Effects of step salinity gradients on salmon lice larvae behaviour and dispersal

Abstract

In trying to deal with the problematic salmon louse Lepeophtheirus salmonis in salmon aquaculture, strategies to better prevent infestations are gaining traction. Successful prevention requires an accurate understanding of the environmental influences that alter the distribution of the planktonic stages of lice in the water column in space and time. Here, we tested the salinity preferences of nauplii and copepodid larval stages using step salinity column experiments. Under consistent temperature and lighting conditions, we created step gradients using a bottom layer of full salinity (34.7 ppt), with an upper layer of equal or lower salinity (~34.7 to 16 ppt). Lice entered the column in the lower layer and dispersed for 1 h before their position was recorded. Both nauplii and copepodids increasingly avoided the overlying layers as they became more brackish. However, the strength of avoidance differed between nauplii and copepodids. Nauplii almost completely avoided salinities below 30 ppt. For copepodids, there was a more gradual decline in the proportion preferring the less saline overlying layer, and the presence of some individuals occurred even at 16 to 20 ppt. Both stages aggregated at or just below the halocline, with no aggregation evident in isohaline columns at the same depth. For nauplii, clustering within the halocline was particularly strong. When integrated into a sea lice dispersal model, the new salinity preferences we determined markedly altered dispersal patterns in scenarios when salinity gradients were present. Our results have implications for the mapping of salmon lice larval behaviour and dispersal, with benefits for aquaculture planning and management.

Impact of thermoclines on the vertical distribution of salmon lice larvae

“…When harvesting eggs from lice, fish were captured and sedated with tricaine methanesulfonate (Finquel, Argent Chemical Laboratories) at a concentration of 1 g per 100 l of water. Thereafter, eggs were transferred to an incubator system (as per Crosbie et al 2019) and the number of egg strings and date of collection was recorded to determine larval age and lifecycle stage. Microscopy was used to identify stages if there was doubt.…”

“…To test larval behaviour around a thermal gradient, we designed a column experiment to simulate step temperature gradients based on previous studies looking at haloclines (Crosbie et al 2019) and measured where the larvae preferred to distribute. We simulated thermoclines with an overlying layer ranging from 6 to 18°C on top of a bottom layer of 12°C and tested the depth preferences of both nauplii and copepodid stages.…”

“…They are phototactic and will migrate upwards towards light if other influences are not present (Heuch et al 1995, Aarseth & Schram 1999, Crosbie et al 2019), yet nauplii react to different wave lengths and are less sensitive to white light than copepodids (Novales Flamarique et al 2000). Salinity also impacts lice behaviour, with both nauplii and copepodids displaying clear preferences for full salinity over brackish water when presented with a step gradient (Heuch 1995, Crosbie et al 2019. Nauplii show greater avoidance of brackish water, while some copepodids may still be present in salinities of 20 ppt and below (Cros bie et al 2019).…”

Snorkel technology to reduce sea lice infestations: efficacy depends on salinity at the farm site, but snorkels have minimal effects on salmon production and welfare

“…Here, we repeated the experiment in an environment with a strong halocline, with the low-salinity layer extending to depths below the bottom of the 3 m deep lice barrier. Infectious lice copepodids actively avoid salinities < 30 ppt , Crosbie et al 2019, thus they were likely positioned deeper than the bottom of the lice barrier. Under these conditions, results reflected our prediction that lice levels per fish would not differ between lice barrier and control cages as host−parasite encounter rates are likely to be similar.…”

“…Knowledge of the behaviour of both sea lice and salmon provides the potential for new approaches that seek to spatially de-couple hosts from parasites and prevent infestations from occurring. The infective copepodid stage of the salmon louse L. salmonis is pelagic (Johnson & Albright 1991), strongly phototactic (Bron et al 1993), and actively avoids lowsalinity waters (Heuch 1995, Crosbie et al 2019. These behaviours typically result in greatest abundances at shallow depths, particularly directly beneath haloclines of 30 ppt in coastal waters (Johannessen 1978, Costelloe et al 1995, 1998, McKibben & Hay 2004, Costello 2006.…”