Food for thought. What do deep-sea fish have for dinner?

Are the dinners of the mid-Atlantic Ridge especially tasty? By studying the diet of the roundnose grenadier, we can learn more about the foodwebs and ecology of the mid-Atlantic Ridge, writes Guro Gjelsvik – our “student of the month”.
Guro GjelsvikStudent of the month
Guro Gjelsvik
Master of Science-student at department of Biology at the University of Bergen

Dietary study of Coryphaenoides rupestris

The roundnose grenadier, Coryphaenoides rupestris, one of several species of fishes known as“rattails”, is one of the most common near-bottomfishes on the northern mid-Atlantic Ridge. The grenadier is quite abundant and therefore also the target of  a commercial deep-sea fishery. We wish to know more about  the diet of this species. That way, we can learn more about the foodwebs and ecology of the mid-Atlantic Ridge.

Uses fish samples from the 2004-expedition

Roundnose grenadier
The roundnose grenadier, Coryphaenoides rupestris. Photo: Guro Gjelsvik 
Fish samples collected and frozen during the 2004 RV G.O. Sars expedition are used for the diets studies. In the laboratory, all the specimens are processed in the same way: The weight and length are measured. The liver is also weighed, as this can provide  information about the nutritional condition of the fish. Sex and reproductive stage (child, teenager, or adult!) is determined by looking at the ovaries and testes. Then I examine the stomach contents.

Grenadier stomach
Grenadier stomach. Photo: Guro Gjelsvik 
The stomach is emptied into a glass dish. I then try to identify what the fish has eaten, when possible to species level. Sometimes the prey animals are easily recognised, other times it is just mush, depending on the level of digestion. The grenadiers studied so far have eaten mostly crustaceans, squid and octopods. Squids  have parrot-like beaks, these cannot be easily digested, so it is possible to determine what species they belonged to.

The age of fish can be determined  using otoliths (ear bones). The otoliths grow by  laying down seasonal growth rings, so to age each fish it is just a matter of counting the rings – just like the rings in a tree trunk!

When all this is done, I will compare my results with similar studies from other areas in the North Atlantic. Then we will know whether dinners of the mid-Atlantic Ridge is especially tasty, or simply the same as anywhere else!