Causes of polar cod stock fluctuations

Photo: Peter Leopold. NPI.

Interactions, drivers and pressures 2019
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The Barents Sea polar cod stock was at a low level in 2017 and 2018. 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.

Causes of polar cod stock fluctuations

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. In 2018 the survey failed to cover the eastern parts of the Barents Sea and a reliable estimate of polar cod could not be obtained. However, the 2019 estimate confirmed that the polar cod stock is presently at a very low level. 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. Since the mid-1990s, there has been a general trend of increase in both air and water temperature in the Barents Sea (See Section 3.1); record high temperatures have been recorded during the 2000s. The areal extent of sea ice coverage has never been lower than 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. It should be noted that during the three last years the temperatures have decreased somewhat, and the ice coverage has increased again. 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 declining stock trend in recent years. In the overlapping area cod feeds efficiently on polar cod (see Section 4.2). The Barents Sea polar cod stock also has had a declining trend in recent years. Less is known about recruitment mechanisms of polar cod than of capelin, but some recent studies of recruitment of polar cod (Eriksen et al., 2019, Huserbråten et al., 2019, and Gjøsæter et al., 2020) may shed some additional light on this topic. Based on a particle tracking model, Eriksen et al. (2019) studied simulated drift patterns of polar cod eggs in the Svalbard area. It has been inferred from 0-group distributions that some spawning must have been taking place near Svalbard, but the location of this spawning is unknown. By releasing “eggs” several places around the Svalbard peninsula, from inner fjords to the outer coast, and letting these “eggs” drift with the currents until late summer and then compare their distribution with observed distributions of 0-group, the authors were able to backtrack the most probable spawning locations. Because there is a clockwise gyre flowing around Svalbard, they concluded that eggs spawned in outer coastal areas both at the western, northern and eastern coasts of Svalbard would be possible spawning areas, but that spawning locations under the ice east of Svalbard was the most probable spawning area for the western component of polar cod. This finding was confirmed by similar studies carried out by Huserbråten et al. (2019), who expanded the particle drift experiment to many more years and included the whole Barents Sea. The data-driven biophysical model of polar cod early life stages used in the latter study predicted a strong mechanistic link between survival and variation in ice cover and temperature; ice cover was positively related to survival of polar cod eggs and larvae, while temperature was negatively related to survival. The backtracking model also suggested a northward retreat of the spawning assemblages in the eastern Barents Sea, possibly in response to warming. Gjøsæter et al. (in review) used the same biophysical model to characterize the environmental and developmental properties of the early life history of individuals that reached the 0-group stage at the time and place of observations, and examined if and how ice cover, ice breakup time, maximum temperature, and spawning stock biomass relate to modelled larval survival. Results indicate that high ice coverage has a significant positive effect and high temperature a significant negative effect on survival of eggs and larvae from an eastern spawning component. No significant effects were found for the western spawning component, possibly because the variations in ice cover has been less noticeable there. These recent studies support earlier findings that successful polar cod recruitment is associated with an ice cover until the eggs hatch. After hatching, however, larval survival depends on available food, which will only be available after ice break-up and onset of primary and secondary production. One may hypothesize, that ice break-up synchronizes these events, since the melting of ice and the associated stabilizing of the water column, warming of the surface layer, and deepening of the photic zone may initiate both hatching of eggs and onset of algal production.