Jassa marmorata is a small tube-dwelling amphipod, 5–10 mm in body length, that constructs mud tubes from fine sediment, organic particles, and a silk-like secretion. On aquaculture nets, these tubes form dense, interlocking mats — sometimes 10–20 mm thick — that coat the mesh surface. The species has a broad distribution across the North Atlantic and Mediterranean, and it thrives in the nutrient-enriched, particle-laden water around fish farms. Populations build rapidly during summer, with females producing multiple broods of 15–30 juveniles over a season.
Jassa mats are among the most effective fouling types at blocking water exchange through cage nets. Even a 10 mm mat can reduce flow through standard mesh by 50–70 %, causing measurable drops in dissolved oxygen inside the pen. The trapped organic matter within the tubes also promotes bacterial growth and can produce localised hydrogen sulphide odours. Fish held in heavily Jassa-fouled cages show reduced feed intake and slower growth — an impact worth quantifying with the cost calculator. Jassa fouling is often associated with prior colonisation by hydroids or filamentous algae, which provide the initial surface complexity the amphipods prefer.
Net cleaning machines with rotating discs or high-pressure nozzles strip Jassa mats effectively, though the fine mud released into the water column can temporarily affect visibility and gill function in caged fish. Scheduling cleans for slack tide minimises sediment issues. Regular net changes every 3–4 weeks during the summer peak prevent heavy mat accumulation. Because Jassa is a secondary fouler, controlling the primary colonisers it depends on also reduces amphipod densities — see the methods comparison for integrated strategies. Explore other small crustacean foulers like the skeleton shrimp in the organisms database.
Control Methods
Net cleaning machines High-pressure washing Regular net changes