4.4 Causes of polar cod fluctuations

Interactions, drivers and pressures 2016
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The Barents Sea polar cod stock is in 2016 at an intermediate level. Norway conducted commercial fisheries for polar cod during the 1970s, and Russia has fished this stock on more-or-less a regular basis since 1970. However, the fishery has for many years been so small that it is believed to have very little impact on stock dynamics. Stock size has been measured acoustically since 1986, and has fluctuated between 0.1–1.9 million tonnes. The stock size declined from 2010 to a very low level in 2015.

However, it then increased again to 0.9 million tonnes in 2016 which is around the average level. This increase is due to the strong 2015 year class. The rate of natural mortality for this stock appears to be quite high. This is related to the importance of polar cod as prey for cod and different stocks of seals.

It appears that mortality has increased in recent years. During the recent period with polar cod, when the Barents Sea has been warm, the distribution of sea ice has decreased, and several boreal species have moved northward while the distribution area of Arctic species like polar cod has decreased.

Since the mid-1990s there has been a general rise in both air and water temperature in the Barents Sea (See chapter 3.1). The 2000s have been record warm. The area covered by sea ice has never been so low in the Arctic and the Barents Sea as in 2016. In the Barents Sea the area of Arctic water decreased while a larger part of the sea has been dominated by warmer Atlantic water. These climatic changes may have affected the distribution and abundance of Arctic species like polar cod.

The reduction of sea ice in winter reduces spawning habitat, leading to unfavourable conditions for polar cod spawning (Eriksen et al., 2015c). The eggs have long incubation time and float near the surface where they may be exposed to unstable temperatures and increased water mixing due to lack of ice. Most of the juveniles are found in waters with temperatures below 5°C and reduction of cold water masses in summer and autumn reduces the nursery area for 0-group polar cod. 0-group polar cod prey on small plankton organisms such as copepods and euphausiids, while adults feed mainly on large Arctic plankton organisms such as Calanus hyperboreus and C. glacialis and hyperiids. The biomass of Arctic forms of zooplankton decreased in recent years and most likely influenced negatively the feeding conditions for 0-group polar cod. However no significant changes in the condition of adults were observed in recent years. This indicates a high degree of adaptability of this species to changes in the environment and enough available food resources.

The current fishing pressure is negligible now compared to the 1970s, when total catches were as high as 350 thousand tonnes. Thus the total mortality is close to the natural mortality. Most likely predation by cod has contributed to the high natural mortality. Cod is a boreal species and associated with the temperate waters. The Barents Sea warming has been beneficial for cod and it has spread further north. In the northern areas cod overlapped with polar cod, and thus predation pressure on polar cod has increased, contributing to the stock decline until 2015. In the overlapping area cod feeds efficiently on polar cod (see chapter 4.2).

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