Friday Parasite #7: Lousy Fish Today

Last week, a new study in PNAS (1) attracted some media attention – the authors presented compelling evidence that juvenile salmon get infected with sea lice as they migrate past fish farms. Most of the news reports I saw focused on the fish. Do the parasites kill the young fish? Can fish farming coexist with wild salmon? So far, none of them have answered one pressing question -- What is a sea louse, anyway?

               

“Sea louse” is a general term that refers to several different species of parasitic crustacean. The culprit in this particular case is Lepeophtheirus salmonis, a parasitic copepod from the family Caligidae. Copepods are tiny crustaceans that look like a cross between a tadpole and a shrimp. The biggest ones are only a couple of milimeters long. Most species are planktonic, spending their lives floating around and sieving even tinier organisms out of the water. But some species eat blood, and they’ve evolved mechanisms that let them hang on to larger animals and get it. L. salmonis is one of the bloodsucking type, and can infect several species of salmon and trout. A heavy infection can slow a host’s growth, make it vulnerable to infection, or kill it from blood loss.

Unlike the human lice I wrote about a few weeks ago, L. salmonis doesn’t depend on a pair of fish bumping into one another to get onto a new host. Adult females release their eggs into the ocean, and newly-hatched copepods live in the plankton for a couple of molts before they have to find a host. When a young parasite comes across an appropriate fish host, it attaches by driving its clawed antennae into the fish's skin and uses its syringe-like mouth to suck up blood (2).

So a group of young wild salmon migrating through an area that already contains a group of infected farmed fish is effectively swimming through a heavy concentration of hungry, floating copepod larvae. It’s like making a group of preschoolers run through a room full of hornets. Some of them are bound to get stung.

References:
(1)Krkosek, M. et al. 2006. Epizootics of wild fish induced by farm fish. PNAS 103(42): 15506-15510.
(2) O’Donohoe, P. et al. 2004. National Survey of Sea Lice on Fish Farms in Ireland 2003. Marine Institute, Galway.

Photo Credits
H. Broman & A. B. Skiftesvik
Fisheries Research Service, Scotland

Comments

[lucy]

i think that farming fish is evil & damented

[sam]

this is so inhumaine to poor little fishies who cant talk for them selves your a very mean person and all the other people who do this to fish!!!!!!!!!!!!!!!!