Causes of polar cod stock fluctuations

Photo: Frederik Broms, NPI.

Interactions, drivers and pressures 2020
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The Barents Sea polar cod stock was above 800 kt in the period 1998-2011 (except from the year 2003 when the stock was probably grossly underestimated) but dropped to below half of that in 2012-15 (Figure 4.4.1). In 2016, the strong 2015-year class brought the stock up to nearly 1 million tonnes but dropped to low levels in 2017-2019.

Causes of polar cod stock fluctuations

In 2020, the exceptionally strong 2019-year class recruited to the stock and the total biomass exceeded 1.7 mill tonnes in the acoustic estimate. Although the one-year-olds normally accounts for a high proportion of the total biomass (nearly 60% in 2020), and a considerable fluctuation in total biomass following fluctuations in recruitment therefore is anticipated, there are reasons to believe that the acoustic stock estimates in some years highly uncertain. The number of two and three years old polar cod in 2020 are for instance 3.2 and 6.8 times larger than the number of one- and two-years old fish in 2019. Either the 2019 (and 2018) estimates are an underestimate (e.g. due to lack of area coverage), or the 2020 estimate is an overestimate. Consequently, the fluctuations in stock size are probably less than what is derived from the estimates of stock size. But even though the variation in stock size may be less than shown in Figure 4.4.1, there are strong reasons to believe that the recruitment in recent years, and therefore also the total stock size, are substantial.

Figure 4.4.1 Total stock biomass and recruitment (acoustic estimate of 1-year-olds) of polar cod in the Barents Sea. Figure 4.4.1 Total stock biomass and recruitment (acoustic estimate of 1-year-olds) of polar cod in the Barents Sea.

The reasons for this are unclear. 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. Reasons for the stock fluctuations must therefore be sought in variable natural mortality and variable recruitment, or a variable part of the total stock may be found outside the Barents Sea.

The rate of natural mortality for this stock appears to be quite high; even in absence of fishing the total reduction in numbers from one age group to the next judged from the acoustic surveys are substantial in some years. It appears that polar cod mortality has increased in recent years, although “negative mortalities” in some years make it difficult to draw firm conclusions, as stated above. Presumably most of the natural mortality is caused by various predators. Although it is generally assumed that polar cod is a key forage species in the Arctic, and that large predatory fishes (like cod), seals, and sea-birds are important consumers, not much is known about the details, like for instance who are the dominant predators on each of the polar cod age groups. Since polar cod are normally found at deeper waters as they grow older and larger, sea birds probably play a minor role in consuming larger polar cod. On the other hand, predatory demersal fish like cod, Greenland halibut, long rough dab and others probably mostly consume larger polar cod. Seals are able to feed over most depths in the Barents Sea, and polar cod is known to be a dominant prey for harp seals (Haug et al. 2021 and references therein). Only for cod do we have a year by year consumption estimate of polar cod, and the consumption estimates varies from less than 100 kt (in the 1980s and in 2014 and 2017) to more than 700 kt (in 2009). Preliminary analysis of cod consumption in the arctic Barents Sea in late summer shows a strong increase in cod’s consumption of polar cod from the period 2004-2007 to 2008-2013, coinciding with a reduction in polar cod biomass. In the later period, the cod stock increased and expanded its distribution to the north and northeast, overlapping more with the polar cod habitat. After this period, consumption of polar cod has been reduced to similar, or slightly higher, levels as before the expansion. The consumption after 2013 has seemingly decreased somewhat in pace with the decrease in cod stock size, and the more southern distribution of cod during the feeding season. However, consumption increased in 2020 following the recruitment of the strong 2019-year class.

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, since 2016, the temperatures have decreased somewhat, and the ice coverage has shown an increasing trend. Nevertheless, the ice coverage in November 2020 was the smallest since 1951, that could affect spawning condition for polar cod in winter 2020/2021. At the same time, there is a lack of knowledge about the spawning area of polar cod. Previously, spawning was in the southeastern part of the Barents Sea, but in the last warm years it shifted to the southwestern part of the Kara Sea. Some spawning places have in 2013 been found in a shallow water area at 77°00’-77°30’ N 75°-80° E (Prokhorova et al., 2013). How spawning places can affect polar cod recruitment and therefore stock size fluctuation is unknown.

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 recruitment (Fig 4.4.1) has been spasmodic in recent years; mostly modest but with very strong year classes in 2014 and 2019. 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 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. (2020) 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.