Standardizing Biological Modelling of Sea Lice Dispersal

1. Standardizing Biological Modelling of Sea Lice Dispersal Alexander G Murray1, Tom Adams2, Lars Asplin3 and Sam Shephard4 Marine Scotland Science, Aberdeen Scottish Association for Marine Science, Oban Institute of Marine Research, Bergen Inland Fisheries Ireland, Dublin

2. Importance of sea lice • Key limitation to sustainable increase in marine salmonid farming • Treatments and controls are expensive and have problems • Transmit to (and from) wild salmonids • Number are up • Treatment rate is up

3. Planktonic larvae • One of the issues that makes sea lice difficult to control are their free living larvae • These are transported with ocean currents • Potentially over long distances • Although activity is relatively limited they are not passive particles • Links farms and between farm and wild populations

4. Coupled modelling of coupled processes • Larval transport depends on movement of water and biological properties of the sea lice • Therefore coupled hydrodynamic-particle modelling is used • Model 1. derives current velocities through simulation of physical processes in response to drivers such as wind and tides • Model 2. describes the biological processes such as mortality or swimming behaviours of sea lice

5. Applications • Scotland: Loch Torridon, Loch Linnhe, Loch Fyne, west coast, Scottish shelf • Norway: Hardanger Fjord, entire Norwegian coast • Canada: Broughton Archipelago BC • Faeroes • Ireland Killary Harbour

6. Multiple applications (nearly) same problem • The models used have been developed by different institutions in different countries • There are different issues in different territories and applications, but generally similar problems addressed • So there are shared problems with potential for collaboration • Collaboration has in fact occurred for some time (particularly between Norway, Scotland and Canada) • However this can be taken further and models standardised

7. Why standardise models? • Not to force round pegs into square holes • Sometimes different models are required for different purposes • However, a flexible standard model (e.g. with temperature dependence) can cover a range of conditions • Standardised models allow an accepted ‘off the peg’ model to be used in new areas • Cheaper and easier • Modelling assumptions already accepted • Standardised models allow simulations of different systems to be compared • different outputs are differences in the system not in the model

8. MASTS funded workshop • MASTS funded workshop held in Aberdeen in February • Meeting of delegates from the institutions on this presentation • Discussed modelling approaches to identify similarities and differences • Outputs relating to biological processes discussed here • www.masts.ac.uk/media/36205/nhmsg5-report.pdf

9. Biological standardization • Physical models need standards too, and these were discussed, but time limits what is presented here • Biological processes are divided into two topics: • (a) processes such as production and mortality which are relatively simple rates • (b) processes related to lice swimming which is more complex issue of behaviour • These introduce a range of different issues

10. The basic rate processes • Egg production • Survival of larvae • Maturation of larvae • Modelling rates requires empirical data • Much of this data already exists from existing studies • Some refinements needed through future research

11. Egg production • It is possible to model relative lice production without explicit egg production model • Needs relative size of production sources • Egg production to estimate absolute lice production • Need estimate of numbers of ovigerous females • Clutch size varies with • Age of female • Farmed or wild fish • Environment • Egg survival high but varies with environment • Still not standardised

12. Survival • Very sensitive to environment • Mortality increases rapidly as salinity drops below 29ppt • Biological mortality, e.g. grazing by plankton or mussels • For full strength sea water mortality of about 1% h-1 is a reasonable standard • Also finite life-span

13. Maturation • Sea lice go through non-infectious stages • Time required is highly dependent on temperature • Use a single value based on average temperature for any particular scenarios e.g. 3.6 days for 10 c • Early Brooks • Boxaspen & Naess low temp • Standard by Stien

14. Effects of rate sensitivities • Shorter cycles with faster maturation as temperature warms • Problem populations can build up faster • Dispersal distances less so smaller management areas may be effective • Populations inhibited if salinity drops

15. Swimming behaviour • Lice are not passive particles • Phototactic tend to be found near surface • This is standard used in Scottish models lice particles are held in surface layer • In Norway more complexity may be required e.g. to avoid haloclines • More sophisticated models therefore being developed

16. Vertical swimming • Sea lice swim towards surface • In downwellings this means rising particles become concentrated • Can be trapped by onshore winds forming concentrations

17. Effect of swimming behaviour • Lice are not scattered through water column • Subject to surface currents which are often stronger and more sensitive to winds, so lice may move greater distances than passive particles • Lice can form concentrations at downwelling locations and shorelines with onshore wind, sometimes distant from the source • Lice can avoid dangerous low salinities (this is not yet standardised) • This is an area in greater need for new data than the rates

18. Also • Standardisation of hydrodynamic modelling approaches • Standardisation of field sampling • Standardisation of outputs

19. Application of standardisation • Modelling for European standard that will take lessons learned from previous models • Partners from organisation involved in this model • Application to Killary Harbour, Ireland