Smoltification is a complex series of physiological changes that allow young Atlantic salmon to adapt from living in fresh water to living in seawater. In salmon farming, this transition from “parr” to “smolt” is controlled using lights or functional feed to ensure a continuous and predictable supply of fish.
Scientists at SINTEF, one of Europe’s largest independent research institutes located in Trondheim, Norway, have investigated hyperspectral imaging (HSI) to study the vital aspects in detecting smoltification, relying in part upon a BitFlow Camera Link frame grabber to grab high-speed video frames for analysis at more than 100 frames-per-second.
The ability to verify smoltification is critical since incomplete seawater adaptation may result in poor animal welfare and increased mortality. Animal welfare is of increasing importance in salmon farming, as the industry is under pressure to improve production and farming operations due to ethical concerns. Conventional smoltification assessments measure chloride content in blood samples after exposing fish to saline water, or by detecting the presence of ion-transporting enzymes through analysis of tissue samples from gills. These methods are time-consuming, so only a few salmon are typically tested from populations of several hundreds of thousands of fish.
To evaluate the robustness of its HSI approach, SINTEF placed emphasis on collecting diverse data with variations in fish colour, patterning, size and shape using three different salmon farming sites. Data were collected weekly in synchronisation with the sites’ respective production and testing schedules. To make all data sets comparable, despite differences in ambient lighting conditions and exposure settings, all were normalised for comparison using white and dark reference images.
The raw data obtained from HSI were multidimensional images of individual fish, including their background. A step-wise procedure was used to process and analyse the data so the low-dimensional spectral characteristics could be observed, and classification of parr or smolt made possible. Wavelengths were optimised by factoring in water temperature, dissolved oxygen, water opacity and colour, as well as lighting and feeding regimes.
Upon conclusion of its study, SINTEF demonstrated a spectral imaging system where only three wavelengths are needed to identify the smoltification status of Atlantic salmon, and that this system could serve either as a supplementary or free-standing verification tool in fish production. In doing so, the researchers also laid a pathway to manufacturing low-cost spectral imaging instruments for use in production tanks or integrated in existing sorting and vaccination systems for faster, wider and more cost-effective population sampling of Atlantic salmon.