Mechanical Delousing - Impacts and Costs

Sea lice treatment vessel - Newfoundland

Howdy all – this is the next installment in my Understanding Fish Farming blog. For first-time readers, the goal of this blog is to provide an overview and analysis of key aquaculture concepts for non-fish farming audiences. I try to publish weekly but the frequency will ebb, and flow based on the demands of my consulting and advisory activities.

For this week’s installment, I’m going to look at mechanical/thermal delousing systems. Salmon farmers have been battling sea lice for decades now and mechanical/thermal systems are, in many respects, the last line of defense when other, less invasive, systems have stopped working. If you are not familiar with sea lice, please refer to my previous blog post, a Simple Primer on Sea Lice.

Types of systems

Mechanical systems come in a variety of shapes and sizes, but the main options involve pressurized streams of water and/or soft brushes targeted to mechanically dislodge sea lice from a salmon as it passes through the treatment chamber. Here are a couple of companies providing mechanical systems – Smir and FLS – I have direct experience with FLS systems but have not worked with any Smir systems.

Thermal systems involve submerging salmon in a bath of warm water (approximately 12ºC warmer than ambient conditions – ie if the sea water is 10ºC, the treatment water is 22º C) for a few seconds to take advantage of a louse’s natural inclination to drop off the salmon when exposed to warm water. For an example and photos of a system, have a look at the ScaleAQ website.

Results from these systems can vary widely based on the condition of the fish prior to treatment, condition of the system itself, the husbandry skills of the operating teams and the marine environment.

Steps in the treatment process

Feeding – it varies from farmer to farmer but normally, fish are not fed for 2 – 3 days prior to a stressful handling event. It’s a bit counter-intuitive for non-fish farmers to grasp but wild salmon do not feed when they enter rivers to spawn and some spawning distances can exceed 1,000 kms. It is a normal and natural reaction to stop feeding when under stress.

Lice count – generally, a pre-treatment lice count of a small number of fish is conducted. The fish are anaesthetized, lice are counted, and then they are returned to the pen.

Pen preparation – feeding systems, photoperiod lighting, mortality removal systems, environmental monitors etc. are removed from the pen to allow the net to be shallowed and made ready for the treatment. This step can take anywhere from a couple of hours to half a day.

Crowding – if the farmer has sufficient mechanical capacity, they may crowd the fish in the stock containment net. A general rule of thumb will be that fish should be crowded for no more than 2 – 3 hours. If they do not have capacity to process the fish at this pace, they will likely split the fish into a few groups with only one group being crowded at any given time. Oxygen is provided in the crowd to try and keep the fish as calm and unstressed as possible.

Pumping – throughput capacities vary widely from system to system. There are general limits to how large a diameter pipe and pumping system can be used, so increasing capacity generally means more treatment lines. The goal here is to get the fish to pass through the pumping system without overstressing them in the crowd and damaging them in the system. Not all systems are creating equal in achieving this goal.

Post-treatment lice count – a small number of fish are captured when they exit the treatment system, anaesthetized and checked for remaining lice. This is a critical step to confirm the system is operating correctly.

Pen re-assembly – once the treatment is complete, the pen must be put back together – feeding, lighting, mortality removal systems all need to be replaced once the net has been returned to normal depth. As with all these steps, efficiency can vary widely from company to company.

The impacts of mechanical delousing

I drafted two scenarios comparing two identical groups of fish – one with a mechanical treatment and the other without. This is done purely for illustrative purposes, if a group of fish requires treatment for sea lice, not treating is not an option. I’m just trying to illustrate the impacts.

Production impacts of having to treat for sea lice with mechanical systems

Feeding and growth – as mentioned above the fish are not fed just prior and during the treatment. Post-treatment, it takes a few days for feeding to return to normal. In a production model I developed for one of my consulting projects, I assumed that an individual pen would feed at 50% of normal for 3 days post treatment, 75% for a further 3 days, and a return to 100% feeding on the 7th day post treatment. In this example, the untreated fish consume an additional 91,588 kgs of feed, weigh 85g more resulting in 163,577 kgs more biomass. These values are rough estimates but I hope the point is clear that these treatments have a significant impact on feeding for considerably longer than the actual treatment period.

Impact on farm feeding rates of mechanical treatments

Mortality – mechanical sea lice treatments are a necessary evil. The example shown compares a mechanical treatment compared to no treatment which is obviously not realistic. If salmon need to be treated for sea lice, not treating isn’t an option.

Here is the incremental mortality I have assumed in the model. As mentioned above, there is no additional mortality when the fish are off feed for a few days prior to treatment. During the actual treatment, I have assumed ½% per day of incremental mortality. This number can vary widely from treatment to treatment but over a 5-day treatment period, an additional 2.5% mortality is a poor outcome but not an unreasonable estimate. Post-treatment, a farmer can expect elevated mortality due to injuries, the stress of the treatment and the fallout of weaker members of the population. I’ve assumed an additional 1.5% mortality in the six days post treatment. I would be curious to hear from active farmers (if any are reading this) to know if this tracks close to their experience. It does with mine, just thinking about it is giving me PTSD.

Assumed incremental mortality

Fish health – the primary elements of a salmon’s immune system are its scales and mucous layer. Some systems are gentler than others, but they all result in a loss of scales and mucous and can open pathways for infections. Thermal systems have been connected to impacts of gill health as well. The alternative, of course, could be to do nothing and allow an unchecked sea lice infestation to destroy your fish. No farmer likes to see these kinds of impacts, but they are better than the alternative.

Harvest impacts – aside from less growth and biomass, the fish are also in poorer condition following a mechanical treatment. The loss of scale and mucous can lead to scarring and sores on the skin and may result in downgrading of harvest fish. Since the advent of mechanical sea lice treatments salmon farmers have seen a significant decline in harvest quality results. This article illustrates the issue. Mechanical treatments are certainly not the only source of injuries and blemishes, but they are a primary driver.

Costs

Production costs have doubled in the last 10 – 15 years. Rising feed costs are an issue but significant drivers are increasing costs related to managing sea lice. Here is how I have modelled the operational cost per day of a mechanical sea lice treatment in Canadian dollars.

Estimated daily vessel cost for sea lice treatment

Actual costs vary widely from company to company, but the bottom line is that managing sea lice is enormously expensive. In my model, I have assumed the primary treatment vessel is supported by two service vessels engaged in rigging and de-rigging the pens, crowding the fish etc. I have also assumed that incremental labour is captured in service vessel costs. I have assumed that costs double if the fish are larger, which is a clumsy way to model it, but there is no question that the effort and resources required to treat go up as the biomass on the farm increases.

Conclusion

As I mentioned earlier in the article, mechanical/thermal sea lice removal systems are a must for most salmon farmers and the non-treatment scenario I have presented isn’t a realistic option. I was using it to illustrate how much managing sea lice with these systems is increasing production costs, diminishing productivity, and creating acute animal welfare pressures. A key challenge, of course, is that sea lice are an incredibly adaptable parasite and farmers/suppliers will need to put incredible energy into developing new solutions to get out in front of the problem and stay there.

If you are still reading, once again, I salute you.

Questions, comments, corrections can be directed to Info@AlanWCook.com or my LinkedIn profile.