Around seamounts large fishes appear to be more abundant than elsewhere – one reason why fisheries tend to focus on these features. Currently it is poorly understood what mechanisms provide the food to these seamount-residents as the open ocean is a comparably food-poor environment.
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| These hatchetfish are about 2 cm long and migrate around 600m vertically every evening and every morning. On a human scale this is like walking roughly 50 km to get food, and then 50 km back to get home. (Photo: Oddgeir Alvheim) |
Phytoplankton are tiny plant-like creatures that dwell in sunlit surface waters across the world's oceans; small jellies and crustaceans graze on them and are in turn eaten by slightly bigger animals, for example inch-long hatchetfish or small squid. In fact, the risk of predation is so large near the surface that many midwater animals hide in the twilight zone many hundreds of meters below the waves during the day, and only come up to feed at night under the cover of darkness. As soon as the sun comes up they disappear into the inky deep again. This daily vertical migration is one of the defining characteristics of midwater life, and might be the largest animal migration on earth both in numbers and in terms of biomass.
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| As the sun comes up, midwater animals migrate into deep water. Green indicates dense aggregations of animals, white indicates their absence. |
Hiding in the deep darkness might seem a good idea when predators lurk at the surface. Around seamounts, however, the downward migration comes with added danger. Having drifted over the summit of a seamount, the midwater animals become trapped on the mountain top when they descend around sunrise. Seamount resident predators like rockfish or alfonsino are only too happy to turn these trapped animals into breakfast.
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| Echogram of an unnamed seamount (brown/grey) protruding into a layer of midwater animals (blue) |
Exactly how much biomass is transferred from the midwater into the stomachs of seamount fish is unknown, and in my PhD research I am trying to gain a better understanding of this process. Earlier in the blog you heard about how Lily is using sound to map the seafloor (Nov. 16th), and essentially I am using the same principle to map the distribution of midwater animals around seamounts. The difference is that the echoes from fish and invertebrates are very faint and thus much more difficult to detect. Also, inferring the type and size of animal from the echo trace is not always possible, as not all animals are equally good reflectors of sound. For example, the gas-filled swimbladder of a small fish might return a much stronger echo than the watery body of a large squid or even a jellyfish (see Clare's blog on Nov. 14th). Nonetheless, even without knowing exactly which species I am observing, I have already learned a lot about the interactions between midwater animals and seamounts.