Otolith Aging and Chemical Analysis

The Brooks Lab has a team of scientists researching and analyzing the slow-growing, deep-dwelling, apex predator fish of the Southern Ocean: the Antarctic toothfish. We are doing this primarily by analyzing otoliths (ear bones) from toothfish caught in the Ross Sea from the late 1970’s to today. There are two parts to this research:
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1) We are processing and aging thousands of otoliths to learn more about life history and population dynamics of these fish caught in the Ross Sea.
 
2) We are analyzing the otoliths for trace elements Mg-25, Sr-88, and Ba-138 (and others) combined with stable isotopes δC-13 and δO-18 to better understand toothfish migration pathways, connectivity, and change. 

Our goal is to better understand how toothfish move throughout their life and how connected populations in the Ross Sea are to other populations across the Antarctic. Additionally, we are examining how the establishment of the 2016 Ross Sea Marine Protected Area (MPA) under the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) is aiding toothfish conservation. Since the toothfish fishery operates adjacent to the MPA borders, we will discern if and how this MPA influences toothfish populations in the Ross Sea, including trying to understand if the MPA is having a spillover effect.
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Aging fish in the Brooks lab
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Otolith processing for age and growth (top). Annual growth zones of Antarctic toothfish in sectioned otolith (bottom).

What is an otolith? 

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An otolith is a fish ear bone comprised of proteins and calcium carbonate that are laid down at various rates in a process that leaves alternating opaque and translucent bands, similar to the growth rings in a tree. We can determine the age of the fish by counting the rings in the otoliths. This is important for the stock assessment process for Antarctic toothfish. See more at CCAMLR.  

Processing and aging otoliths

Document and Weigh Otolith

1) First, we organize and document data from the otoliths that we received from various national fishing associations. Commercial toothfish fishing vessels are required to have observers onboard to collect scientific data, including otoliths from toothfish caught. We have otoliths from 1970s (caught by scientists before commercial fishing began) to present day. 

2) We take one of the two otoliths from each fish, randomize between right and left,  and weigh it.

Grind Otolith

3) Processing continues by baking the otoliths for approximately 4 minutes in order to better expose the translucent and opaque growth zones. 

4) Next, we grind the otolith down to the nucleus, affix it to a microscope slide and then grind the other side down to the nucleus. This allows for us to view the growth annuli (rings) via a stereomicroscope. 
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Antarctic toothfish caught by New Zealand in Subarea 88.1 in 2007. Age 17 (error +/- 2). Method: bake and embed (different from our method).

Age Otolith

5) We age the otoliths by counting growth annuli through a microscope. To learn more how this is done, check out the CCAMLR otolith library. We're also investigating if this process can be automated through machine learning.  

In-depth methods can be read here. Please contact us if you want to discuss methods! 
You can reach us or find more information through our email or social media pages below. ​