Use of bubble flotation to improve copepod fisheries: laboratory studies on the physical and behavioural interactions of Calanus finmarchicus and air bubbles

Abstract

The current study presents a novel approach to zooplankton harvest in the ocean, where copepods are lifted through the water column and concentrate them at the ocean surface. There they are harvested with a surface skimmer or shallow trawl. The method can potentially reduce fuel costs and unwanted by-catch compared to a conventional plankton trawl.

The optimal bubble size for attachment to Calanus finmarchicus was determined to 125-225 µm. Attachment was found on 331 of 604 studied copepods (55%), and the majority (88%) had an attached air volume equal to a 50-300 µm bubble. The attachment ratio has been estimated to 20% for 275-400 µm bubbles, i.e. 20% collisions resulted in attachment (N=40). Copepod behaviour, i.e. escape jumping, is the major cause for detachment. Female C. finmarchicus are very resilient jumpers, performing on average 95 jumps, during 2 minutes, before becoming passive. Males are significantly less active.

Bubble driven upwelling contributed more than attachment to lifting copepods through the water column. An attached 500 µm bubble gives the copepod an estimated rise velocity of 4 cm/s, while upwelling velocities (Vup) of up to 35 cm/s were created in bubble plumes, 30 cm above the bubbler. The relation between air flow (Q) and created upwelling flow was determined to Vup~Q^0,23 for small bubbles (mean size ~150µm) and Vup~Q^0,40 for large bubbles (mean size ~1500 µm).

For flotation to be used successfully in a zooplankton harvest system, much higher attachment levels must be achieved in the ocean than what was done in this study. Bubble driven upwelling provides an alternative, but has several serious disadvantages.