Changes in pH during transport of juvenile cod Gadus morhua L. and stabilisation using buffering agents

Salmonids have been transported from hatchery to ongrowing units for many years, but little information is available on optimal conditions for the transport of marine finfish species. This study examined the changes in pH that occur under various simulated transport conditions of stocking density and fish size, and thereby recommends protocols for live marine fish transportation. The pH of the transport water declined rapidly due to the respiration of carbon dioxide. Various buffers (sodium bicarbonate, TRIS, Magnaspheres) were compared and maintained pH close to normal seawater levels. The decline in pH was greater with small (18 g) rather than larger (32 g) fish, at identical loading biomass in tanks, indicating that fish size should be considered when planning transport. Although unionised ammonia level increased with increasing stocking density (10 to 30 kg m− 3) the pH did not decline significantly. The use of air aeration as well as oxygenation reduced CO2 levels indicating that aeration stripped the CO2 from the water. Non invasive measurements of free cortisol release into the tank water by juvenile cod indicated primary stress response to handling events. This study demonstrates that cod juveniles will be exposed to fluctuating pH in transport tanks, and buffering of the pH and stripping of CO2 by aeration to reduce fluctuations is recommended.

► Tank pH declined rapidly in simulated cod transport.
► 1 g l− 1 Tris (hydroxymethyl) methylamine or 2 g l− 1 sodium bicarbonate buffered.
► 5 g l− 1 magnesium hydroxide spheres with mixing most effective.
► Aeration reduced CO2 and maintained pH compared with only oxygenation.
► Tank pH significantly lower in small cod than same mass of larger cod.