Sea otter decline accelerating effects of climate change

Sea otters. Image credit: Rick Derevan, Flickr, CC BY-NC-ND 2.0.

In a new study published by the Bigelow Laboratory for Ocean Sciences, researchers have found that the loss of key predators may be accelerating the effects of climate change in their habitats.  The study focussed on the kelp forests of the Aleutian Archipelago off the western  coast of Alaska, which have experienced a sharp decline in sea otter population since the late 20th century. 

Kelp forests are made up of algae on the shallow sea floor, in this case the Clathromorphum nereostratum algae. The forests are extremely delicate ecosystems, with sea otters being the keystone predators (a predator that maintains balance by preying on a herbivore which would otherwise eliminate an important plant species) in the Alaskan region. The otters feed on sea urchins, which in turn feed on the algae. Without a steady population of sea otters, the sea urchin population will continue to grow and decimate the algae. 

Researchers studied grazing scars on the algae to reconstruct the rates of bioerosion over a forty year period beginning in 1965, comparing locations with different population densities of sea otters. A direct correlation was found between sea urchin biomass and bioerosion at the different sites – less otters meant more sea urchins, and therefore greater bioerosion of the forests. Climate change has been weakening these forests, and this over-grazing may just push them over the edge.

Where have the sea otters gone?

In the industrial age, otters in the region were hunted to near extinction due to the fur trade, but the population expanded again after trade declined. Unfortunately this recovery was short-lived. A 2003 study investigating the decline in sea otter population in the North Pacific concluded that the industrial whaling seen in the 20th century forced the killer whales in the region to turn to alternative sources of food – the sea otters – and their numbers remain low today. 

The effect of warming oceans

Global distribution of kelp forests. Image credit: Maximilian Dörrbecker (Chumwa), Wikimedia, CC BY-SA 2.0

Warming oceans, which have increased an average of 0.5 degrees Celsius since 1965, have been found to weaken the algae found in the kelp forest and make them more susceptible to grazing by sea urchins. Weaker algae means the sea urchins can penetrate deeper into the algae when feeding. Another study investigated the effect of marine heat waves on kelp forests in Northern California, made up of the same algae species as their Alaskan counterparts. Worryingly, during warm bouts this Californian kelp died, or failed to reproduce. Applying these findings to the Aleutian Archipelago, which has experienced several marine heatwaves in the past decade, there is concern that the algae here may face the same fate, as it too may not be adaptable to such rapid rises in ocean temperature.

 Why and how has overgrazing occurred?

The disappearance of their main predator, sea otters, has allowed the sea urchin population to grow without regulation. But sea urchins aren’t the only creatures to snack on the kelp – rising temperatures have been found to increase herbivores’ rate of consumption of the Clathromorphum nereostratum algae that form the base of the Alaskan kelp forests. This, combined with the warm water weakening the algae, has resulted in overgrazing and the near complete decimation of the kelp forests. 

What does this mean for the future of the Alaskan Kelp Forests?

The level of overgrazing that has occurred due to the loss of sea otters in the region has several consequences for the forests. The Bigelow study found that at each site studied across the archipelago, at least 40 percent – and up to 85 percent – of the algae colonies had been bio-eroded by sea urchins in 2014. Many of the colonies had lost so much living tissue that they were unable to regrow. A continued decline in the sea otter population in the Aleutian Archipelago could mean the entire eradication of the kelp forests, if they cannot adapt to the warmer waters and marine heat waves that are sure to reoccur in future years.

Written by Lara Watson and edited by Ailie McWhinnie.

Lara’s thoughts… The archipelago is currently under the Alaskan Maritime Refuge, designed to protect wildlife and conserve the habitats. The decline in great whales as food sources for killer whales is something that is most likely out of the control of national governments, but I think there is still something that governments and organisations can do to help prevent the further decline of the kelp forests. Perhaps an extensive otter breeding program, if such a thing is possible on such a wide stretch of coast, is one possible solution. The gray whale population has not recovered from industrial whaling, which only began to come to a close in the 1970s. It may be years before a solution is found, and the forests may not have that much time. 

Find her on… Twitter @lara_bethan and Instagram @lara_bethan.

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