The term “sea lice” describes two entirely different biological phenomena. The first refers to parasitic copepods, small crustaceans that feed on the skin, mucus, and blood of marine fish, particularly salmon and sea trout. The second refers to the microscopic larvae of cnidarians, such as thimble jellyfish, which cause a temporary, intensely itchy human skin rash known as seabather’s eruption.
Sea Lice Affecting Fish: Habitat and Host Environment
The parasitic crustaceans known as sea lice belong to the family Caligidae, with the species Lepeophtheirus salmonis and various Caligus species being the most significant to marine ecosystems. These ectoparasites are found across the Northern Hemisphere, inhabiting the full-salinity, temperate coastal waters of the North Atlantic and North Pacific Oceans. They are a naturally occurring parasite of wild salmonid populations, but their numbers become a major problem in areas of intensive aquaculture.
High concentrations of these parasites occur primarily in regions with large-scale salmon farming, such as the fjords and sea lochs of Norway, Scotland, Ireland, and the coast of British Columbia. The densely stocked net pens used in commercial farming create an unnaturally high concentration of hosts, which fuels the rapid reproduction and proliferation of the parasites. This elevated parasite load in the immediate vicinity of the farms then poses a threat to migrating wild salmon and sea trout populations.
The life cycle of the salmon louse includes two free-swimming larval stages, called nauplii, which drift passively in the water column before developing into the infective copepodid stage. Once a copepodid encounters a suitable host, it attaches to the fish’s skin, fins, or gills using a specialized structure. As the parasites mature into pre-adult and adult stages, they become mobile, typically grazing on the host’s head, back, and perianal areas. While L. salmonis is largely restricted to salmon and related species, Caligus species are more generalist parasites that can infest a wider variety of marine fish.
Sea Lice Causing Human Irritation: Coastal Location and Seasonality
The condition commonly called “sea lice” by swimmers, or seabather’s eruption, is caused by the larval stage of cnidarians, most frequently the thimble jellyfish (Linuche unguiculata). These tiny larvae inhabit warm, tropical, and subtropical waters across the globe. The areas where human encounters are most common include the coastal waters of Florida, the Gulf of Mexico, and the wider Caribbean Sea, including locations like Bermuda.
The larvae are planktonic, drifting near the surface in the upper layers of the water column, often concentrated by wind and surface currents in shallow coastal areas. Swimmers encounter them when they become trapped against the skin, particularly in areas covered by swimwear, hair, or wetsuits. The friction from the swimsuit fabric or the osmotic change when a person exits the saltwater and dries off triggers the microscopic stinging cells, called nematocysts, to fire.
These outbreaks of seabather’s eruption are highly seasonal, coinciding with the reproductive bloom of the adult jellyfish populations. The period of highest risk for swimmers is typically from late spring through the summer months, spanning roughly from March or April to August. The peak incidence of human irritation frequently occurs in May and June when the larval concentration is at its highest in the nearshore environment. A lack of adult jellyfish in the area does not guarantee safety, because the larvae remain active in the water after the adults have died following spawning.
Environmental Conditions Governing Population Density
The presence and concentration of both the fish parasite and the human irritant are governed by a few common, overarching environmental factors. Water temperature is perhaps the most influential variable, as it dictates the life cycle duration for both marine organisms. For the parasitic copepods, rising temperatures accelerate development, meaning they reach the infective stage and reproductive maturity faster, increasing population turnover and density.
Both types of larvae are planktonic, and their dispersal and congregation are heavily influenced by local hydrodynamics, including tides, currents, and flushing rates. Strong ocean currents can transport the larvae of the fish parasite over long distances, connecting farm sites and wild populations. Similarly, surface currents and wind-driven action concentrate the jellyfish larvae in the warm, nearshore waters where swimmers are present.
Salinity is another ecological factor, particularly for the fish parasite, which thrives in full-salinity seawater. Low salinity, often found in estuarine mixing zones, is detrimental to the free-swimming larval stages of the copepod. The vertical stratification of the water column, where freshwater sits atop denser saltwater, can also affect the distribution and movement of the larvae as they seek optimal conditions for survival and host detection.