Ecological role of the Arctic’s most abundant cephalopod, Gonatus fabricii

Research notes

Recent studies provide new insights into the ecology of the squid Gonatus fabricii in the Arctic, revealing its ecological role to be more important than previously believed, and substantially clarifying its distribution patterns, reproductive biology and trophic ecology.

1.

Gonatus fabricii (Cephalopoda) is a common, widespread squid species which – despite its English name (boreoatlantic armhook squid) – has most of its range in the Arctic. It is the most abundant species among Arctic cephalopods, and its biomass reaches 8 million tonnes in the Nordic Seas. It is also the only squid amongst the 10 species of cephalopods that live permanently in the Arctic. It is therefore an important link in the polar ecosystem as both prey and predator. However, its trophic ecology, reproductive biology, and biomass distribution in the Barents Sea were mostly unknown until recently, when our studies filled those gaps. We also demonstrated that G. fabricii is much more important in the Arctic ecosystems than previously believed.

Despite the wide range of G. fabricii, our findings confirmed that this squid spawns within a geographically restricted area (as initially hypothesised by Herman Bjørke in 1995). Moreover, we found evidence for one new breeding area southeast of Greenland. Such geographically localised reproduction is relatively common in shallow-water squids, but is much less known in deep-water squids. A localised reproduction linked to an increased food availability in the top layers of the open ocean may be especially important for G. fabricii because it would likely increase the survival of juveniles, while the surface currents might aid in their dispersal. Interestingly, no differences in sizes at maturity were found between the breeding areas, even between those located in the low sub-Arctic and in the Arctic.

Gonatus fabricii apparently does not breed in the Barents Sea, as the breeding areas are always located in the deep-sea parts of the Arctic and northern North Atlantic Oceans.

Previously G. fabricii only inhabited the western part of the Barents Sea, reaching ~40°Е, during the non-breeding stage of its life cycle. However, due to climate warming, G. fabricii has expanded since 2004 to inhabit the entire Barents Sea and the western part of the Kara Sea. Still, only immature and early-maturing specimens are found in the Barents Sea. The maximum biomass of G. fabricii in the Barents Sea was nearly 25 000 tonnes with an abundance of 1.7 billion specimens in 2011. The areas where biomass density exceeded 100 kg/km2 and abundance exceeded 10 000 specimens/km2 were concentrated in deep-sea troughs in the marginal parts of the Barents Sea and in adjacent areas with depths over 500 m. But this spatial concentration of G. fabricii could not be correlated with the climatic state of the Barents Sea in 2009–2012.

Gonatus fabricii descend from surface water layers to the deep sea during ontogenesis, i.e. as the individual grows and matures. Thus, small juvenile squid live near the surface, and large adult squid dwell in the deep sea. During this descent, females’ tissue becomes jelly-like; they lose their tentacles and most of their ability to move around, and they cease feeding. We found that the gonads of female G. fabricii contained between 8 862 and 16 200 oocytes – precursor cells from which eggs develop. Younger females had more egg precursors, as up to 23.5% of all oocytes are resorbed into the body during the later phases of ontogenesis.

2.

As mentioned earlier, the larger a squid is, the deeper it lives. Gonatus fabricii changes its diet from crustaceans to fish as it grows. Our stable isotope analysis showed that G. fabricii transitions 2.6 trophic levels higher in the food web through its life cycle, from surface-dwelling juvenile forms to large deep-dwelling adults.

The trophic level of G. fabricii was assessed using the most abundant herbivorous copepods as a baseline, Calanus finmarchicus in Greenland and C. glacialis in the Barents Sea. The trophic levels of squid range between 2.5 and 5.1. Thus, large adult specimens living in the deep sea are comparable to the Arctic’s top vertebrate predators, such as seals, toothed whales, and large fish-eating or benthic scavenging fishes.

This means that G. fabricii is a top invertebrate predator in the Arctic. At the same time, G. fabricii is also important prey: small juveniles living near the surface are preyed upon by many fishes and seabirds; large adults living at depth are preyed upon by toothed whales and large specimens of seals that are able to dive deep enough. Overall, 46 species of predators are known to feed on G. fabricii, although most feed on the small stages.

To understand the marine food web, it is important to estimate the stages at which squid are eaten by predators, and their total biomass. Fortunately, squid beaks remain intact for long periods in predators’ stomachs, from which they can be collected, analysed, and measured. That means that equations can be used to estimate the size and biomass of G. fabricii based on its beak size, for all the Arctic, and specifically for West and East Greenland, and the Barents Sea.