Is ocean acidification knocking the scents out of salmon? | Science News for Students

Is ocean acidification knocking the scents out of salmon?

In more acidic water, the smell of danger doesn’t trigger salmon’s fear factor like it should
Feb 28, 2019 — 6:30 am EST
a photo of a salmon jumping out of water

A salmon’s sense of smell is critical for its survival. But rising acidity in the world’s oceans could hamper the fish’s ability to detect danger, a new study finds.

John Pennell/iStockphoto

Salmon’s ability to sniff out danger seems to nosedive as seawater becomes more acidic. That’s the finding of a new study. It assessed the fear response of salmon at different levels of pH. That’s the scale that ranks how acidic or alkaline something is.

Human activities have been adding more and more carbon dioxide, or CO2, to the atmosphere. This has been causing global warming. But not all of the CO2 emitted stays in the air. The world’s oceans absorb some of it. That, in turn, has been lowering the water’s pH, a process known as acidification (Ah-SID-ih-fih-KAY-shun).

Chase Williams is a toxicologist at the University of Washington in Seattle. Such scientists study how chemicals and other things might cause harm. Williams and his colleagues knew that a lot of research had linked lower pH to altering smell-based behaviors of tropical fish. They wanted to probe for similar impacts in salmon, a species that many people eat.

“Salmon rely heavily on their sense of smell for survival and successful reproduction. So it was important to investigate how low-pH water might impact their sense of smell,” he explains.

Williams and his team tested coho salmon in the lab living in water with different pH levels. And the more acidic the water, the less the fish fled the smell of danger, the researchers now report. They shared their findings December 18, 2018 in Global Change Biology.

Fear factor

“The smell of salmon skin should set up major alarms in a salmon’s brain. It warns them that their school mate is being eaten,” Williams says. “You can imagine that’s very powerful in the wild.” Normally, one whiff of salmon skin will send other fish scurrying for safer waters.

For their new tests, the researchers placed eight young coho salmon in each of three separate tanks. Then they gave the fish 14 days to get used to the ocean water, which had been harvested from nearby Puget Sound. It’s the large saltwater body in western Washington through which adult coho pass on their way to the Pacific Ocean.

One tank was a control. It contained water with a pH of 7.8. That is the natural average pH in Puget Sound. The second tank contained water with a pH of 7.5, making it slightly more acidic. That is also the projected pH of Puget Sound in 50 years, if CO2 emissions continue to rise. The third tank contained water with a pH of 7.2. That’s the projected pH in Puget Sound a century from now.

After two weeks, the researchers pumped an “alarm scent” laced with salmon skin extract into each tank.

Fish in the control tank fled the alarm scent, as expected. They spent 80 percent of their time in a “safe” portion of the tank. But fish didn’t seem to care where they hung out in the tank with the most acidic pH. “In other words,” said Williams, “they were indifferent to the alarm odor.”

The team then analyzed olfactory (scent) receptors in the cohos’ noses and brains. “We found no real signal interference in the nose,” said Williams. “But the way the brain is processing [the signal] seems different.”

Put simply: The salmon no longer recognized that the smell meant danger.

Upstream battle

Salmon depend on smell for more than evading predators. Like most salmon species, coho are anadromous. That means that they hatch and grow in freshwater rivers, then migrate to the sea as they mature. At the end of their lives, the salmon return to the streams of their birth. They fight their way there to spawn — then die.

Smell is critical for these fish finding their way back to “home” streams to reproduce. It’s also crucial for identifying other coho, including potential mates.

The results of the new study don’t bode well for these fish, Williams says. Already, he notes, these fish and their ecosystems face a host of other pressures. Dams on some rivers, for instance, make it difficult for coho to return to spawn in their home streams. And even should they make it upstream to spawn, dams might kill young coho as they tried to migrate downstream to the sea. Polluted storm water flowing into Puget Sound is also proving toxic to these fish.

James Helfield agrees that the study is bad news for coho. He is a fisheries biologist at Western Washington University, in Bellingham. Helfield studies salmon ecology but was not involved in the study. “This study illustrates another way that climate change might harm salmon populations,” he says.

In many waters, salmon are more than just another fish in the sea. In the ocean, they provide essential food for other species, including seals, orcas, other fish species and, of course, people. In rivers, they have a trickle-up effect on the food web and the ecosystem. Once salmon die in those rivers, their decaying bodies add nutrients to the forest.

And what’s bad news for salmon is probably bad news for other marine species, too. In fact, says Helfield, the study’s findings are not unexpected. “Other recent studies,” he notes, “have shown that some marine fish become ‘drunk and disoriented’ when exposed to too much carbon dioxide in sea water.”

Power Words

(more about Power Words)

acidic     An adjective for materials that contain acid. These materials often are capable of eating away at some minerals such as carbonate, or preventing their formation in the first place.

acidification     A process that lowers the pH of a solution. When carbon dioxide dissolves in water, it triggers chemical reactions that create carbonic acid.

average     (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.

carbon dioxide (or CO2)     A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter burns (including fossil fuels like oil or gas). Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.

climate     The weather conditions that typically exist in one area, in general, or over a long period.

climate change     Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.

colleague     Someone who works with another; a co-worker or team member.

control     A part of an experiment where there is no change from normal conditions. The control is essential to scientific experiments. It shows that any new effect is likely due only to the part of the test that a researcher has altered. For example, if scientists were testing different types of fertilizer in a garden, they would want one section of it to remain unfertilized, as the control. Its area would show how plants in this garden grow under normal conditions. And that gives scientists something against which they can compare their experimental data.

ecology      A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.

ecosystem     A group of interacting living organisms — including microorganisms, plants and animals — and their physical environment within a particular climate. Examples include tropical reefs, rainforests, alpine meadows and polar tundra. The term can also be applied to elements that make up some an artificial environment, such as a company, classroom or the internet.

factor     Something that plays a role in a particular condition or event; a contributor.

food web     (also known as a food chain) The network of relationships among organisms sharing an ecosystem. Member organisms depend on others within this network as a source of food.

forest     An area of land covered mostly with trees and other woody plants.

freshwater     A noun or adjective that describes bodies of water with very low concentrations of salt. It’s the type of water used for drinking and making up most inland lakes, ponds, rivers and streams, as well as groundwater.

global warming     The gradual increase in the overall temperature of Earth’s atmosphere due to the greenhouse effect. This effect is caused by increased levels of carbon dioxide, chlorofluorocarbons and other gases in the air, many of them released by human activity.

marine     Having to do with the ocean world or environment.

mature     (adj.) Connoting an adult individual or full-grown and fully developed (non-juvenile) form of something. (verb) To develop toward — or into — a more complex and  full-grown form of some individual, be it a plant, animal or microbe.

migrate     To move long distances (often across many countries) in search of a new home. (in biology) To travel from one place to another at regular times of the year to find food or more hospitable conditions (such as better weather). Species that migrate each year are referred to as being migratory. Some species also migrate occasionally to find environments suitable for growth or reproducing.

nutrient     A vitamin, mineral, fat, carbohydrate or protein that a plant, animal or other organism requires as part of its food in order to survive.

olfaction     (adj. olfactory) The sense of smell.

orca     The largest species of dolphin. The name of this black-and-white marine mammal, Orcinus orca, means killer whale.

organism     Any living thing, from elephants and plants to bacteria and other types of single-celled life.

pH     A measure of a solution’s acidity or alkalinity. A pH of 7 is perfectly neutral. Acids have a pH lower than 7; the farther from 7, the stronger the acid. Alkaline solutions, called bases, have a pH higher than 7; again, the farther above 7, the stronger the base.

population     (in biology) A group of individuals from the same species that lives in the same area.

receptor     (in biology) A molecule in cells that serves as a docking station for another molecule. That second molecule can turn on some special activity by the cell.

salmon     A popular game fish that tends to live most of its life in the ocean, then enters coastal rivers (and freshwater) to breed and lay eggs.

sea     An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.

seawater     The salty water found in oceans.

spawn     (in biology) To release or fertilize eggs in an aquatic environment.

species     A group of similar organisms capable of producing offspring that can survive and reproduce.

toxicologist     A scientist who investigates the potential harm posed by physical agents in the environment. These may include materials to which we may be intentionally exposed, such as chemicals, cigarette smoke and foods, or materials to which we are exposed without choice, such as air and water pollutants. Toxicologists may study the risks such exposures cause, how they produce harm or how they move throughout the environment.

upstream     The direction from which water flows, or portions of a stream from which water has flowed.

Citation

Journal: C.R. Williams et al. Elevated CO2 impairs olfactory‐mediated neural and behavioral responses and gene expression in ocean‐phase coho salmon (Oncorhynchus kisutch). Global Change Ecology. Published online December 18, 2018. doi: 10.1111/gcb.14532.

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