Eleven years (2006-2016) of capelin diet were examined from the Barents Sea where capelin is a key forage species, especially of cod. The PINRO/IMR mesozooplankton distribution shows low plankton biomass in the central Barents Sea, most likely due to predation pressure from capelin and other pelagic fish. This pattern was also observed in 2017. In the Barents Sea, a pronounced shift in the diet from smaller (<14cm) to larger capelin (=>14cm) is observed. With increasing size, capelin shift their diet from predominantly copepods to euphausiids, (mostly Thysanoessa inermis - not shown), with euphausiids being the largest contributor to the diet weight in most years (Figure 4.1.1).
Interactions, drivers and pressures 2017
Feeding, growth, and maturation of cod
Cod is the major predator on capelin; although other fish species, seabirds and marine mammals are also important predators. In the last 6-7 years, cod stock levels have been extremely high in the Barents Sea. Estimated biomass of capelin consumed by cod in recent years has been close to the biomass of the entire capelin stock (Fig. 4.2.3). Abundance levels of predators other than cod are also high and, to our knowledge, stable.
Cod-capelin-polar cod interaction
The interaction cod-capelin-polar cod is one of the key factors regulating the state of these stocks. Cod prey on capelin and polar cod, and the availability of these species for cod varies. In the years when the temperature was close to the long term mean, the cod overlap with capelin and polar cod was lower than in the recent warm years. Cod typically consume most capelin during the capelin spawning migration in spring (quarters 1+2), but especially in recent years the consumption has been high also in autumn (quarters 3+4) in the northern areas (Fig. 4.2.3).
Important indirect effects of fisheries on the ecosystem
In order to conclude on the total impact of trawling, an extensive mapping of fishing effort and bottom habitat would be necessary. In general, the response of benthic organisms to disturbance differs with substrate, depth, gear, and type of organism (Collie et al. 2000). Seabed characteristics from the Barents Sea are only scarcely known (Klages et al. 2004) and the lack of high-resolution (100 m) maps of benthic habitats and biota is currently the most serious impediment to effective protection of vulnerable habitats from fishing activities (Hall 1999).
Causes of capelin 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.
Snow crab effect on benthos
In most of the measured years, the biomass in the northeast part of the Barents Sea was above the total Barents Sea mean (see Fig. 3.4.7). But from 2013 and ongoing, the mean biomass was reducing, and was record low (<20 kg/n.ml) in 2016, and below the total Barents Sea mean. This decrease could be explained by the maximum distribution of the snow crab predating on the benthos, and with increasing bottom temperatures (chapter 3.1). But in 2017 the biomass increased to 116 kg/nml, the highest value recorded both with and without snow crabbiomass.
Benthic habitat integrity and benthos vulnerability
With retreating sea ice, new areas in the northern Barents Sea become available for fisheries, including bottom trawlers. Of special interest to WGIBAR is therefore the vulnerability analysis. Current knowledge on the response of benthic communities to the impact of trawling is still rudimentary. The benthos data from the ecosystem survey in 2011 has been used to assess the vulnerability of benthic species to trawling, based on the risk of being caught or damaged by a bottom trawl (see WGIBAR report 2016).
Causes of polar cod fluctuations
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.
Environmental impact of fisheries
The impact of fisheries on the ecosystem is summarized in the chapter on Ecosystem considerations in the AFWG report (ICES 2016c), and some of the points are: