Distribution and biology of the colossal squid, Mesonychoteuthis hamiltoni: New data from depredation in toothfish fisheries and sperm whale stomach contents

The colossal squid Mesonychoteuthis hamiltoni (Robson, 1925) is the largest living invertebrate, and one of the largest cephalopods ever to live on the planet (Vermeij, 2016). It attains dorsal mantle lengths of ∼250 cm, and a maximum recorded total weight of 495 kg (Jereb and Roper, 2010). It is an important element of the Antarctic ecosystem, occurring in waters between the Sub-Antarctic Front and Antarctic continent (Rosa et al., 2017). In this region, M. hamiltoni has been reported mixed with Antarctic toothfish in the stomach contents of cetaceans, suggesting that these two prey species co-occur in the same habitat (Yukhov, 2013) and represent the dominant toppredators across the 1000–2000 m depth range. This co-occurrence leads to intensive trophic interactions between these two species: the colossal squid and toothfish occasionally prey on each other as adults. Antarctic toothfish likely attack dead and dying colossal squid, whereas the squid prey on toothfish captured by commercial longlines (as well as possibly on weakened and dying individuals; Remeslo et al., 2015). Sperm whales in Antarctic waters appear to be the only major predators of healthy, full-grown M. hamiltoni, consuming around 9 million tons of this squid per year (allowing tentative stock estimates of 45–60 million tons; Nesis, 1985). Because of their potential commercial importance, data on M. hamiltoni distribution and biology (feeding, reproduction) were collected through intensive studies during the whaling era, as its potential as a food and/or pharmaceutical resource was considered. A range of Ukrainian studies, dedicated to exploring the commercial potential of both giant and colossal squids, were partially published in grey literature (Yukhov, 2012, 2013, 2014). Fishing gear targeting colossal squid was also developed but never deployed in practice (Yukhov, 2013).

Captures of juvenile colossal squids, which inhabit surface waters down to 500 m, are reasonably common (see review in Rosa et al., 2017), whereas adults are generally too large to be caught by trawl. Only one known report of a large trawl-caught M. hamiltoni individual exists, in 1981 by RV "Evrika" (Fig. 1 – Remeslo et al., 2015); most information about adults' distribution and biology has instead been inferred from sperm whale stomach contents. The cessation of commercial whaling and inability to collect any meaningful information about colossal squid from any of the modern routinely deployed scientific and fishing gears have considerably hindered the progress of knowledge about this species. However, complex interactions between M. hamiltoni and the Antarctic toothfish can provide some further insight into the squid's distribution and abundance, both from its occurrence in toothfish stomachs, and from the frequency of colossal squid attacks on captured toothfish.

To expand our knowledge of the colossal squid's biology, we analysed diverse historical data collected during the commercial whaling years (1967–1974) and published in Ukrainian grey literature (Yukhov, 2012, 2013, 2014) together with newer information collated by CCAMLR observers in 2012–2017. In addition, the reproductive systems of two large females captured by a New Zealand longliner in 2007 and 2013 were opportunistically examined, providing some insight into the species' fecundity and reproduction.