Can ballan wrasse keep up with salmon?

Image by Geir Friestad via Flickr

Ballan wrasse are crafty fish that eat sea lice off swimming salmon. While we know they are useful cleaner fish, we don’t know much about how well they can swim in different currents and at different temperatures. If we did, we could establish better deployment strategies and predict when their welfare may be at risk. Jeffrey Yuen, an honours student from the University of Melbourne, tested the standard and maximum metabolic rate, aerobic scope, and critical swimming speed of ballan wrasse acclimated at 5, 10, 15, 20, and 25°C, to see how performance differed. The wrasse were generally inactive and had low metabolic rates at lower temperatures. They also did not swim continuously between 5 and 20°C. Only at 25°C did they swim continuously, with an average critical swimming speed of 27 cm s-1. This is a fraction of the critical swimming speed of salmon. The much weaker swimming capacity of ballan wrasse means they won’t cope well in salmon farms with moderate to strong current speeds. Further, their low metabolic rates and inactivity at 5-10°C suggests that they won’t do their lice eating job well at these temperatures, limiting them to warmer places and times. Jeffrey’s results can be used by farmers to make sure they are stocking ballan wrasse in the right times and places in order to make sure cleaner fish are doing their jobs.

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