Temperature drives depth of salmon lice nauplii but not copepodids

Pinpointing the whereabouts of free-swimming salmon lice larvae is vital to successfully formulating lice prevention strategies that reduce contact between them and farmed salmon. Biophysical models are used to estimate salmon lice larvae locations, but model accuracy could be improved by more precisely coding how larvae change depth in response to environmental conditions. In a first set of experiments, we determined larvae swimming depth changes during salinity stratification, to fill in knowledge gaps from previous studies (see “salinity-mediated depth of salmon lice” article in Norskfiskeoppdrett nr 3/2018).

Another variable of interest is temperature. Field data from plankton net sampling has suggested that salmon lice larvae, particularly at naupliar stages, actively seek out warmer depths that optimise development, and attempts have been made to incorporate this information into lice dispersal modelling. However, experimental evidence for this behavioural response to temperature is lacking.

Temperature choice experiments

To uncover the temperature preferences of nauplii and copepodid salmon lice larvae, we produced vertical temperature gradients in 80 cm deep columns. The columns consisted of an inner column housing the larvae and an upper and bottom outer water jacket which could be filled with different temperatures so stable temperature stratification was created in the inner column. Using a salinity of 32 psu in the top and 34 psu in the bottom of the inner column, we were able to create both a warmer and a cooler top layer. The bottom temperature was set at 12 °C, and we varied the top temperature by -6, -4, -2, 0, +2, +4 and +6. Larvae were release at the bottom of columns and their depth distribution was recorded after 1 h (Figure 1).

Figure 1. Photo of Tom Crosbie marking of salmon lice larvae depth positions

Warmer or cooler surface conditions did not alter the depth distribution of infective copepodid larvae (Figure 2b). However, increasing surface layer temperature relative to underlying waters resulted in progressively fewer nauplii entering the surface layer (Figure 2a). Lowering the top layer temperature compared to bottom layer caused increasingly more nauplii to move into the top layer (Figure 2a).

Figure 2. The proportion of a) salmon lice nauplii and b) copepodids in the top layer of columns under varying temperature change. Exponential curves explaining the relationship between larvae in the top layer relative to temperature change are shown.

Implications

Despite the importance of temperature in controlling Atlantic salmon swimming depth, our results suggest this variable is of little or no effect to infective copepodid swimming depth. Infective copepodids may therefore rely more on other environmental (e.g. light and salinity) and host cues (e.g. semio-chemicals and flow) to find their fish hosts. From our results, no depth adjustments to copepodids should be made based on vertical temperature stratification alone in lice dispersal models.

In contrast, nauplii altered their swimming depths in response to vertical thermal gradients. The temperature-induced changes to nauplii depth we observed differed to the prevailing view that nauplii select warmer depths. Instead, we observed nauplii being pushed below a warmer surface layer.

Our column experiment results so far have shown that nauplii avoid surface waters as its density decreases with lower salinity or higher temperature relative to deeper layers. Transitioning into a surface layer of lower water density would require more energy for upward swimming. Nauplii may avoid water density transitions, staying in deep water to conserve energy stores for the energy-intensive host-finding copepodid stage.

We plan validate our results in future column experiments that test combined temperature and salinity stratification and different column depths. Collectively, the information will be coded into new and improved lice dispersal models that will continue to guide the way salmon lice infestations in farmed salmon are managed into the future.

Authors: Daniel Wright, Thomas Crosbie, Sussie Dalvin, Frode Oppedal, Tim Dempster