Vertical salmon lice behaviour as a response to environmental conditions and its influence on regional dispersion in a fjord system

Abstract

The salmon louse Lepeophtheirus salmonis is a major parasite of salmon, and is able to travel between farms during its pelagic phases. We investigated the spatial dispersion of L. salmonis planktonic stages in the Hardangerfjord, Norway, using an individual-based model and a fjord circulation model. The models allowed us to investigate how assumptions about swimming responses to environmental cues affect vertical distribution, development and horizontal transport. The rules governing vertical distributions include passive particles remaining fixed at constant depths, but also prescribed responses of active particles to environmental cues such as ambient light level, salinity and temperature. Horizontal dispersion was affected by the vertical distribution scheme (rules) of the particles (each representing an individual planktonic-stage L. salmonis). When particles were held fixed in the surface layer, the horizontal dispersion and the area potentially affected by a source of lice decreased relative to the distribution that was predicted when lice had vertical migration behaviours. The simulations also showed that swimming triggered by both light and temperature may result in a diel migration pattern. If the particles sought out the warmest areas during juvenile stages, development to the infectious stage was reduced by up to 1 d. Better information is required on the actual response of lice to a set of vertical environmental factors to improve predictions of lice dispersal in fjords.

Statistical and ecological challenges of monitoring parasitic salmon lice infestations in wild salmonid fish stocks

“…Such variations that cannot be fully predicted are common in many ecological studies and although not all relevant details may ever be available, it would likely have been possible to reduce the noise and improve the models by having a more thorough sampling design and higher number of data points. Further, recent approaches of spatiotemporal models of salinity, temperature and currents in the fjords may improve the availability of relevant environmental data (Asplin et al 2011, Salama & Rabe 2013, Johnsen et al 2014. Although statistical models based on rather coarse-scale proxy data can be used to identify factors influencing the infection pressure on wild salmonids, as we have tried here, this approach may not be that useful for determining whether individual areas have a lice problem owing to finer-scale environmental variation.…”

Salmon lice (Lepeophtheirus salmonis) development times, body size, and reproductive outputs follow universal models of temperature dependence

“…A degree-day is the product of time and temperature (e.g., 30 days at 10°C = 300 degree-days), and it is a useful index of "physiological age" or the temperature required for growth and development of ectotherms within their range of tolerance (Costello 2006). Moreover, this index is used as a temperature-dependent development time in salmon lice dispersal models Johnsen et al 2014). Here, we calculated degree-days by multiplying the mean temperature of the incubators by development time in days.…”

“…To test for differences in distribution patterns due to variations in development times to the infective stage, we used simulations from an individual-based coupled biological-physical dispersal model (described in Johnsen et al (2014). In our simulation, 10 particles, representing salmon lice during its planktonic stages, were released every hour from two sites (coastal and fjord site) around the Hardangerfjord area (60°N, 5.5°E) on the west coast of Norway for a winter month in January 2015.…”

“…For marine animals that develop in the water column, larval stage duration determines the length of time that larvae are subject to transport by currents and exposed to sources of mortality (O'Connor et al 2007;Treml et al 2008). Therefore, predicting disease transmission of marine pathogens requires a considerable understanding of the biology of the agent and its interaction with the surrounding physical environment (Murray 2009;Asplin et al 2014;Johnsen et al 2014).…”

Identifying salmon lice transmission characteristics between Faroese salmon farms

“…Gillibrand & Willis (2007) and Johnsen et al (2014) showed that relatively small vertical movement behaviors had a significant effect on the dis persion pattern in stratified waters, emphasizing the need for a more comprehensive model in estuarine-driven areas. However, outside the Faroese fjords, where the circulation is dominated by the tides (Larsen et al 2008), the connectivity between the salmon farming locations will those also be dominated by tides.…”