Capelin: a “Sea Canary” for Marine Ecosystem Change in Response to Heatwaves

Twenty years ago, Dr. George Rose with Memorial University of Newfoundland called capelin "a sea canary for marine ecosystem change." A new study reinforces its role as a pillar of healthy marine ecosystems. It shows that heatwaves can dramatically affect capelin populations. Commercially important fish species like salmon and halibut rely on capelin as a food source-as do whales, seals, sea lions, and seabirds. And because they are not commercially harvested in the northeast Pacific Ocean, studying capelin in Alaska offers a unique opportunity to investigate population changes relative to environmental factors.

Between 2002 and 2022, Alaska Fisheries Science Center scientists observed capelin's response to two marine heatwaves.They found declines in capelin abundance reduced energy available to predators to less than 1 percent of pre-heatwave values.

Capelin: an Indicator Species for Marine Ecosystems in Alaska

Lead author of the study Rob Suryan explains, "We consider capelin, along with herring, sand lance, and krill, to be the 'big 4' pelagic schooling forage species in Alaska. They are incredibly energy-rich-meaning they have a high lipid, or fat, content. And when they school, they form dense aggregations, which greatly increases foraging efficiency for predators. These two factors make them very valuable food for marine mammals, seabirds, and many commercial fish species."

Abundant prey is critical for these predators. Decreased capelin populations, in particular, have been linked to declines in cod, birds, and seals and sea lions in both the North Atlantic and North Pacific Oceans.

In the North Atlantic Ocean, capelin are harvested for food, bait, and fishmeal by multiple countries. They are not commercially harvested in the northeast Pacific Ocean. Therefore, changes in capelin abundance and condition in the waters off Alaska primarily reflect natural changes that are independent of potential effects by commercial harvesting. This "canary" for ecosystem change in Alaska provides an important, unbiased indicator for understanding how environmental changes influence population fluctuations of capelin and other species throughout the ecosystem.

Heatwaves Have Major Impacts on Capelin and the Ecosystem

During this study, we investigated data from autumn surface trawl surveys in Alaska. We evaluated how size, body condition (lipid content), abundance, and distribution of age 1+ capelin responded to two marine heatwaves between 2014 and 2019. We analyzed data that spanned two decades (2002-2022), and across three large marine ecosystems in Alaska :

• Gulf of Alaska
• Bering Sea
• Chukchi Sea

We found that capelin abundance declined dramatically during heatwave years, and they were found in far fewer areas. During heatwave years we also found them in deeper, cooler water. In fact, they were absent in the surface waters (0-20 meters) in the Gulf of Alaska and Chukchi Sea regions-and less abundant in the Bering Sea. This dramatic change in prey distribution had important implications for capelin predators, especially those that primarily feed in surface waters, like salmon and many seabirds.

We also found declines in capelin body condition (less lipid content) in warmer years in the Chukchi and Gulf of Alaska regions. Together with lower abundance, we estimated that-during heatwaves-the energy available to predators from capelin declined to less than 1 percent of the maximum in the Bering Sea. In addition to Pacific cod , the prey collapse during the multi-year 2014-2016 marine heatwave in the Gulf of Alaska was linked to mass mortality events of seabirds and marine mammals .

Pinpointing Vulnerable Life Stages for Capelin

Overwinter mortality is one of the greatest concerns for high-latitude fish. Food is scarce during the winter and this study was a first step toward exploring which life stages of capelin are most vulnerable. From the survey data, we evaluated pre-winter body condition as an important metric to forecast survival and future abundance.

We found no consistent correlation between pre-winter body condition of age 1+ capelin and overall abundance in the following year. This was consistent across all three large marine ecosystems in the study. This suggests the environmental conditions that led to mortality likely occurred during a different time of year, or during the first winter of life. Smaller age-0 fish are particularly susceptible to overwinter starvation because they have relatively low energy stores to sustain them.

Capelin have relatively complex spawning patterns and life histories, which influences our ability to track vulnerabilities by life stages.

Capelin Spawning Behavior

• Age and frequency: Spawn at 2 years old and die after spawning; they may spawn more than once in the same season
• Location: Typically on beaches, but also offshore in deeper water
• Timing: May through July, but spawning can extend into September in some parts of Alaska

Capelin are also relatively unique in that they can overwinter in their larval stage. These adaptations suggest that sexual maturity occurs at 2 years under normal conditions.

We've learned that warmer ocean conditions appear to disrupt the typical capelin life history. During the 2014-2016 heatwave in the Gulf of Alaska, capelin spawned at 1 year old instead of two. In such conditions, they appear to prioritize reproduction over survival by spawning earlier.

Future Research Focus

Understanding why drastic changes in population abundance occur helps us evaluate current ecosystems, and forecast the future. In particular, linking environmental conditions to fish health provides insights into mechanisms of change.

But marine ecosystems are complex. While we have documented large swings in capelin abundance, we lack a clear understanding of how those changes came about. We also want to understand if pre-winter conditions are an important factor for age-0 capelin overwinter survival.

Future studies will focus on the youngest age classes of capelin and what triggers the older age classes to spawn. This will include a more holistic approach combining field, laboratory, and modelling efforts to provide an improved understanding of what drives capelin abundance, reproduction, and energetics. Capelin is a key component to marine food webs in Alaska. Being able to forecast this "sea canary's" health and productivity provides important indicators and insight on the ecosystem as a whole.