Stock size fluctuations
The Barents Sea capelin has undergone dramatic changes in stock size over the last three decades. Three stock collapses (when abundance was low and fishing moratoriums imposed) occurred during 1985–1989, 1993–1997, and 2003–2006. A sharp reduction in stock size was also observed during 2014-2016; followed by an unexpectedly strong increase during 2016-2017. Observed stock biomass in 2015 and 2016 was below 1 million tonnes, which previously was defined as the threshold of collapse. Despite indications that capelin stock size was underestimated in 2016, at present 2015-2016 is recognized as a ‘mini-collapse’. However, the stock size development during the next couple of years will reveal whether this is indeed the case.
Previous collapses have had serious effects both up and down the food web. Reduced predation pressure from capelin has led to increased amounts of zooplankton during periods of capelin collapse. When capelin biomass was drastically reduced, its predators were affected in various ways. Cannibalism became more frequent in the cod stock, cod growth was reduced, and maturation delayed. Sea birds experienced increased rates of mortality, and total recruitment failures; breeding colonies were abandoned for several years. Harp seals experienced food shortages, , and recruitment failure, and increased mortality; partly because they invaded coastal areas, and were caught in fishing gear. The effects were most serious during the 1985–1989 collapse, whereas, the effects 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 most recent collapses (1990s and 2000s).
These collapses were caused by poor recruitment, most likely in combination with low growth and increased predation pressure. It is likely that high levels of fishing pressure during 1985-1986 amplified and prolonged the first collapse. After each collapse, the fishery has been closed and the stock has recovered within a few years due to good recruitment. Several authors have suggested that predation by young herring has had a strong negative influence on capelin recruitment and, thus, has been a significant factor contributing to these capelin collapses (Gjøsæter et al. 2016).
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 poor recruitment (Fig. 4.3.1).
Mean length of 0-group capelin has varied somewhat during the data time series. From a biological perspective, one may hypothesize that survival rates from age 0 to age 1 might be correlated with lengths at age 0. However, a plot of mean length at age 0 and total mortality, from age 0 to age 1, shows no such correlation; rather, this plot shows that 0-group and/or 1-group abundance estimates and, therefore also, mortality estimates from age 0 to age 1, are noisy; this could possibly mask possible relationships that might exist.
Fig. 4.3.2 shows a stock-recruitment plot from Gjøsæter et al. (2016) going back to 1987. This plot shows that 1989 is still the strongest year class (age-1). An estimation of breakpoint from this plot could be attempted. This figure has not been updated since the 2016 report. Fig. 4.3.3 shows an alternative approach where recruitment at age 0 is used, and SSB is estimated as the mature stock (> 14 cm) in autumn (with fishery removals taken in January-March subtracted).
Fig. 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. (Fig. 7. in Gjøsæter et al. 2016).
Fig. 4.3.3. Relationship between mature stock biomass (> 14cm) with spring fishery subtracted (biomass at 1 Oct. Y, total landings from 1 Jan to 1 Apr.Y+1 are subtracted, 1000 tonnes) and 0-group index in billions (Y+1), covering the cohorts 1980—2017. The size of bubbles indicates the biomass of herring at age 1-3 (ICES WGIBAR data). Minimum diameter of bubble corresponds to 0.02 million tonnes of herring (1983), the maximum - 5.02 million tonnes. (1994). The red point is the 1989 cohort which is the basis for the current reference point (Blim).
The Barents Sea polar cod stock also had a declining trend until 2015 (described in the next section). The decrease in polar cod abundance may have contributed to increased predation pressure on capelin since polar cod is also prey for cod. Predation pressure from seals and whales may also have changed, but data are limited. Assuming that the occurrence of predators, such as harp seal and minke whale, has been stable, their steady feeding on capelin would come in addition to the heavy predation by cod.