Feature Image: The Northeast Arctic cod stock is the largest in the world and serves as an important fishery to Norway and surrounding regions. Photo credit: https://pixabay.com/en/fish-cod-smoked-steckerlfisch-107708/
Langangen, Ø., Färber, L., Stige, L.C., Diekert, F.K., Barth J.MI., Matschiner, M., Berg, P.R., Star, B., Stenseth, N.C., Jentoft, S., and Durant, J.M. 2019. Ticket to spawn: Combining economic and genetic data to evaluate the effect of climate and demographic structure on spawning distribution in Atlantic cod. Global Change Biology 25: 134-143. https://doi.org/10.1111/gcb.14474
The Northeast Arctic cod stock
The Northeast Arctic cod stock (Gadus morhua) is the largest in the world and serves as an important fishery to Norway and surrounding regions. Cod are migratory fish, moving from their feeding grounds to spawning grounds seasonally. Seasonal migration is energetically costly, but may benefit individual survival by lowering the risk of predation and disease and offspring survival and/or growth. In the Northeast Arctic, cod use the Barents Sea as their feeding ground and migrate along the Norwegian coast to spawning ground in the spring. Fish mature after 7-11 years and spawn from February to May with peak spawning during March and April.
The timing and duration of spawning in the Northeast Arctic cod stock has remained relatively stable over time. During spawning, eggs are released into the water column and then fertilized. After fertilization, eggs develop in to larvae and then juveniles that drift along the Norwegian coastal current towards the Barents Sea. Healthy spawning grounds and spawning populations are vital to maintain the population of fish stocks and ensure sustainable management.
Changing distribution of Northeast Arctic cod stocks at spawning grounds
Recently, there has been evidence that suggests that the distribution of cod among the main spawning regions in Norway is changing. While spawning in the central Lofoten area has remained relatively stable over time, there has been variability in the spawning intensity in the northern Finmark and southern Møre regions (See Figure 1 of Langangen et al. 2019 for a map of the spawning regions). Two different hypotheses have been proposed to explain the changes in spawning of Northeast Arctic cod: climate and harvesting (i.e. fishing). Climate change can impact the spawning grounds of cod, primarily through warming ocean temperatures, due to temperature restrictions of spawning grounds or shifts in the distribution of feeding grounds. Harvesting can impact spawning by selectively removing larger individuals with higher reproductive potential, leading to decreases in the age of maturity and size at maturity in fish populations. In addition, larger fish are able to migrate further due to their energy reserves, so removing large individuals from the population through fishing can impact the migratory distance.
Is climate or harvesting driving changes in the distribution of Northeast Arctic cod in spawning grounds?
In order to determine whether climate or harvesting were driving the changes in distribution of Northeast Arctic cod amongst spawning ground, Øystein Langangen and a team of scientists from the University of Oslo (Norway), Heidelberg University (Germany), University of Basel (Switzerland), and University of Agder (Norway), examined Norwegian fisheries data to determine the size of spawning fish in different spawning regions (from north of south: Finmark, Lofoten, and Møre). They analyzed over 1 million records of cod during the spawning season from 2008-2016. From this, they determined that the size of spawning fish increased from south to north. In other words, spawning fish were smaller in the Møre region and larger in the Finmark region. The Møre region is farthest away from the feeding ground in the Barents Sea, indicating that spawning fish size decreased with distance from the feeding grounds.
Langangen and colleagues also examined whether ocean temperature could be driving the distribution of cod among spawning grounds. They found a significant correlation between the winter-spring temperature of the water and the weight of spawning fish. Looking at data from 2000-2016, they found that in years with warm winter-spring temperatures, more northern spawning occurred. Taken together with the smaller size of fish further from feeding grounds, Langangen and colleagues suggest that climate is a major driving factor in the distribution of Northeast Arctic cod spawning grounds, while harvesting is not a dominant factor.
Conclusions and future directions
In order to ensure a sustainable fishery, managers must understand the factors that influence fish populations and their reproductive success. This study suggests that climate change is altering the distribution of Northeast Arctic cod spawning grounds, which has important implications for fishery managers. However, climate and harvesting are not the only factors that can influence populations and spawning. Future research is necessary to investigate other potential influences such as species interactions, geographic patterns, and competition.