Power of the plankton pt.1

Anton and a shorthorn sculpin (true size 14 mm) from a Bongo haul, magnified under the sorting scope and shown enlarged on-screen. Photo: Jan Dierking

Today scientific crew member Anton is going to introduce you to the importance and fascination of larval fishes in the Baltic Sea. Here is his blog entry:

Most of the fish species in the Baltic Sea spawn in spring. This is the time when life starts to bloom not only on land but also in the sea. Increasing zooplankton due to algae blooms build a food foundation for freshly hatched fish larvae. On our cruise we catch the larvae with a bongo net which filters the plankton out of the water column, and conserve them for later analyses in the laboratory.

Now in April, in Bornholm Basin in the central Baltic Sea, the most likely larvae to catch are those of flatfish and sprat, but there are also other fishes like shorthorn sculpin or longspined bullhead. If we are lucky we can even catch a few very early cod larvae which normally occur in the summer months. These can be recognized immediately by two black stripes on the body.

Picture of a 6 mm long cod larva caught in one of our Bongo hauls, taken under the sorting scope. Photo: Anton Höper

Larvae are tiny, many of them just a few mm long, but a look under the microscope reveals their beauty and fascinating characteristics. Some larvae look also surprisingly different from what you would imagine. Everybody knows the tasty flounder which has the unique asymmetrical and flat shape of all flatfishes. One would expect their larvae to look about the same but they have a shape which is quite similar to “normal“ looking fish. Their body gradually flattens when they grow older. The second picture shows a few flatfish larvae which still have the common fish shape.

The examination of the larvae is an important part of the long-term data series because it shows the success or failure of the new cohort and it allows approximate conclusions about how many young fish will join the adult fish populations in subsequent years (“stock recruitment”). Based on conserved samples, we can also analyze the condition, feeding ecology, genotype and other larval characteristics. In the end, understanding the ecology of these early life stages is essential to predict how adult fish stocks will fare in the future.

Flounder larvae (ca. 5 mm long) from the Bongo hauls. All flatfishes still have the typical bilaterally symmetrical body shape of “normal” fishes as small larvae, but then gradually become asymmetric as they grow. Photo: Anton Höper

3 thoughts on “Power of the plankton pt.1

  1. Hi Jan, Thank you and your team members for the impressions. Very interesting! Have a nice trip and stay safe and healthy. See you soon. Best wishes, Sandra 🙂

    • Hi Sandra, thanks a lot for your feedback and the good wishes! Greetings from on board Alkor, Jan

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