Evaluation and use of the Lactate Pro, a portable lactate meter, in monitoring the physiological well-being of farmed Atlantic cod (Gadus morhua)

Measurement of blood lactate in teleost fish is a common procedure in assessing the impacts of aquacultural procedures such as crowding and pre-harvest handling, or the metabolic consequences of swimming and fatigue, but processing of blood and laboratory assays is not easily applicable at aquaculture sites or in the field. Lactate meters offer an attractive alternative, but few studies have assessed their viability for analysis of fish blood. We have carried out the first evaluation of the Lactate Pro™ meter for on-farm determination of the blood lactate of teleost fishes, specifically the cod, Gadus morhua. Blood lactate of farmed cod, caught by rod and line, was below detection limits of the meter (< 0.8 mM), and confirmed by laboratory assay as 0.459 ± 0.037 mM (mean ± SEM, n = 34). After 20 min crowding in a sweep net, as used in grading or harvesting, blood lactate was measurable with the meter. Mean blood lactate during crowding for 120 min was 2.723 mM ± 0.122 (n = 102). In the two crowded cages, blood lactate concentrations was 4 mM or above in 9% and 24% of cod, indicating possible exhaustion. Meter readings were stable for at least 8 h in blood samples held on ice in tubes containing sodium fluoride and oxalate as an anticoagulant and antiglycolytic mix. The coefficient of variation of lactate readings was 4.35% at 1.26 mM, 4.53% at 2.96 mM, 2.89% at 3.94 mM. Lactate Pro™ meter readings were compared to blood or plasma lactate concentrations measured by laboratory enzymatic assay. Regression analysis gave an R2 value of 0.9315 for meter readings and laboratory-derived values for whole blood, (mean ±SE slope 1.138 ± 0.082), and an R2 value of 0.943 for meter readings against laboratory-derived plasma lactate concentrations (slope 0.942 ± 0.024). Laboratory-derived plasma lactate concentrations of crowded cod were ∼ 5% above meter readings of whole blood lactate concentrations for the same samples (P < 0.003, n = 95), but haematocrit did not significantly affect meter readings. Our data lead to the conclusion that the Lactate Pro™ meter is a valuable tool for easy on-farm measurement of blood lactate in aquaculture and that the meter can be employed in assessing the welfare of farmed cod.