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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 on-board algorithm.

Homing and site fidelity of Atlantic wolffish Anarhichas lupus was studied at the main spawning ground and four different feeding grounds in Icelandic waters. A total of 883 fish were tagged with T-bar anchor tags, whereof 349 fish also received an internally implanted data storage tag (DST). The DSTs data revealed consistent year-to-year migrations between spawning grounds and feeding grounds. Arrival time at the main spawning ground was similar for males and females, but males departed two months later, suggesting that peak of hatching of larvae was in late February. Homing was estimated to be 93% for the main spawning ground and 87% for feeding grounds. Fish tagged at the main spawning ground did not exclusively migrate to one feeding ground during the feeding season, and fish tagged at each feeding ground migrated to more than one potential spawning ground. Fish tagged at the southeast feeding ground appear to be isolated from the other areas investigated. Atlantic wolffish in Icelandic waters is currently managed as one single stock, hence the single stock management approach might need to be reconsidered.

Biological data were retrieved from 3694 female Atlantic wolffish Anarhichas lupus collected in Icelandic waters at four locations during the breeding season using a long–term study spanning from 2002 to 2013. The main objective was to investigate reproduction investment and timing. In the main spawning ground, little temporal differences were observed. In contrast, the peak of the spawning season was different among spawning grounds, suggesting spatial differences in the timing of reproduction. The size of females A. lupus was related to the spawning time with the larger fish spawning earlier than smaller ones. In addition, no significant pattern was found between temperature and spawning time. Except at its main spawning area where spawning begins in late September, spawning usually began in late August or beginning of September and was completed in early November.

Pampoulie, C., Skirnisdóttir, S., Daníelsdóttir, A. K., and Gunnarsson, Á. 2012. Genetic structure of the Atlantic wolffish (Anarhichas lupus L.) at Icelandic fishing grounds: another evidence of panmixia in Iceland? – ICES Journal of Marine Science, 69: 508–515. The stock structure of the Atlantic wolffish was investigated at Icelandic fishing grounds, using 16 microsatellite loci. Despite the potential of the Atlantic wolffish to exhibit genetic structure (lack of eggs/larval dispersal and adults are sedentary), none of the genetic tests applied in this study detected significant genetic differentiation among the contemporary samples as well as among the contemporary and archived samples. The results of this study therefore suggested a lack of genetic structure among the populations of Atlantic wolffish in Icelandic waters and temporal stability over a period of ∼10 years. These results are discussed in terms of biological characteristics of Atlantic wolffish, recent isolation of populations, and their application to sustainable fisheries management issues.

The effect of temperature on growth of butterfish, Pholis gunnellus on the west coast of Iceland was studied from September 1995 to March 1997. Butterfish were collected monthly at two sites with different temperature regimes. According to otolith readings butterfish can reach 12 years. The fish in the area with higher sea temperature were larger at all ages but had similar growth rates. Higher temperature seems to result in a shorter incubation time of butterfish eggs and/or higher growth rate of the larvae, leading to larger larvae size at settlement.