Salmon jumping in a cage. Photographer: Eivind Senneset / Institute of Marine Research (Havforskningsinstituttet)

'Hoppenet' can contribute to better salmon welfare

NORWAY
Wednesday, September 11, 2024, 04:00 (GMT + 9)

Jump nets are small floating net cages designed to catch fish that voluntarily jump into them. They both increase welfare compared to traditional methods, and are also affordable and easy to place.   

Today's methods for catching fish from a cage are dependent on squeezing part of the fish together. It can expose fish to damage and unnecessary stress. The jumping net is a kind of mini-cage inside the large cage in which the fish are kept. By the fact that the fish themselves jump into the jumping net, the need for handling is reduced.  

"It is an advantage for fish welfare, we get the opportunity to handle the fish without stress and the risk of causing damage", says HI researcher Frode Oppedal.

Jump net mounted in a breeding pen where white floats hold the net in the surface and one wall is suspended on the railing. Photo: Amy Loebmann/The University of Melbourne

Can help monitor lice 

In the study, salmon were caught with jump nets compared to salmon that were caught with the solution that is most common today; orkast note. It is a type of net used to catch fish alive. 

The comparison showed that the skip net caught fish of the same length and weight as the orcasnet, but the salmon caught with the skip net had more lice.  

"This agrees with previous observations which show that salmon with many lice jump more than salmon with few or no lice. The jumping increases in particular when the louse first attaches itself to the fish and then when it goes from being fixed to being mobile", says Oppedal 

Since fish with many lice more often end up in the jump net, it can give a wrong picture of how many lice there actually are if the numbers and method are not used correctly. 

"The amount of lice can be better controlled if the breeders use mare nets. For example, they can count the fish with the most lice and move them directly to treatment", says Oppedal. 

The fish are more active in the evening and early morning 

The research has been carried out as part of a master's thesis. 

To evaluate how the size of the jump net affects the catch rate, student Amy Loebmann (University of Melbourne) observes salmon that jumped into floating surface rings of different sizes without a net. These were supposed to imitate jumping nets in various sizes. 

"We recorded when fish jumped during short observation periods in order to analyze how the jump speeds varied over different time intervals - hours, days and weeks", says Oppedal.

This is what it looked like inside the cage when student Amy Loebmann tested the size of the jump net's opening in relation to the number of salmon that jump in. Photo: Frode Oppedal/Marine Research Institute

The results show that skip nets placed in the middle of the cage catch more fish than those placed at the edges. Larger jump nets also give better catches, and the fish jump most in the evening and early morning. 

"With the right placement and timing, larger jump nets can utilize the fish's natural behavior pattern and give a significantly higher catch when the fish are at their most active", says Oppedal. 

Reference

Loebmann, A., Barrett, L., Oppedal, F., Dempster, T., 2024. A passive fish sampling method is representative for size but selective for parasite load. Aquaculture 593, 741295.

Author/Source: Pauline Paolantonacci/Institute of Marine Research (Havforskningsinstituttet)

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