LAB - Sample processing

Otoliths (ear bones) are made of calcium carbonate and protein laid down at different rates throughout a fish's life.

(Go to the ON BOARD page to discover more about otolith extraction)

Counting growth lines in otoliths is one of the most common methods applied for age estimation. The interpretation can be performed on whole or sectioned otoliths.

I would genuinely be curious to know the scientist who discovered this. Chapeau.

Once age is estimated, it can be used for different purposes: age composition of a population, age at first maturity, draw the growth curve (length-age), and many more.

1) The gonad (or part of it) is prepared to be frozen in a paraffin block. This very sophisticated machine does the job during the night so that the blocks are ready for the oven in the morning.

2) After a its stay in the oven and later on in the freezer, here it is: a wonderful block with a piece of a gonad of a blackspot seabream caught during the Azorean bottom longline survey in 2019.

3) The steel blade of the microtome cuts the block into sections that, after proper treatment, are gently set on the microscopic slide. After that, the slide is sent back to the oven.

4) After a brief xilol bath, slides are ready for staining. We used a well-known system: H&E staining. Hematoxylin is alkaline and stain the nucleus in blue, while eosin, which is acidic, makes the extracellular matrix and the cytoplasm pink.

5) Just few drops of DPX mounting medium and the cover glass can be set on top of the sample.

6) Almost ready, these microscopic slides are peacefully resting to get dry. Many hours under a microscope will follow for sure!