Do young salmon with sea lice grow more slowly?
2017-06-01
2017-06-01
Godwin, S.C., Dill, L.M., Krkošek, M., Price, M.H.H., Reynolds, J.D. In press. Reduced growth in juvenile sockeye salmon infected with sea lice. Journal of Fish Biology. [PDF]
Map of the study area (center black box).
Pathogens and parasites threaten wildlife around the world. Sometimes outbreaks of wildlife disease are prominent and lead directly to large die-offs, but pathogens and parasites can also indirectly reduce wildlife survival - for example by decreasing their ability to compete for food or making it more likely that they get eaten by a predator. Often, outbreaks of pathogens and parasites in wildlife are caused or exacerbated by humans.
Here on the west coast of North America, we have Pacific salmon, which is a group of species with incredible ecological, economic, and cultural importance. Pacific salmon may be at particular risk to these indirect - or sub-lethal - effects of infection, because they are commonly parasitized by small marine animals called sea lice that live and feed on the surface tissue of fish.
Sea lice are native to British Columbia (BC), but in the wild they normally infect adult salmon rather than the juveniles. This is a good thing because it’s the juveniles that are more vulnerable, since they are smaller and lack fully developed scales to help protect them. Unfortunately, salmon farms along the migration routes of wild juvenile salmon create reservoirs for sea lice that then spill the parasites back onto the vulnerable juveniles. Right now, over 98% of juvenile sockeye salmon migrating through the BC’s Discovery Islands (a salmon farming hotspot) are infected by sea lice.
Here on the west coast of North America, we have Pacific salmon, which is a group of species with incredible ecological, economic, and cultural importance. Pacific salmon may be at particular risk to these indirect - or sub-lethal - effects of infection, because they are commonly parasitized by small marine animals called sea lice that live and feed on the surface tissue of fish.
Sea lice are native to British Columbia (BC), but in the wild they normally infect adult salmon rather than the juveniles. This is a good thing because it’s the juveniles that are more vulnerable, since they are smaller and lack fully developed scales to help protect them. Unfortunately, salmon farms along the migration routes of wild juvenile salmon create reservoirs for sea lice that then spill the parasites back onto the vulnerable juveniles. Right now, over 98% of juvenile sockeye salmon migrating through the BC’s Discovery Islands (a salmon farming hotspot) are infected by sea lice.
Catching baby salmon is all work no play.
For this study, we tested whether juvenile sockeye salmon that are heavily infected with sea lice grow more slowly than uninfected individuals. To do this, we captured juvenile sockeye around the Discovery Islands, BC, during their early marine migration along the coastline from the Fraser River towards the open ocean. After catching the fish, we removed their otoliths - earstones that have daily growth rings similar to the annual rings in tree trunks - and assessed how much they grew in the ten days before we caught them.
We found that when juvenile sockeye are highly infected with sea lice, they grow 8% more slowly than uninfected individuals, on average. Smaller sockeye may be especially affected by sea louse infection, because smaller fish experienced a greater growth reduction with high sea lice numbers than larger fish did. Consequently, there’s the potential for a vicious cycle where fish with lots of sea lice grow more slowly and become smaller relative to uninfected fish, so they grow even more slowly, and so on.
So why should care that salmon with sea lice are slower growers? Well, during this early marine migration, juvenile sockeye salmon need to grow quickly to avoid getting eaten by predators and feed on larger prey. Simply put, juvenile salmon need to get big, fast, and if they don’t, they are far more likely to die.
We found that when juvenile sockeye are highly infected with sea lice, they grow 8% more slowly than uninfected individuals, on average. Smaller sockeye may be especially affected by sea louse infection, because smaller fish experienced a greater growth reduction with high sea lice numbers than larger fish did. Consequently, there’s the potential for a vicious cycle where fish with lots of sea lice grow more slowly and become smaller relative to uninfected fish, so they grow even more slowly, and so on.
So why should care that salmon with sea lice are slower growers? Well, during this early marine migration, juvenile sockeye salmon need to grow quickly to avoid getting eaten by predators and feed on larger prey. Simply put, juvenile salmon need to get big, fast, and if they don’t, they are far more likely to die.
A whole juvenile sockeye otolith (left) with daily growth rings at 400 times magnification (right).
Currently, sea lice outbreaks on salmon farms are managed with a pesticide in the food of the farmed salmon. These treatments occur when sea lice numbers on farms get too high, and they have resulted in some positive conservation outcomes for wild salmon populations that had been severely affected by sea lice in the past. Unfortunately, farm treatments only target one species of sea lice - Lepeophtheirus salmonis - but the vast majority of sea lice infecting our juvenile sockeye salmon are an entirely different species - Caligus clemensi. As a result, massive outbreaks of C. clemensi can occur on farms.
Our results add to a growing body of work suggesting that we should start considering C. clemensi in parasite management on salmon farms and, more broadly, that understanding the sub-lethal effects of parasites like sea lice may be essential for effective marine conservation of vulnerable fish species.
Our results add to a growing body of work suggesting that we should start considering C. clemensi in parasite management on salmon farms and, more broadly, that understanding the sub-lethal effects of parasites like sea lice may be essential for effective marine conservation of vulnerable fish species.