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Background: Archival tags have been used on fish for a number of years to measure temperature, pressure and salinity, among other parameters. Measurements of heart rate in fish can be used in a wide variety of biological and ecological studies, e.g., for evaluating short-and long-term effects of stressful conditions involved with fishing and farming practices, or events that are non-related to human activity, such as predator-prey interactions. Furthermore, if a relationship between temperature, heart rate and metabolic rate has been established there are potentials to establish long-term bioenergy models that include seasonal effects on feeding, locomotion, spawning and migration. However, heart-rate measurements in fish have been difficult to conduct in the wild due to lack of commercial availability, large size of the recording systems, difficulties with proper placement of external electrodes and restriction in setup with external systems. This work presents a validation of a commercial ECG (Electrocardiogram)-derived heart rate data logger on farmed Atlantic cod. This validation focuses on assessing the logger's signal processing algorithm and recording quality. Results: The eight fish studied showed both individual periodical variation, as well as individual baseline variation of the heart rate during the experimental period. Mean heart rate ranged from 29.9 ± 6.3 to 48.3 ± 4.9 bpm, at 10.2 ± 0.1 °C. This is comparable with previous research on Atlantic cod. The loggers were set up to record heart rate and temperature every 5 min and to store the ECG records used to calculate the heart rate every 30 min. The quality index (QI), which is an algorithmic assigned value of the quality of the ECG, was used to filter the data set eliminating a total of 21,643 measurements (19% of the recorded heart rate data). It was possible to manually calculate 18,490 out of 19,042 ECG records (97%). Conclusions: This study presented a validation of using a commercial implantable data logging device in Atlantic cod that records ECG, and can calculate both heart rate, and a novel quality index for the ECG record with an onboard algorithm.

The effect of thermal environment on condition factor was examined for six different size-classes of Atlantic cod Gadus morhua fed to satiation. A weight-length relationship for 8 to 1303 g fish reared at 4-20°C indicated an allometric growth (W = aL b , a = 0.0045, b = 3.257) of cod. Changes in relative condition factor (K rel ) with temperature were described with a second order polynomial. The most pronounced effect of temperature on body condition was found in the smallest size-classes, but the curves flattened with increased size. Temperature had size-dependent effects on the relative condition factor obtained from an overall weight-length relationship for all fish in the experiment, i.e. K rel increased with weight at 4°C, but decreased with weight at 16 and 20°C. K rel remained high for most size-classes at 8 and 12°C. The slopes (b-values) of the weight-length relationships decreased linearly with temperatures from 4 to 16°C.

Arctic charr Salvelinus alpinus of the Hólar strain (mean ± s.e. body mass = 152·1 ± 3·1 g) were reared at four different salinity regimes at a constant temperature of 7·4° C. Two groups were given a three-month acclimation in salinity 18 before the salinity was increased to either 25 or 29 (groups called A25 and A29), and two groups were reared in salinities 25 or 29 over the full experimental period of 409 days (groups called F25 and F29). In the first 3 months, the A25 and A29 groups had the highest growth rates. By October 2011, there were no significant differences (two-way nested ANOVA, P > 0·05) in the mean body masses among A25, F25 and F29 (c. 1450 g), whereas A29 had a lower mean mass (1282 g). The growth in the last period from October 2011 to January 2012 was reduced by sexual maturation in the highest salinity regimes (A29 and F29), whereas fish in groups A25 and F25 showed high growth throughout the study. Males in all salinity groups had higher growth rates than females for the most part of the study, but the divergence between the sexes was most pronounced in the highest salinity regimes. All salinity groups showed distinct changes in Na(+) , K(+) -ATPase activity, with high activity in spring and summer, and lower activity in the autumn. Plasma sodium (Na(+) ) levels were stable indicating that none of the experimental groups had problems in maintaining hydromineral balance during the study. While plasma leptin levels were not affected by salinity regimes, it was noted that these levels were 13-30% higher in fish with empty guts compared with those having food in their gut at the time of sampling. This suggests a link between leptin levels and food intake, indicating that this hormone may play a role in food intake and energy allocation in fishes.