Both commercial but
also non-target fish are heavily impacted by fishing, the dominant activity in
the ecoregion. Due to more stringent fisheries management in past decades
resulting in a reduction in fishing mortality rates, natural mortality is now
back to being the main source of mortality for many commercial species. The
mean relative spawning-stock biomass (SSB) has increased since 2000 and is now
above the reference point that can provide the maximum
sustainable yield (i.e.the SSB to BMSY trigger ratio > 1) for most
stocks in the ecoregion (Figure 16). Among those with a high ratio are hake (Merluccius merluccius) and plaice
(Pleuronectes platessa), while four
stocks (North Sea cod, saithe, witch [Glyptocephalus
cynoglossus], and whiting [Merlangius merlangus]) have an SSB below the
sustainable threshold and a fishing mortality that exceeds it. More detailed
information is provided in ICES Greater
North Sea ecoregion Fisheries Overview.
The commercial fish species are connected through predator-prey relationships. In the ecoregion the main forage fish (herring, sandeel, sprat, and Norway pout) feed predominantly on plankton and are an important food source in the North Sea foodweb. Smaller piscivorous fish (e.g., whiting, haddock [
Melanogrammus aeglefinus], and grey gurnard [
Eutrigla gurnardus]) and stocks that enter the ecoregion only in specific seasons such as western horse mackerel (
Trachurus trachurus) and mackerel, all eat forage fish and juvenile gadoids. Benthic-feeding fish include various flatfish species (e.g. plaice and sole) feeding on prey at or near the bottom. Top predators that eat large fish (> 25 cm) are mainly fish like large cod, saithe, and some shark species but also include marine mammals like seals and harbour porpoise. The depletion of larger predatory species in the ecoregion has likely perturbed the structure and function of the ecosystem by reducing predator top-down controls on certain lower tropic level species.
The Greater North Sea is an important migratory corridor for Atlantic salmon. Juvenile smolts migrate from rivers to northern oceanic feeding grounds, and the adults migrate back to natal rivers throughout almost all of the ecoregion. Knowledge of the role of the species in the ecosystem is limited. The North-East Atlantic Commission (NEAC) area has seen a general reduction in catches in both fresh and marine waters since the 1980s, which reflects a decline in fishing effort as a consequence of management measures as well as a reduction in the size of stocks. Environmental conditions in both freshwater and marine environments have a marked effect on the status of salmon stocks. In the marine environment, return rates of adult salmon have declined since the 1980s and, for some stocks, are now at their lowest levels in the time-series, even after the closure of marine fisheries. Climatic factors modifying ecosystem conditions and the impact of salmon predators at sea are considered to be the main contributing factors to lower productivity, which is expressed almost entirely in terms of lower return rates.
