4.4 Causes of polar cod fluctuations

Interactions, drivers and pressures 2017
Typography
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The Barents Sea polar cod stock was at a low level in 2017. Norway conducted commercial fisheries on polar cod during the 1970s; 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. Stock size declined from 2010 to a very low level in 2015, increased to 0.9 million tonnes in 2016, and again declined to 0.4 million tonnes in 2017. The rate of natural mortality for this stock appears to be quite high; relative to its importance as prey for cod and different stocks of seals.

It appears that polar cod mortality has increased in recent years. The recent warming in the Barents Sea has resulted in decreased sea ice distribution; several boreal species have moved northward; while the distribution area of Arctic species, such as polar cod, has decreased.

Since the mid-1990s, there has been a general trend of increase in both air and water temperature in the Barents Sea (See chapter 3.1); record highs have been recorded during the 2000s. The areal extent of sea ice coverage has never lower, for both the Arctic and the Barents Sea, as in 2016. In the Barents Sea, the area of Arctic water decreased, while a larger portion has been dominated by warmer Atlantic water. These climatic changes have likely 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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