The spawning migration of the Northeast Arctic cod
Many animals migrate for various reasons, for instance to find new feeding grounds, survive harsh climate, or reproduce. Northeast Arctic cod is a fish that is of high economic importance to Norway and Russia and which undertakes annual migrations. Here, I will talk about its spawning migration and highlight some of its costs and benefits.
In the spawning migration of the Northeast Arctic (NEA) cod, the mature adults migrate against the currents to reach spawning grounds along the Norwegian Coast (see Map). They start their annual migration in winter when they reach maturity at around 6-9 years. The cod reach the spawning grounds around March and start spawning. This spawning event lasts until around May, when the fish start their return migration into the Barents Sea. The spawning locations of the NEA cod have changed in the last couple of decades. Earlier records show, that there was more spawning further South, even further than Møre (see Map).
But nowadays the majority of the spawning takes place in spawning locations North of Møre. Currently two hypotheses exist trying to explain this shift in spawning ground use. One is saying the shift is due to a change in temperature. In warmer years, the fish tend to spawn further up North than in colder years . The other hypothesis states that the shift is due to an overfishing of large-sized cod. Smaller fish are supposedly not able to swim that long distances due to their energetic requirements, thus, the spawning grounds further south along the coast are hardly used anymore . But even so, the fish still spawn at different spawning grounds, closer to the Barents Sea and further away. So why, are some parts of this population migrating longer distances for example to Møre in the South, which is around 1500 km from the Barents Sea instead of spawning in Finnmark, much closer with an approximate migration of 400 km (see Map)?
With the migration to and back from the spawning grounds the fish spend energy. This energy needs will reduce their energy available for the production of eggs, thus the longer the fish migrate, the fewer eggs a female can produce and spawn. There should be some benefit involved to counterbalance those extra costs for the parents. Usually, in the spawning migration, the benefits are mediated through the offspring.
Previous studies investigated already this system and the costs and benefits. They concluded that southern spawning grounds provide a survival benefit for the offspring, thus southern spawned larvae, have a higher survival compared to the northern spawned ones, which would then offset the female migration costs . In this case, fewer spawned eggs, due to higher migration costs, are counterbalanced by higher survival and thus, the longer migration distance is still worth the effort. However, it was suggested that the main benefit is rather an increase in offspring length at the more southern spawning grounds . In the southern areas the water temperature is warmer and that increases the early growth of southern spawned offspring. An increase in offspring size will lead to several benefits for the population including increased survival.
Understanding the trade-offs associated with the spawning migration may be very important for our ability to sustainably manage this stock and keep it in a healthy state, especially as climate change affects this stock especially through warming temperatures. This might affect early survival of the offspring differently than early growth.
 Sundby, S. & Nakken, O. 2008 Spatial shifts in spawning habitats of Arcto-Norwegian cod related to multidecadal climate oscillations and climate change. ICES Journal of Marine Science: Journal du Conseil 65, 953-962. https://doi.org/10.1093/icesjms/fsn085
 Opdal, A. F. & Jørgensen, C. 2015 Long-term change in a behavioural trait: truncated spawning distribution and demography in Northeast Arctic cod. Global Change Biology 21, 1521-1530. https://doi.org/10.1111/gcb.12773
 Jørgensen, C., Dunlop, E. S., Opdal, A. F. & Fiksen, Ø. 2008 The evolution of spawning migrations: state dependence and fishing-induced changes. Ecology 89, 3436-3448. https://doi.org/10.1890/07-1469.1
 Langangen, Ø., Ottersen, G., Ciannelli, L., Vikebø, F. B. & Stige, L. C. 2016 Reproductive strategy of a migratory fish stock: implications of spatial variations in natural mortality. Canadian Journal of Fisheries and Aquatic Sciences 73, 1742-1749. https://doi.org/10.1139/cjfas-2015-0321.