For salmon, the transition from freshwater to the sea is a crucial transformative act. Salmon smolts undergo physiological changes to adapt to the challenges of life in saltwater.
A key player in this metamorphosis is the Sodium-Potassium ATPase (NKA) enzyme, the tireless “gatekeeper” controlling ion flow across cell membranes.
Traditionally, NKA activity in the gills was considered the benchmark indicator of a smolt’s readiness for saltwater immersion. However, with modern aquaculture pushing size and growth limits, this solitary measure has started to fall short. The increased size of smolts and changes in rearing conditions can alter the NKA-gill relationship, making it a less reliable indicator of overall smolt quality.
In response, scientists from the University of Bergen, NORCE, and the University of Gothenburg published a scientific review proposing modifications and optimization of NKA enzyme activity protocols for gill, intestine, and kidney tissues. They describe how better utilizing NKA activity measurements is part of a holistic approach to assessing smolt quality in the modern salmon industry.
The Na/K Pump: A Conductor of Cellular Harmony
Imagine a tiny gatekeeper tirelessly shuttling sodium and potassium into cells. That’s the Na+/K+ ATPase (NKA) enzyme, crucial for maintaining cellular balance and organ function. During the parr-smolt transformation in salmon, NKA activity in the gills increases, allowing them to thrive in the sea.
However, the once-standard NKA activity measurement in the salmon industry came under scrutiny due to inconsistencies and the emergence of alternative methods. Large smolts (>200 g) raised in flow-through systems (FTS) may show a greater increase in gill NKA activity than cohorts raised in recirculating aquaculture systems (RAS).
In teleost fish, NKA is particularly abundant in ion-transporting organs like gills, intestines, and kidneys, opening up new possibilities.
Three Osmoregulatory Organs
The scientific review argues that focusing solely on gills to analyze smolt transformation is misguided. The intestine and kidneys also play vital roles in salt regulation, and their NKA activity deserves attention.
The study highlights the limited attention paid to the role of the intestine and kidneys in the osmoregulation of smolts and post-smolts raised in industrial conditions.
In this regard, the authors propose modified protocols for NKA analysis in all three organs (gills, intestine, and kidneys), making it accessible and relevant for modern aquaculture.
Beyond the Enzyme
The scientific review doesn’t stop there. It recognizes the potential of new tools, such as molecular markers analyzing gene expression patterns. However, it also warns against relying solely on these tools. Like NKA, gene expression may not always directly translate into functional protein activity, especially under different environmental conditions and breeding practices.
Conclusions
So, what’s the key takeaway? According to the scientists, the secret lies in combining NKA activity in all three organs with physiological markers and, when possible, molecular tools. This creates powerful indicators of smolt quality.
They modified the McCormick NKA microassay protocol for use in gills, intestines, and kidneys in salmon smolts and post-smolts. They conclude that “Applying the NKA assay in all three organs has the potential to detect alterations in overall osmoregulatory capacity and how it may respond to environmental changes (temperature, salinity, photoperiod, suboptimal water quality, etc.).”
This information equips fish farmers with a comprehensive understanding of their smolts, ensuring successful freshwater to saltwater transitions. It can also be used to develop indicators for nutrition and digestion studies, toxicology, stress physiology, fish health and welfare, and water chemistry.
The study was funded by the Norwegian Research Council (NFR).
Contact
Marius Takvam and Tom Ole Nilsen
Department of Biological Sciences,
University of Bergen
Bergen, Norway.
Email: m.takvam@uib.no and tom.nilsen@uib.no
Reference (open access)
Takvam M, Sundell K, Sundh H, Gharbi N, Kryvi H, Nilsen TO. New wine in old bottles: Modification of the Na+/K+-ATPase enzyme activity assay and its application in salmonid aquaculture. Rev Aquac. 2023; 1-12. doi:10.1111/raq.12887
