Interactions, drivers and pressures 2016

Sorting trawl catch. Photo: Norwegian Polar Institute

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 (Jørgensen et al., 2015). Current knowledge of 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 (WGIBAR report 2016).

In most of the measured years, the benthos biomass in the northeast part of the Barents Sea (NE) was above the total Barents Sea mean (Figure 4.6.1). But from 2013 and ongoing, the mean biomass (see also black arrows in Figure 3.4.3 lower panel) was reducing, and was record low (<20 kg/n.ml) in 2016, and below the total Barents Sea mean.

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.

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 substratum, depth, gear, and type of organism. Seabed characteristics from the Barents Sea are only scarcely known 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.

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 (Figure 4.2.3).

Cod is the main predator on capelin, although other fish species as well as seabirds and marine mammals are also important predators. In the last 6-7 years there has been an extremely high cod stock level in the Barents Sea. Estimated biomass of preyed capelin by cod in recent years has been equivalent to the biomass of the entire capelin stock (Figure 4.2.3). Under good conditions the capelin stock tolerated a high grazing pressure; the biomass produced during the year was equivalent to the standing stock biomass measured in autumn. The abundance of predators other than cod is also at high and, to our knowledge, stable levels.

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:

The Barents Sea polar cod stock is in 2016 at an intermediate level. Norway conducted commercial fisheries for polar cod during the 1970s, and 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. The stock size declined from 2010 to a very low level in 2015.

Polar cod (Boreogadus saida). Photo: Institute of Marine Research, Norway

Ten years (2006-2015) of capelin diet was examined from the Barents Sea where capelin is a key forage species, especially of cod (Gadus morhua). The PINRO/IMR mesozooplankton distribution shows low plankton biomass in the central Barents Sea, most likely due to predation pressure from capelin. In the Barents Sea, a pronounced shift in the diet from smaller (<14 cm) to larger capelin (≥14 cm) is observed.