The salmon louse larval black box: evaluating fecundity and enumerating planktonic stages with an aquaculture management perspective
MetadataVis full innførsel
Modern salmon aquaculture began in 1970 with the innovation of at-sea fish pens which precipitated a rapid growth in production. The expansion of the industry and increased number of farmed fish concentrated within the open net-pens has produced conditions that foster environmental and disease problems. Among the various pathogens impacting the industry, the salmon louse (Lepeophtheirus salmonis) presents a unique challenge due to its proliferation on farms, welfare impacts on host fish, the threat it poses to wild populations of salmonids, and for the cost of and its resistance to control efforts. Norway, the world leader in salmon production, has responded to the persistent challenge of salmon lice with the implementation of a management regime (Traffic Light System) that links permitted aquaculture production to louse induced mortality of wild Atlantic salmon populations. Those management decisions are reliant on an understanding of salmon louse distribution throughout the Norwegian coast, but aspects of the copepod’s life history and biology which determine their planktonic abundance remain understudied. Nevertheless, to meet the needs of the management regime modelers must forecast salmon louse reproduction and planktonic dispersal from salmon farms. Although these models are validated with observations of salmon louse infections on fish, there is a lack of empirical evidence on the distribution and abundance of planktonic stages. Due to the difficulty of enumerating planktonic lice in a mixed zooplankton sample they are almost unobservable and thus exist in a ‘black-box’. This thesis seeks to shed light on the salmon louse larval black-box within the context of the aquaculture management in Norway through two approaches. A greater knowledge of the planktonic stages can be gained through a better understanding of the salmon louse’s life history, and through empirical data on their planktonic abundance and distribution. This thesis addresses the first approach by refining the current understanding of salmon louse fecundity and the second through the development of a novel method for enumeration of planktonic stages. In paper I, we investigated fecundity by examining egg clutch size of salmon lice collected from farmed salmon, wild salmon, and sea trout from multiple farms and fields sites throughout Norway. The investigation revealed the predominant determinate of clutch size is the body size of females, which is dependent on rearing temperature. We further found that a third of adult female lice on farmed salmon were not sexually mature and 10% of the mature females were not egg-bearing. The female lice parasitizing sea trout were less fecund then those on Atlantic salmon with lower rates of egg-bearing and smaller clutch sizes. In papers II and III, we develop a novel method of planktonic salmon lice enumeration which used fluorescence to differentiate th e lice within a mixed zooplankton sample so they could be rapidly identified. First the fluorescence profiles of lice and non-target copepods were examined to identify a unique and reliable fluorescence signal, then a methodology using that signal was developed and tested. The fluorescence signal was found to be strongest using an excitation wavelength of 470 nm and an emission filter of 525 nm. After storage in formalin preservation the salmon lice copepodids had a fluorescence intensity that was 2.4 times greater than non-target copepods. When a mixed zooplankton sample was illuminated with the excitation light the salmon lice would fluoresce brighter than most other animals in the sample and could be quickly discovered. Participants in a blind trial processed standard zooplankton samples in a mean of 31 minutes and identified the lice with an accuracy of 82%. Compared to traditional taxonomic identification, the novel method was 20 times faster, thus providing a practical tool for the study of lice and monitoring of their planktonic stages. The management of salmon aquaculture is dependent on accurate understanding and modeling of the distribution of planktonic salmon lice. The work of this thesis can reduce the inherent uncertainties of those models through better parameterization and through a new tool which enables validation with direct observation of planktonic abundance. However, for the aquaculture industry to continue to grow in Norway this thesis concludes that a prevention priority must replace the current paradigm of salmon louse control through treatment.
Består avPaper I: Thompson, C.R., Bui, S., Dalvin, S., and Skern‐Mauritzen, R. Disentangling the key drivers of salmon louse fecundity through an examination of field samples, with a focus on the role of temperature and body size. The article is not available in BORA.
Paper II: Thompson, C.R., Bron, J.E., Bui, S., Dalvin, S., Fordyce, M.J., Furmanek, T., á Norði, G. and Skern‐Mauritzen, R., (2021). Illuminating the planktonic stages of salmon lice: A unique fluorescence signal for rapid identification of a rare copepod in zooplankton assemblages. Journal of Fish Diseases, 44(7): 863-879. The article is available in the thesis file. The article is also available at: https://doi.org/10.1111/jfd.13345
Paper III: Thompson, C.R., Bron, J., Bui, S., Dalvin, S., Fordyce, M.J., á Norði, G. and Skern‐ Mauritzen, R., 2022. A novel method for the rapid enumeration of planktonic salmon lice in a mixed zooplankton assemblage using fluorescence. Aquaculture Research, 53(6): 2317-2329. The article is available in the thesis file. The article is also available at: https://doi.org/10.1111/are.15750