Pioneering study on pumping stress carried out in a commercial RAS 

The authors, from Spain and the Faroes, write that although pumps are often used to move fish in recriculating aquaculture systems (RAS), the effect on the fish is poorly understood.

They therefore believe it is of great interest to gain a precise understanding of how common commercial practices affect the overall stress and welfare status of fish, in order to develop and implement mitigation measures, both from an economic and ethical perspective.

“According to previously published research, the practice of moving fish triggers activation of the stress response, with significant differences in intensity between different techniques,” they write in the paper, which can be read here.

It has been reported, among other things, that vacuum pumping (different to centrifugal pumping) is more stressful than other methods, such as netting, and can significantly extend the recovery period, in addition to reducing product quality by shortening the time to rigor mortis onset. 

According to the paper, previous research on the effects of pumping in fish has mainly examined biochemical parameters in plasma, behaviour, muscle pH, survival, or time to onset of rigor mortis under laboratory conditions.

Centrifugal pumping

“However, to the best of the authors’ knowledge, no studies have focused on the effect of centrifugal pumping on biochemical parameters and molecular endpoints in salmon smolt directly in a commercial RAS environment,” they write.

They point out that conducting this type of research in commercial operations, where ordinary operations are followed without any form of intervention from the researchers, makes replica studies in tanks impossible.

“At the same time, it provides valuable insight into what happens in a real operational situation and increases the applicability of the results. Previous RAS studies have mainly focused on the response of mucosal surfaces (especially gills and skin) to changes in the system,” the researchers write.

This case study is therefore the first to investigate the effect of a standardised centrifugal pump transfer operation (where the main purpose is stocking density reduction) on the general stress, welfare, and immune status of salmon in an industrial RAS, as well as the fish’s ability to recover after the event.

Two hypotheses

They describe that they investigated two hypotheses. First, it was expected that transfer from one tank to another by pumping would trigger a stress response in salmon (reflected by an almost immediate increase in plasma cortisol, followed by an increase in glucose and changes in both biochemical parameters and gene expression), in an organ-specific manner. This, according to the researchers, indicates potential compensatory mechanisms between peripheral and central systems.

Second, it was assumed that all parameters examined would return to control levels shortly after the recovery period began (up to 7 days after stress).

The researchers write that overall, the data from this case study indicate that centrifugal pumping triggered activation of primary and secondary stress responses, particularly at the peripheral level, characterised by increases in circulating cortisol levels and a somewhat delayed increase in plasmatic glucose.

Stress response

“The results show that centrifugal pumping in this specific industrial-based case triggered a stress response in salmon smolts, but that this response did not appear to be maladaptive, as the fish were able to maintain homeostasis.”

Nevertheless, some of the investigated endpoints (e.g. mRNA levels of relevant genes in the head kidney and plasma ALT) remained significantly altered seven days after stress, which they believe indicates that full homeostasis had not yet been achieved and that additional time is required for full recovery after the transfer.

They point out that having to expend energy to activate stress responses can lead to increased susceptibility to further stress. It may therefore be critical to ensure stability in the farming environment in the days following such an event to prevent increased mortality.

Real-world impacts

The importance of the findings is also emphasised by the fact that the case study was conducted in a facility with full commercial operation and is thus very representative of the conditions in a RAS facility with salmon, while most of the available literature deals with small-scale, controlled laboratory experiments that at best mimic industrial conditions.

“It is possible that prolonged exposure to common stressors in the RAS (e.g. high density, acoustic environment and constant light) can lead to reduced responses in the HPI (hypothalamus-pituitary-interrenal) axis and associated inflammatory reactions upon subsequent stress, without significantly harming the fish. This is supported by the fact that the response, although significantly lower than expected, was partially restored towards the end of the experimental period,” they write.

The scientists conclude that further research that follows the stress and immune status of salmon in industrial settings throughout the production cycle is necessary to investigate whether farming conditions in commercial RAS actually alter the HPI axis response and the immune-endocrine interactions. The authors thank Hiddenfjord for making the study possible in their facility and for practical support on site.