Even fish don’t swim well when they’re young!

Surely, you think, fish must be born as expert swimmers. Actually, fish larvae are pretty feeble when it comes to an efficient mode of swimming called ‘burst-and-coast’. Dr Ulrike Müller from Wageningen University studies how fish swim: “Fish larvae are the most critical life history stage. In marine fish more than 99% of the mortality occurs in the larval stage so anything that might explain what holds them back is useful”. Müller will present her research on Monday 3rd April, at the Society for Experimental Biology’s Annual Main Meeting in Canterbury [session A5].

The ‘burst-and-coast’ method of swimming involves alternating stages: the fish power forward in a ‘burst’ and then hold their body straight to ‘coast’. Dr Müller and her colleagues might have found what it is that prevents larvae of the zebrafish from reaching top speeds: the larvae cannot maintain their body in a horizontal position while coasting. The research team think this is because larvae don’t have the help of an adult swimbladder and have only small, ineffective side-fins. This lack of control in early days means that the larvae suffer a lot of drag during the coasting phase and this could be something that affects other types of fish. “Many fish larvae hatch without fully formed pectoral fins and all hatch without a swimbladder, so similar problems could occur for them”, predicts Müller.

This conclusion is all the more profound because it refutes two previous theories of poor hatchling swimming, one focusing on the burst phase and the other on the coast.

The former of the two explanations suggests that because the larvae are not very heavy they may not gain much momentum in the burst phase to carry over into the coast phase, while the latter theory states that when the larvae are very small they experience the water in a different way; it appears thicker, like syrup and harder to swim in. “Momentum can explain some of the poor swimming in larvae, but not all, and the difference in coasting ability cannot be explained by differences in body length either”, Müller argues.

Hopefully Müller’s findings about pectoral fins and swimbladders can be used to reduce fish mortality rates. “Fish breeding programs cannot offer their larvae swimming lessons, but once we understand which behaviours cause the fish larvae the most trouble, then it is easier to adjust rearing schemes in aquaculture and to lend a hand to endangered species”, she says.