Watermark

Morning! How's life in land? Although in a second thought, we prefer not to know... We keep our course steady and we are looking forward to start working. We have had time to rest from the busy days prior to departure, seasickness is finally vanishing and we are ready. But we still have this stubborn easterly wind, delaying us nearly since we left, although its intensity varies widely. Our position is 43º18.4500 – 037º23.6700.

As usual we have a few ships on sight in the radar, six today, but they are at 20-50 nm and we cannot see them. With Spanish name there is the Navíos Antares and yesterday we saw another called Valencia Express. I think the biggest so far was a 290 m tanker we detected in the radar a couple of days ago. We also see everyday a few whales blowing here and there, but the distance and  waves do not give the chance to identify them. 

But we promised we would tell you about otoliths, and that's what we intend to do. Otoliths are calcified structures found in the fish ear (rays and sharks also have otoliths but they are cartilaginous and cannot be used for age reading), comparable to the small bones in our inner ear. Otoliths are surrounded by sensorial tissue and allow the fish to detect sound, acceleration and gravity. They are three pairs, but we only use the largest of them, called sagittae.  On board we extract, clean and store them in small paper bags where we record the species, survey, haul, and sex and length of the fish. In Platuxa we usually collect otoliths from cod, American plaice and Greenland halibut, but this year we will extract only cod and Greenland halibut otoliths, although sampling effort on these species will remain. Last year, 2000 pairs of otoliths were collected in Platuxa, nearly half of them from cod. 

Greenland halibut otoliths

Otolith analysis must necessarily wait to reach land. Apart from a very well trained eye to read them, otoliths must be prepared to be analysed under the microscope. Shape and size of sagittae otoliths varies a lot among species and also the difficulty to read them. As otoliths are formed, translucent (hyaline) and opaque layers are deposited in alternation, generally in a cycle lasting a year. Thus, as it is done with rings in trees, this alternance of hyaline and opaque bands can be used to estimate fish age. Some otoliths can be read simply cutting them in two and charring them a bit. However, most often otoliths are mounted in resin, their surface polished and cut into very thin layers before analysing them under the microscope to count the number of hyaline or opaque bands. It sounds easy, but it is not. Actually labs around the world meet every now and then to discuss their methods and homogeneize criteria.

When our sample of several hundreds of say, cod otoliths, has been analyised, the result is what we call an ALK, or Age – Length – Key: a table with size in cm in the rows and age in years in the columns, where we register the number of cod of each age at each size. Here we show only part of a key:

 

	Edad / Age						Edad / Age
Talla / Length	0	1	2	3	Total	Talla / Length	0	1	2	3	Total
8					0	8
9					0	9
10					0	10
11					0	11
12					0	12
13		1			1	13		1,0	0,0	0,0	1,0
14		1			1	14		1,0	0,0	0,0	1,0
15		1			1	15		1,0	0,0	0,0	1,0
16		6			6	16		1,0	0,0	0,0	1,0
17		4			4	17		1,0	0,0	0,0	1,0
18		5			5	18		1,0	0,0	0,0	1,0
19		6			6	19		1,0	0,0	0,0	1,0
20		3	2		5	20		0,6	0,4	0,0	1,0
21		3	2		5	21		0,6	0,4	0,0	1,0
22		1	5		6	22		0,2	0,8	0,0	1,0
23		1	5		6	23		0,2	0,8	0,0	1,0
24			9		9	24		0,0	1,0	0,0	1,0
25			4	1	5	25		0,0	0,8	0,2	1,0
26			5	2	7	26		0,0	0,7	0,3	1,0

The table on the left is transformed into proportions (see table on the right) before we apply it to our length distribution data to find out how much fish of each age we have in our samples. This information is highly relevant, because reproductive potential increases with age and knowing the demographic structure of the population tells us about its health, so to speak. In our example we see that 60 % of fish measuring 20 cm is one year old, and the remaining 40%, two years old. And so on.

We also know that molecular composition of otoliths depends of the water mass where the fish has grown. The oceans are made up of many different water masses, with diverse salinities, temperatures and ionic concentrations. Luckily for life on Earth, these water masses maintain their respective currents -their “travelling routes”- without mixing. This does not mean, however, that a water mass does never change. We are not going to dwell very much into this, but there is a very close interaction between the atmosphere and the oceans. Oceans are very good at trapping carbon dioxide (one of the gases which presence has increased significantly since the Industrial Revolution), but at the expense of alterations in the carbonates chemical cycle. A direct consequence is the decreased availability of carbonate ions to those animals that need it to build exoskeletons and other calcium carbonate structures. These changes can be detected in otoliths, thus transformed in highly valuable tools to obtain information on physical conditions in the oceans and climate change  in decades past. There are important otolith collections in research institutes worldwide, IEO included, counting tens of thousands of samples from many species and fishing grounds, waiting for the necessary economic resources to unleash their research potential.

This is not everything about otoliths, not at all, but hopefully you are now smitten with enough curiosity as to learn more about this fascinating research field on your own.

 See you tomorrow...