Pacific herring, an important food source for salmon, show sensitivity to marine heatwaves

Pacific herring are known as one of the ‘great fishes of the North Pacific Ocean’ as they are inextricably connected via complex food webs and overlapping habitats with Pacific salmon species, such as Chinook and Coho, sea lions and orcas. 

Despite the importance of Pacific herring, the consequences of climate change and ocean acidification on this species remain poorly understood. The Washington Ocean Acidification Center (WOAC) would like to change that. WOAC postdoctoral researcher Chris Murray is the lead author on a new paper, which investigated how Pacific herring respond to the co-occurring stressors of high temperatures and increased levels of CO2. The paper was published in the Journal of Experimental Biology on March 10, 2022.

“In the past decade, the North Pacific Ocean has experienced two significant heatwave events, including the event known as ‘the blob’ between 2014-2016. We also know that ocean acidification, which is linked to increased carbon dioxide levels in the ocean, doesn’t exist in a vacuum,” said Murray. “We wanted to better understand the implications of heat stress on this important fish while incorporating ocean acidification into the framework.” 

The research team designed an experiment to mimic environmental fluctuations predicted over the next 100 years in Padilla Bay, a Puget Sound estuary that is broadly representative of Pacific herring spawning habitat. The researchers tested the upper limit of projected carbon dioxide in combination with a simulated heatwave event to determine how herring embryos reacted to rapid warming, increased carbon dioxide and to a combination of the two stressors.

They measured oxygen consumption, developmental rates and energy efficiency– typical measures for basic survival and growth–under these conditions. In general, the results showed that Pacific herring embryos are largely tolerant of increased CO2, both as a single stressor and as a compound stressor. This may be due to the fact that Pacific herring populations have over time adapted to life in the Salish Sea, which is relatively acidified from natural processes. 

However, the authors found that the heat wave on its own did produce a number of adverse effects. The heat wave caused a sharp increase in metabolic rate and caused embryos to expend a greater amount of their fixed energetic reserves provisioned in their yolk sacs to maintain homeostasis. This resulted in smaller larvae at hatch with less remaining yolk, which could yield a higher mortality rate for larvae in the wild.

“While the herring embryos have a wider thermal tolerance than expected, this experiment showed that there may be a limit to what the species can withstand,” said Murray. “This has opened the door to other research that is necessary in order to better understand the Pacific herring.”

Murray noted that this experiment didn’t test what is potentially the most sensitive developmental stage, which occurs shortly after fertilization before the embryo develops functioning organ systems. Additionally, there are multiple environmental factors that are difficult to replicate in a lab, such as high-frequency fluctuations of temperature, pH, and dissolved oxygen and intense solar radiation during low tide, as environmental conditions in shallow seagrass meadows may fluctuate widely over a single tidal or day/night cycle.  

University of Washington, 10 March 2022. Press release.

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