high levels of algae proliferation. Overgrowth of seagrasses by macroalgae enhances seagrass mortality due to light reduction and smothering. Since the 1980s, mats of macroalgae have dramatically increased on the Swedish west coast and at the same time, 60% of the seagrass has disappeared mainly due to human caused shifts in the marine foodweb structure.

Similar ecological processes have been reported in many other parts of the world with different habitat types, such as coral reefs and kelp forests. In tropical waters, overfishing of important grazing fish has released macroalgae from grazer control, enhancing algae bloom and massive coral mortality due to light reduction and smothering. In other regions, removal of large predators has caused a trophic cascade releasing sea urchins from predator control. As the population of these voracious herbivores increased, so did herbivory pressure, which reduced entire kelp forests to barren wastelands.

To the left is a healthy seagrass ecosystem where top predators control the abundance of intermediate predators. To the right is the overfished system, where top predators are removed so intermediate predators can thrive and suppress the abundance of their prey (the grazers), which cannot control the growth of algae. As seagrasses disappear, so does their functions and services. Arrow thickness indicates the strength of trophic interactions, and dashed lines indicate weak effects. Plus and minus signs indicate positive and negative effects respectively.

All these studies show that overfishing is causing a global shift from a highly biodiverse and productive ecosystem to species-poor alternate state, which provide fewer ecological services and fewer benefits to the associated organisms, including humans. Despite the fact that many efforts are already underway to protect and restore these ecosystems, evidence show that it is not sufficient. Their successful maintenance will depend on the conservation of key predators and healthy fish communities, which will reduce the expansion of macroalgae and prevent entire kelp forests to be grazed away. Additionally, regulations on agricultural practices to reduce the run-off of fertilisers into the seas would help decrease algae proliferation and improve water quality. Finally, getting the people to be more sensitive and aware of the importance of coral reefs, seagrasses and kelp forests through education would lead to more careful and responsible practices from all. For instance, a reduction of seafood consumption would have a major impact on these coastal ecosystems.

 

References

Baden S, Emanuelsson A, Pihl L, Svensson CJ, Åberg P (2012). Shift in seagrass food web structure over decades is linked to overfishing. Mar Ecol Prog Ser Vol. 451: 61–73

Infantes E, Crouzy C, Moksnes PO (2016). Seed Predation by the Shore Crab Carcinus maenas: A Positive Feedback Preventing Eelgrass Recovery? PLoS ONE 11(12): e0168128. doi:10.1371/journal.pone.0168128

Scheffer M, Carpenter S, de Yound B (2005). Cascading effects of overfishing marine systems. TRENDS in Ecology and Evolution Vol.20 No.11

Cheal, A.J., MacNeil, M.A., Cripps, E. et al (2010). Coral–macroalgal phase shifts or reef resilience: links with diversity and functional roles of herbivorous fishes on the Great Barrier Reef. Coral Reefs 29, 1005–1015

Smale DA,  Burrows MT,  Moore P, O’Connor N, Hawkins SJ (2013). Threats and knowledge gaps for ecosystem services provided by kelp forests: a northeast Atlantic perspective. Ecology and Evolution, Volume 3, Isssue 11, pages 4016-4038.

Jackson JBC, Kirby MX, Berger WH, Bjorndal KA, Botsford LW, Bourque BJ, Bradbury RH,  Cooke R, Erlandson J, Estes JA, Hughes TP, Kidwell S, Lange CB, Lenihan HS, Pandolfi JM, Peterson CH, Steneck RS, Tegner MJ, Warner RR (2001). Historical Overfishing and the Recent Collapse of Coastal Ecosystems. Science 293 (5530), 629-637.

Picture references

http://www.hawaii.edu/reefalgae/invasive_algae/chloro/cladophora_sericea.htm

https://oceanconservationtrust.org/projects/reforestation-project/Harz 2015

Coral Reef at Palmyra Atoll National Wildlife Refuge, https://commons.wikimedia.org/w/index.php?curid=18806932

Kelp: Jim Patterson Photography

Article written by Margaux d’Ambly