By Rebecca Hunter, University of Glasgow
Today was the final trawl at the NW station and the third day of continuous fish processing. It’s interesting to see the difference in the catch now we have moved north; most striking is the huge abundance of the grenadier Coryphaenoides brevibarbis. So after collecting hundreds of these fish in the past 48 hours, it seemed appropriate to dedicate my blog entry to them.
Coryphaenoides brevibarbis, rat-tail.
This species is found in the Atlantic Ocean at depths of 1700 – 4300 m and they are quite small compared with most other rattails. They are demersal, which means they generally live just off the bottom, and have a diet based on crustaceans. We dissect out the stomachs which can then be opened and the contents examined at a later date by poor, unsuspecting honours students. Knowing what the fish eat can give us clues about their lifestyle. For instance, a belly full of invertebrates that live in the mud indicates a bottom dwelling habit, or a diet change between juvenile and adult stages could mean a habitat shift with age. Fish also differ in their food selection; some eat anything they can find, where as others are more fussy.
We also remove the otoliths from the head. These are tiny bones in the ear that act to maintain balance and position. When these structures are cut open rings can be seen, like the rings in a tree trunk, which can be counted to determine the age of the fish. To do this, the otolith must be embedded and cut into very thin sections (less than 1 mm). These sections are then viewed under a microscope where patterns of light and dark rings are visible. There have been numerous studies on shallow-water fish to show that these rings represent fast growth periods (light areas) and slow growth periods (dark areas) due to the seasons of the year. Therefore, one light area and one dark area are counted as one year in the life of the fish. Although fish living in the deep ocean are not directly affected by the seasons, scientists believe there are delayed seasonal patterns in these regions influenced by the waters above.
Removing the otoliths stage 1...
...removing the otoliths, stage 2.
A section from a C. brevibarbis otolith, age 11.
I looked at the otoliths from this species last year and found an age range on the Mid-Atlantic Ridge from 1 – 13 years. Again, unlike many other rattails, this is quite low. The abyssal grenadier was aged up to 40 years and previous studies on the roundnose grenadier had a maximum age of 72! Knowing the age of a fish can be used to calculate growth rates as well and this can give an indication of the amount of energy being supplied to the area.
Energy supply for scientists.
Fish gonads.
I have also been taking gonad samples to look at the reproductive biology on my return to dry land. We haven’t come across any mature females with eggs yet but with another trawl planned at the NE station there are still hours of ‘brevi’ processing to come. It’s funny to see the different ways people employ to keep themselves going on these long days. A constant supply of fizzy drinks and chocolate hobnobs is not always enough, sometimes we need to break out the group huddle to revive ourselves.
Science.............
....HO!!!!!
