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How old are abyssal fish?

Living below 2000 meters is far from easy.  Animals have to be adapted to survive long periods of time with little or no food, relying on the sporadic and desultory fall of food from above.  There is no natural light at these depths so they must also be successful at detecting large food falls and finding mates.  It is generally thought that these conditions cause fish living in the abyssal plains to have slower growth rates and to live for longer than their shallow water relatives.  However, comparatively little is currently known about the ages of deep-ocean fish.

Analyzing otoliths 1
Analyzing otoliths

To try and address this problem, an age and growth workshop was held in January 2009 at the Institute of Marine Research, Flødevigen, Norway.  Dr. Nicola King from Oceanlab, University of Aberdeen, joined Rebecca Hunter, a Master’s student from the University of Glasgow and Dr. Odd Aksel Bergstad (IMR, Flødevigen) to investigate the age and growth of the abyssal grenadier Coryphaenoides armatus.   

This species comes from the family Macrouridae, better known as the rat-tails due to their characteristic long, tapering tail.  There is a commercial interest in a few species from this family, mainly for the roundnose grenadier (Coryphaenoides rupestris) which has been fished since the 1970’s.  Although the abyssal grenadier is not commercially exploited they are an invaluable species for research by allowing insights into the ecosystem of the abyssal realm.

The abyssal grenadier is a cosmopolitan species, being one of the most abundant fish found below 2000 meters worldwide.  They can grow over 100 cm in total length, with females generally reaching a greater size than males; a trend clearly seen in many other species of this family.  Whether there is a difference in maximum age or growth rate between the sexes is another area to be examined in this study, along with general observations on the age range of the species as a whole.

Analyzing otoliths 2
Counting the patterns of light and dark ‘rings’ found in the otholiths

By extracting a bone from the inner ear of the fish referred to as the otolith, rings can be counted, somewhat like those seen in tree trunks, to estimate the age of the individual.  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’ can be seen.  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 abyssal depths of the ocean are not directly affected by the seasons, scientists believe there are delayed seasonal patterns in these regions, influenced by the waters above.

In Flødevigen, discussions were held after all the otoliths had been looked at once to determine a common practice of what to count as 1 year.  Once this had been agreed, Nicola and Rebecca read all the slides twice and a selected number were read by Dr. Bergstad.  This will allow for comparisons between results and should highlight if there is any imprecision or bias in the reading. 

This work is still in progress, but initial results have estimated a maximum age of no more than 30 years.  This is less than would be expected for such a deep living fish and suggests a faster rate of growth than has previously been attributed to the species.  Therefore, these results could indicate that more food, and therefore more energy, is reaching the abyssal plains than current scientific opinion allows. 

By Rebecca Hunter, University of Glasgow

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