Atlantic Salmon (Salmo salar) Cage-Site Distribution, Behavior, and Physiology During a Newfoundland Heat Wave

Background: Climate change is leading to increased water temperatures and reduced oxygen levels at sea-cage sites, and this is a challenge that the Atlantic salmon aquaculture industry must adapt to it if it needs to grow sustainably. However, to do this, the industry must better understand how sea-cage conditions influence the physiology and behavior of the fish.Method: We fitted ~2.5 kg Atlantic salmon on the south coast of Newfoundland with Star-Oddi milli-HRT ACT and Milli-TD data loggers (data storage tags, DSTs) in the summer of 2019 that allowed us to simultaneously record the fish's 3D acceleration (i.e., activity/behavior), electrocardiograms (and thus, heart rate and heart rate variability), depth, and temperature from early July to mid-October.Results: Over the course of the summer/fall, surface water temperatures went from ~10–12 to 18–19.5°C, and then fell to 8°C. The data provide valuable information on how cage-site conditions affected the salmon and their determining factors. For example, although the fish typically selected a temperature of 14–18°C when available (i.e., this is their preferred temperature in culture), and thus were found deeper in the cage as surface water temperatures peaked, they continued to use the full range of depths available during the warmest part of the summer. The depth occupied by the fish and heart rate were greater during the day, but the latter effect was not temperature-related. Finally, while the fish generally swam at 0.4–1.0 body lengths per second (25–60 cm s−1), their activity and the proportion of time spent using non-steady swimming (i.e., burst-and-coast swimming) increased when feeding was stopped at high temperatures.Conclusion: Data storage tags that record multiple parameters are an effective tool to understand how cage-site conditions and management influence salmon (fish) behavior, physiology, and welfare in culture, and can even be used to provide fine-scale mapping of environmental conditions. The data collected here, and that in recent publications, strongly suggest that pathogen (biotic) challenges in combination with high temperatures, not high temperatures + moderate hypoxia (~70% air saturation) by themselves, are the biggest climate-related challenge facing the salmon aquaculture industry outside of Tasmania.

Validating Star-Oddi heart rate and acceleration data storage tags for use in Atlantic salmon (Salmo salar)

Background Data storage tags (DSTs) record and store information about animals and their environment, and can provide important data relevant to fish culture, ecology and conservation. A DST has recently been developed that records heart rate (fH), electrocardiograms (ECGs), tri-axial acceleration and temperature. However, at the time of this study, no research using these tags had been performed on fish or determined the quality of the data collected. Thus, our research asked: do these DSTs provide reliable and meaningful data? To examine this question, Atlantic salmon (1.4 ± 0.7 kg) were implanted with DSTs, then swam at increasing speeds in a swim tunnel after 1 week of recovery. Further, in two separate experiments, salmon (2.4 ± 0.1 kg) were implanted with DSTs and held in a large tank with conspecifics for 1 week at 11 °C or 6 weeks at 8–12 °C. Results External acceleration (EA) and variation in EA (VAR) increased exponentially with swimming speed and tail beat frequency. The quality index (QI) assigned to ECG recordings (where QI0 means very good quality, and QI1, QI2 and QI3 are of reduced quality) did not change significantly with increasing swimming speed (QI0 ~ 60–80%). However, we found that the accuracy of the tag algorithm in estimating fH from ECGs was reduced when QI>0. Diurnal patterns of fH and EA were evident from the time the salmon were placed in the tank. Heart rate appeared to stabilize by ~ 4 days post-surgery in the first experiment, but extended holding showed that fH declined for 2–3 weeks. During extended holding, the tag had difficulty recording low fH values < 30 bpm, and for this reason, in addition to the fact that the algorithm can miscalculate fH, it is highly recommended that ECGs be saved when possible for quality control and so that fH values with QI>0 can be manually calculated. Conclusions With these DSTs, parameters of acceleration can be used to monitor the activity of free-swimming salmon. Further, changes in fH and heart rate variability (HRV) due to diurnal rhythms, and in response to temperature, activity and stressors, can be recorded.

Differential effects of internal tagging depending on depth treatment in Atlantic salmon: a cautionary tale for aquatic animal tag use

Electronic tags are widespread tools for studying aquatic animal behavior; however, tags risk behavioral manipulation and negative welfare outcomes. During an experiment to test behavioral differences of Atlantic salmon Salmo salar in different aquaculture cage types, including ones expected to elicit deeper swimming behavior, we found negative tagging effects depending on whether cages were depth-modified. In the experiment, data storage tags implanted in Atlantic salmon tracked their depth behavior and survival in unmodified sea-cages and depth-modified sea-cages that forced fish below or into a narrow seawater- or freshwater-filled snorkel tube from a 4 m net roof to the surface. All tagged individuals survived in unmodified cages; however, survival was reduced to 62% in depth-modified cages. Survivors in depth-modified cages spent considerably less time above 4 m than those in unmodified cages, and dying individuals in depth-modified cages tended to position in progressively shallower water. The maximum depth that fish in our study could attain neutral buoyancy was estimated at 22 m in seawater. We calculated that the added tag weight in water reduced this to 8 m, and subtracting the tag volume from the peritoneal cavity where the swim bladder reinflates reduced this further to 4 m. We conclude that the internal tag weight and volume affected buoyancy regulation as well as the survival and behavior of tagged fish. Future tagging studies on aquatic animals should carefully consider the buoyancy-related consequences of internal tags with excess weight in water, and the inclusion of data from dying tagged animals when estimating normal depth behaviors.

Otolith shape differences between ecotypes of Icelandic cod (Gadus morhua) with known migratory behaviour inferred from data storage tags

Otolith shape can be used to identify ecotypes of the Icelandic cod (Gadus morhua) stock. The use of data storage tags has increased our knowledge of the stock structure of Icelandic cod. The profiles of tagged cod reveal different migratory strategies. This has led to the definition of two ecotypes within the cod stock. Frontal ecotypes reside in deep waters during feeding season and express a highly variable temperature profile associated with thermal fronts, while coastal ecotypes stay in shallow waters all year round. In this study, the data storage tag profiles were analysed with cluster analysis, which revealed the existence of an intermediate behaviour that expresses a variable depth profile and feeding migration that is both shorter in time and not as deep. The main objective was to develop a morphological key based on otoliths to distinguish the ecotypes. The shape of the otoliths was extracted with shape measurements and fast Fourier transforms. A discriminant function analysis indicated a difference in morphology between the ecotypes, resulting in successful classification.

Connectivity among offshore feeding areas and nearshore spawning grounds; implications for management of migratory fish

Knowing movement and structure of fish populations is a prerequisite for effective spatial fisheries management. The study evaluates migration patterns and connectivity of two groups of cod (Gadus morhua) associated with offshore feeding and nursery grounds. This was achieved by investigating (i) migration pathways of cod tagged at the feeding areas, (ii) immigration of cod to the areas based on mark-recapture data covering a period of two decades, and (iii) depth and temperature data from data storage tags (DSTs). Despite undertaking long-distance migrations after attaining sexual maturity, the cod aggregations in the two study areas appear to be largely separated from each other. This conclusion is supported by DSTs, indicating that mature fish associated with the two areas occupy different thermal-bathymetric niches. Low levels of connectivity suggest that effective spatial management in the two study areas would preserve fish of different origin. For the highly migratory adults, however, spatial management would need to focus on migration pathways and the areas where the fish are particularly vulnerable to fishing.

Observations of vertical movements and depth distribution of migrating female lumpfish (Cyclopterus lumpus) in Iceland from data storage tags and trawl surveys

Lumpfish (Cyclopterus lumpus) is a high latitude species most abundant in Arctic and sub-Arctic waters of the North Atlantic. Vertical behaviour of this fish is unclear as it is often caught by both pelagic and demersal trawls. To gain greater insight into its behaviour, 41 female lumpfish caught during the Icelandic Groundfish Survey (IGFS) in March were tagged with data storage tags (DSTs); the IGFS finishes ∼1 week before the beginning of the lumpfish fishing season (20 March). Data retrieved from returned tags were compared with information on depth and distribution of catches of lumpfish from the IGFS. Thirteen tags were returned with days at liberty ranging from 20 to 61 d. Maximum depth recorded was 308 m (maximum depth of the tag) but based upon interpolation of temperature recordings, one fish may have descended to ∼418 m. Lumpfish displayed a range of vertical behaviours termed demersal, surface, and pelagic. During March, most exhibited either demersal or pelagic behaviour but the time spent in surface behaviour increased from March to April. During demersal behaviour, depth was rarely constant indicating the fish were not stationary. Both DST and catch data from the IGFS indicate that lumpfish exhibit diel patterns in vertical behaviour. As lumpfish frequently exhibit demersal behaviour, the use of the IGFS to monitor changes in abundance is justified. As lumpfish spend a significant amount of time in both the pelagic and demersal zone, they should be considered as a semi-pelagic (or semi-demersal) fish during this life stage/time of year.