4.3 Causes of capelin decline

Interactions, drivers and pressures 2016
Typography
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Previous stock collapses

The Barents Sea capelin stock has undergone drastic changes in size during the last three decades. Three stock collapses (when the abundance was low and fishing has been stopped) occurred in 1985–1989, 1993–1997, and 2003–2006. A strong reduction in stock size was also observed in 2014-2016, and in 2015 and 2016 the stock biomass has been below 1 million tonnes which earlier has been defined as a threshold for collapse.

The previous collapses have caused evident effects both downwards and upwards in the foodweb. The reduced predation pressure from capelin has led to increased amounts of zooplankton during the collapse periods. When capelin biomass was drastically reduced, its predators were affected in various ways. Cannibalism became more frequent in the cod stock and cod growth was reduced and maturation delayed. Seabirds experienced increased rates of mortality and total recruitment failures, and breeding colonies were abandoned for several years. Harp seals experienced food shortage, increased mortality partly because they invaded the coastal areas and were caught in fishing gears, and recruitment failures. The effects were most serious during the 1985–1989 collapse, whereas they could hardly be traced during the third collapse. Gjøsæter et al. (2009) concluded that these differences in effect likely resulted from increased availability of alternative food sources during the two last periods of collapse (1990s and 2000s).

The collapses were caused by poor recruitment, most likely in combination with low growth and increased predation pressure. High level of fishing pressure in 1985–1986 also probably amplified and prolonged the first collapse. After each strong stock decline the fishery has been stopped and the stock has recovered in few years due to good recruitment. Predation by young herring has been suggested by several authors to have strong negative influence on capelin recruitment and thus to be a significant factor in capelin collapses (Gjøsæter et al., 2015).

Recruitment

Capelin is a short-lived species and thus the stock size variation is strongly influenced by the annual recruitment variability. This may indicate that the main reason of capelin stock collapses is a poor recruitment (Figure 4.3.1).

Figure 4.3.1. Fluctuation of capelin at age 0 (blue line) and 1 (red line) for the cohorts 1980–2016.Figure 4.3.1. Fluctuation of capelin at age 0 (blue line) and 1 (red line) for the cohorts 1980–2016.

Recruitment of capelin measured as 0-group during the ecosystem survey has been at an average or high level from 2006 onwards. 0-group abundance gives a first indication of spawning success, while abundance of age 1 indicates recruitment to the adult stock. Recruitment as 1-year olds was also stable and around average for the cohorts 2006– 2013, while the 2014 and 2015 cohorts were poor at the 1-group stage. Survival from age 0 to 1 declined considerably during 2014–2016.

Most of the 0-group capelin originates from the spawning in spring. The 0-group from summer spawners is distributed mostly in the southern Barents Sea. Abundance of this portion (3 cm body length or less in August-September) has been relatively low compared with the total abundance of 0-group, and was estimated to make up 15% in 2013, 10% in 2014, 2% in 2015 and 1% in 2016. These small 0-group capelin likely are less able to survive the first overwintering since they have less time to grow during the first- feeding season.

The capelin stock age composition has varied considerably between years but has generally been dominated by age groups 1 and 2 (Figure 3.5.6). The observed increase in older fish (age 3) and relatively high abundance of capelin of age 2 during the period 2008–2013 have contributed in keeping the stock at a relatively high level and provide a good recruitment. A severe decrease in abundance of the age groups 1, 2, and 3 in 2014 and 2015 preceded the present capelin stock collapse.

The mean lengths of 0-group capelin have varied somewhat during the time-series. From biological reasoning, one may hypothesize that the survival rates from age 0 to age 1 might be correlated with the lengths-at-age 0. However, a plot of mean length- at-age 0 and total mortality from age 0 to age 1 reveals no such correlation. From the plot it is evident that 0-group and/or 1-group abundance estimates, and therefore also mortality estimates from age 0 to age 1, are noisy, and this might mask possible relationships that might exist.

Figure 4.3.2 shows stock–recruitment plot from Gjøsæter et al. (2015), going back to 1987. This plot shows that 1989 is still the strongest year class at age 1. An estimation of breakpoint from this plot could be attempted. Figure 4.3.3 shows an alternative approach where recruitment-at-age 0 is used and SSB is calculated as mature stock (>14 cm) in autumn (with fishery in take January-March subtracted). These plots are not updated from last year’s report.

Figure 4.3.2. SSB/R plot for capelin. Cohorts 1987–2012. Points coded according to herring biomass age 1 + 2 in spawning year. Circles—herring biomass <450 000 tonnes, crosses—herring biomass between 450 000 tonnes and 1.3 million tonnes, triangles-herring biomass >1.3 million tonnes. (Figure 7. in Gjøsæter et al., 2015).Figure 4.3.2. SSB/R plot for capelin. Cohorts 1987–2012. Points coded according to herring biomass age 1 + 2 in spawning year. Circles—herring biomass <450 000 tonnes, crosses—herring biomass between 450 000 tonnes and 1.3 million tonnes, triangles-herring biomass >1.3 million tonnes. (Figure 7. in Gjøsæter et al., 2015). Figure 4.3.3. Relationship between mature stock biomass (>14 cm) take of spring fishery (biomass at 1 Oct. Y, total landings from 1 Jan to 1 Apr.Y+1 are subtracted) and 0-group index (Y+1), covering the cohorts 1980–2016. The size of bubbles indicates the biomass of herring at age 1 and 2 (ICES WGWIDE data). Minimum diameter of bubble corresponds to 0.01 billion tonnes of herring (1982), the maximum - 3.61 billion tonnes. (1993). The red point is the 1989 cohort which is the basis for the current reference point (Blim).Figure 4.3.3. Relationship between mature stock biomass (>14 cm) take of spring fishery (biomass at 1 Oct. Y, total landings from 1 Jan to 1 Apr.Y+1 are subtracted) and 0-group index (Y+1), covering the cohorts 1980–2016. The size of bubbles indicates the biomass of herring at age 1 and 2 (ICES WGWIDE data). Minimum diameter of bubble corresponds to 0.01 billion tonnes of herring (1982), the maximum - 3.61 billion tonnes. (1993). The red point is the 1989 cohort which is the basis for the current reference point (Blim).

Natural mortality

The estimated capelin mortality based on the survey results has shown a marked increase in the last years (Figure 4.3.4). This corresponds in time to the strong decline of the capelin stock.

Figure 4.3.4. Natural mortality of age 1-2 capelin.Figure 4.3.4. Natural mortality of age 1-2 capelin.

The stock of polar cod in the Barents Sea also declined until 2015 as described in the next section. The decrease in polar cod abundance during this period may have contributed to increased predation pressure on capelin since polar cod serve as additional prey for cod. The predation pressure from seals and whales may also have changed, but there is little information available regarding this. Assuming that predators such as harp seal and minke whale have a more stable occurrence in the Barents Sea, their food demand by feeding on capelin would come in addition to the heavy predation by cod.

Reasons for the recent collapse

The strong decline in the capelin stock in the last three years appears to be caused by a combination of the same factors as in the previous capelin collapses but with different relative contributions. We have witnessed a good recruitment as 0-group, indicating that predation by herring on 0-group capelin has not reduced the year class considerably, this is also consistent with the low to intermediate herring abundance in the Barents Sea in recent years. However, there has been increased mortality both from age 0- 1 and on older capelin. Observations during the Joint winter survey indicate that both the 2015 and 2016 year classes of capelin were observed in large quantities as 1-group during this survey, although no estimates of abundance were made. The 2015 year class seems to have been strongly reduced as 1-group between this survey and the ecosystem survey in autumn. It should be noted that the 2016 year class was even more abundant than the 2015 year class at age 0. This is detailed in the subsections below. Despite the capelin biomass decrease, the weight percent of capelin in cod diet remains practically on the level of previous years with high capelin stock. Similarly, the estimated annual consumption of capelin by cod was around 4 million tonnes from 2009, which is of the same magnitude as the stock size. A decline in the consumption was observed in the second half of 2015 and in 2016, but the decline is less strong than the decline in the stock estimates.

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