Counting eggs at sea

Thus we could do a plankton haul with the torpedo almost every hour. We were able to sort the problems with the starboard plankton winch, thus we are again at full power!

Why and how do we count eggs at sea. For some pelagic species we carry out acoustic surveys. Mackerel does not have a swim bladder and it therefore difficult to recognize on acoustics. Also egg surveys are the only fisheries independent stock assessment, since there is no fisheries on the fish eggs.

Mackerel spawns in May-June in the Northern and Central North Sea. We try to cover the whole spawning area tree times. At each station we deploy the plankton torpedo in an oblique haul to the bottom and back again. During the haul we have a live view of the torpedo in the water column. When the torpedo is back on deck the sample is brought to the lab on board. The plankton sample is fixed for 24 hours on formaldehyde.

After that the eggs are sorted from the remaining plankton using the ‘spray’-method. Seawater with fine air bubbles is sprayed into the samples. All plankton with legs sticks to the air bubbles and floats to the surface, while the round eggs sink and are thus easily separated.

We take a picture of all the fish eggs and than measure, count and identify the eggs to species. All eggs of the three periods together give us the production of mackerel eggs this spawning season. So far all samples, but one, contained fish eggs and most contained mackerel eggs.