NOAA Scientists Discover Repeat Spawning in Pacific Capelin

For certain fish species, like Pacific salmon, reproduction is a once-in-a-lifetime event, known as semelparity. Many other fish are iteroparous-meaning they can spawn repeatedly throughout their lives. New research by NOAA's Alaska Fisheries Science Center reveals the first evidence that Pacific capelin can be repeat spawners.

Over the past decade, the North Pacific Ocean has faced extreme climate variability, including marine heatwaves that severely impacted capelin populations . Understanding capelin's reproductive flexibility under these fluctuating conditions is crucial for predicting their annual reproductive potential and long-term resilience. Capelin abundance is also an important indicator of the marine ecosystem health .

Examining Reproductive Flexibility in Fluctuating Environments

Historically, the life history, ecology, behavior, and population dynamics of Pacific capelin have not been studied as well as their commercially important counterparts in the Atlantic. In North Pacific and Arctic ecosystems, Pacific capelin serve as a major forage fish and primary food source for Steller sea lions, humpback whales, and other marine mammals. They also are a key subsistence resource and an important food source for commercially important groundfish.

Capelin are short-lived-typically surviving only 1 to 4 years. Their abundance fluctuates wildly in changing environments, leading to multiyear population crashes during marine heatwaves. Recent research highlights that these heatwaves shrink capelin distribution and deplete their energy reserves. Ultimately, these combined stressors leave capelin highly vulnerable to mortality and put the predator populations that rely on them at risk of decline.

In this new experimental study of captive fish, NOAA scientists looked more closely at the spawning potential of Pacific capelin. They researched their life history and how they respond to shifting environments.

Microscopic Observations Lead to Big Insights

To investigate whether females could spawn repeatedly, our scientists examined capelin oocytes under a microscope. Oocytes are unfertilized female germ cells that develop into eggs. Among many types of mammals and fish, oocytes remain in ovaries for months or decades-suspended in animation. Cell maturation starts before ovulation.

When the oocytes mature in fish, they balloon in size and take on large quantities of protein and lipids. When almost all of the oocytes are in this state, the capelin will only spawn once. If there is one group of oocytes that mature and another that does not, the capelin has the capability to spawn more than once. These microscopic observations are essential for determining the reproductive future for a capelin, and this method can be applied to many other species as well.

A Closer Look at Spawning Scenarios and Habitats

While living longer to spawn repeatedly might seem like the ideal strategy, there are trade-offs. Putting all available energy into a single, massive reproductive event (semelparity) gives offspring the best chance to survive when environmental conditions are ideal. However, if conditions turn harsh-such as during a marine heatwave-that strategy can fail. Instead, maintaining the flexibility to survive and spawn again in later years (iteroparity) allows adults to "hedge their bets." Ultimately, populations that use a mix of both strategies have the greatest resilience and stability in highly variable environments.

This balance varies heavily by population. Our study found an iteroparity rate of 41 percent in Pacific capelin. For comparison, beach-spawning Atlantic capelin in northern Norway can show iteroparity rates up to 60 percent. Atlantic demersal spawners-which lay their eggs on the seafloor rather than the beach-are entirely semelparous. They die after a single spawn .

Looking Ahead: Future Research

Instances and rates of iteroparity likely shift based on environmental variability. This new evidence of iteroparity in Pacific capelin opens a wealth of new field and laboratory opportunities and approaches. They will be aimed at addressing several key questions to help predict capelin population dynamics in response to shifting environmental conditions. These include:

• Evaluating the potential of subtidal spawning habitats
• Tracking the rates and productivity of each reproductive strategy in wild populations
• Uncovering the mechanisms that determine the number of reproductive cycles
• Measuring how often maturing fish skip spawning altogether in varying environments

For field work, data collection remains a major hurdle. Pacific capelin lack a commercial fishery and are notoriously irregular in their spawning timing and locations. Researchers will carefully design field sampling around spawning windows, with expert knowledge and observations from Alaskans around the state. Laboratory studies will complement the field work and offer a controlled environment to isolate variables like temperature to observe stress responses.

Together, these paired approaches will help advance NOAA Fisheries' ecosystem research goals. They may reveal how both individual fish and the broader capelin population adapt to environmental fluctuations.