Meet the ghost of the dark and deep

The rabbit fish, a mysterious-looking cartilaginous fish, is one of many species expected to shift their distribution poleward in response to climate change. Despite its wide distribution in the Northeast Atlantic Ocean, it remains poorly studied; its habitat use, distribution, and biology are largely unknown.

By: Romaric Jac and Claudia Junge //Institute of Marine Research

The rabbit fish got its nickname from its oversized head with large eyes, nostrils, and tooth plates, which result in a rabbit-like appearance. Its scientific name (Chimaera monstrosa¸Linnaeus, 1758) combines the name of a mythological creature composed of parts of multiple animals, and the Latin word for “strange” or “grotesque”. Both words refer to the rabbit fish’s odd mix of characteristics from both bony and cartilaginous fishes and its strange appearance. It belongs to the only living order within the phylogenetic subclass Holocephali (complete heads) namely the chimaeriformes, also known as ghost sharks, rat fish, or rabbit fish.

A cold and dark way of life

The rabbit fish has been found down to 1420 m in Nordic waters, but is most abundant at depths between 200 and 400 m. In Norwegian fjords, it can be found in waters as shallow as 50 m, which is highly unusual compared to other parts of its range. It appears to prefer water temperatures around 6°C, although they can also be found in waters as cold as 2.7°C and as warm as 8.6°C, at least along the Norwegian coast.

Its life history characteristics are typical for cartilaginous fishes: slow growth, late maturation, and a relatively low fecundity. The largest individual found in Norwegian surveys was a female that measured 67 cm from the tip of the snout to the start of the fin that runs along the tail. The total length including the tail has been reported to be up to 1.5 m, but this can be an underestimate given that the exceedingly long tail of a rabbit fish is often injured due to predation or catch events. Although the species is not fished, it is a very common bycatch.

Heading north

During the past three decades, the coastal near-bottom water temperature has increased by an average of approximately 1°C in the Norwegian and the Barents Sea ecosystem. These changes in temperature are even more pronounced in the deep waters of the coastal regions and fjords.

Such warming of northern waters seems the main driver of a northward shift in the abundance of the rabbit fish. The analysis of survey time series data back to the 1990s showed that the rabbit fish has adjusted its distribution northward by more than 73 km per decade and has now been found as far north as 73.44°N. This northward expansion of a boreal species and potential habitat deterioration for Arctic species are consequences of the ongoing Atlantification.

Summary and outlook

Climate change is likely a major driver of large-scale abundance patterns, and changes thereof, even among deep-sea species. Therefore, understanding how climate change and warming waters impact the distribution and population dynamics of species at the northern fringe of their ranges, is much needed and critical knowledge to inform species and ecosystem management. This is especially true for deep-sea cartilaginous fishes which remain relatively understudied compared to both their bony fish counterparts and to cartilaginous fish living at shallower depths.

With forecasts indicating even warmer waters in the years ahead, monitoring and managing the rabbit fish, and other fringe species, in the vast Arctic is a real challenge for the future. These changes present an emerging frontier for the adoption of new and integrative tools for studying rapid change and its consequences for vulnerable species, by combining genetics, spatial ecology, and oceanography, to identify patterns of dispersal, adaptation potential, and gene flow under extreme environmental conditions. In a global warming context, survey data, long-term data series, and international cooperation are required to record such extended shifts and better conserve transboundary species.