The high cost of treatment and monitoring of salmon lice especially in salmon farming industries is a worrying trend that requires an urgent solution. These marine ectoparasites have a significant impact on the health and growth of fish. Today’s treatment approaches mostly target the adult life stage of the salmon lice. However, monitoring of the free swimming larval stage of salmon lice can help improve treatment planning and preventive steps. This requires better knowledge of the abundance and distribution of the planktonic larval stages which is currently limited. Automated real-time methods such as optical plankton counters, video plankton recorder, and digital holography systems have the potential for larval salmon lice detection. Unfortunately, all these methods lack chemical and spectroscopic specificity that forms the basis for the detection and identification of salmon lice individuals at low abundance. Recently, a combination of cross-section and fluorescence detection has been proposed to address the issues by adding molecular information for the detection of zooplankton with improved specificity. Even though autofluorescence has been used in the taxonomical classification of marine phytoplankton, its use in larval salmon lice has not been fully explored.
To this note, Josefine Nielsen (PhD candidate), Christian Pedersen, Thomas Kiørboe, and Peter John Rodrigo from the Technical University of Denmark together with Thomas Nikolajsen from Fauna Photonics ApS and Mikkel Brydegaard from Lund University investigated the autofluorescence in zooplankton and its application in the classification of larval salmon lice. The autofluorescence spectra of six zooplankton species including salmon lice were measured. Additionally, they demonstrated the feasibility of the autofluorescence spectroscopy in the separation of feeding and nonfeeding crustacean zooplankton. Their research work is published in the journal, Applied Optics.
For all the examined zooplankton species, the presence of cyan fluorescence centered around 510 nm – 520 nm was observed. Moreover, salmon lice exhibited distinct spatial distribution in its fluorescence desirable for identification. The herbivorous zooplankton exhibited red autofluorescence from the undigested chlorophyll in the gut. Furthermore, the authors used a dual-band analysis of the fluorescence spectra to demonstrate the ability to distinguish the algae-eating species from the noneating species like the salmon lice. The possible classification of the noneating zooplankton from herbivores depended on the ratio of fluorescence strengths from two 50-nm wide fluorescence bands centered at 515 nm and 686 nm.
Unlike the previous conventional methods, the dual-band method is advantageous in terms of simplicity, reliability, and speed. As such, it can be utilized as the basis of an optical system for real-time detection and monitoring of larval salmon lice. Considering a case study showing that Norwegian salmon farms are dominated by algae-eating species, the classification of algae-eating and noneating zooplankton was noted as a vital step in the classification of salmon lice. According to the authors, these results provide an important step towards the development of advanced real-time automated optical detection systems. This approach is versatile and can be employed for the classification of other species of noneating or parasitic zooplankton. In summary, the autofluorescence research paper has been identified by Advances in Engineering as a useful classification tool in marine environments.
Nielsen, J. H., Pedersen, C., Kiørboe, T., Nikolajsen, T., Brydegaard, M., & Rodrigo, P. J. (2019). Investigation of autofluorescence in zooplankton for use in classification of larval salmon lice. Applied Optics, 58 (26), 7022.