The Barents Sea cod change spawning location, becomes smaller, and may struggle to survive their first winter.

With temperature warming, fish population are not only changing their foraging zone but also where they are spawning their eggs. While temperature can be beneficial for high latitude species it may also lead to some constraints. In their paper, Langangen et al (2025) studied the effect of changing spawning location for Atlantic cod in the Barents Sea.

Figure 1: Climate change and population dynamics. Effect of temperature and spawning location of Atlantic cod Gadus morhua in the growth of survival during their firsts years of life.

By Langangen, Durant

Published July 3, 2025

As climate warming becomes more prominent, it has become clear that fish species tend to shift their distribution polewards. This also applies to the cod Gadus morhua in the Barents Sea. It has been suggested that adult cod tend to follow their main prey, capelin Mallotus villosus, in the summer. With the climate warming, fish in the Barents Sea have moved more north and east compared to colder periods. However, as the adult fish sets out on their migrations towards the spawning grounds, the distance to suitable areas may have become larger. What is known is that cod has tended to spawn further north over the recent period.

Bildet kan inneholde: kart. — **Figure 2:** shows the region of study. The hatched area indicates the distribution of cod in the Barents Sea. Black and grey polygons indicate the spawning grounds of cod, and the boxes illustrate the data resolution. The inlet shows the recruitment (new 3 years of age young fish entering the population) in grey and the spawning stock size in back. The dotted line shows the size of the juveniles in fall. Grey background indicates the focal time of this study.

At the spawning grounds in spring around the Lofoten archipelago and more recently farther north in Finnmark (Figure 2), female cod release pelagic eggs that are fertilized in the water column. After 2-3 weeks, the eggs hatch into small larval fish that, like the eggs, are carried with the currents towards the Barents Sea. As larvae and later as juveniles, the young cod feed and grow as they ride the currents. However, what awaits them is a long, cold, and dark winter in the Barents Sea. Survival through this first winter is a major challenge for these small fish and seems to be strongly dependent on the size of the fish in the fall before the winter. In a recent study, we investigated long time series for spawning location, temperatures, and size of the spawning stock to unravel the causes of changes in juvenile size in the fall. We found that the juveniles tend to become smaller as the cod adults spawn farther north (Figure 3).

Bildet kan inneholde: hvit, tomt, diagram. — **Figure 3:** *shows the effect of spawning location on the size of juveniles in the fall (left). We also found positive effects of temperature (right) and a dome-shaped effect of spawning stock biomass (central).*

This can be caused by the later start of spawning in the northern areas, so that the juveniles have less time to grow by the fall. It may also be caused by slower growth, which may be because the eggs and larvae spawned in Finnmark often experience colder water compared to offspring from Lofoten. Fish growth is typically temperature-dependent, and slower growth is frequently observed at lower temperatures. However, growth is also food-dependent, and we cannot fully separate these effects. However, we have vividly demonstrated that the size of cod juveniles depends on where they were spawned. As a consequence of the smaller size of the offspring in fall, we expect there to be fewer fish that grow up to become adults. This may be part of the reduction observed in the historically very large cod stock in the Barents Sea (Figure 2).

Reference:

Langangen Ø, Olsen T, Ottersen G, Eriksen E, Durant JM (2025) Consequences of climate-related poleward shifts in cod spawning. Can J Fish Aquat Sci