Fishing Net Mesh by Fish Size

Fishing Net Mesh Size by Target Fish

You measure mesh size in millimetres, knot to knot, with the mesh pulled flat. The handy selectivity rule most people fall back on is mesh_mm ≈ fish_length_cm × 2. A net holds onto fish whose girth roughly lines up with the stretched mesh, while smaller ones slip through and live long enough to spawn.

In Brazil, IBAMA sets the legal minimum mesh sizes, and they change with species and state. A few examples: corvina needs a minimum stretched mesh of 60 mm in São Paulo coastal waters, tilapia in Minas Gerais reservoirs asks for at least 30 mm, and for tambaqui in the Amazon the floor climbs to 100 mm. During the closed season (período de defeso), net fishing of certain species is off-limits while they spawn. IBAMA and the Polícia Militar Ambiental (PMA) handle enforcement, with fines and gear seizure waiting for anyone caught fishing without authorisation.

Picking the right gear is what keeps a fishery sustainable. Arrasto (trawl) tears up the bottom and is heavily restricted. Espinhel (longline) goes after pelagic species with hooks, which makes it more selective. Tarrafa (cast net) is the traditional hand-thrown option, with its diameter and mesh regulated. And rede de espera (gillnet) sits in place and sorts fish by mesh size. Before you put any net in the water, check the current IBAMA Normative Instructions.

Applications

Use this calculator to size your mesh for a target species without stepping over the legal limits. Licensed artisanal fishermen rely on it, so do fish farms working out pond harvests and researchers running selective sampling. Whatever the result, cross-check it against the IBAMA rules in force for your state and water body.

FAQ

Is mesh measured between knots or full diamond? Brazilian regulations go by the stretched-mesh measurement taken between knot centres, with the diamond pulled taut.

Can I use any net in my own fish pond? For the species you're actually farming, private aquaculture is off the hook on minimum-mesh rules. You'll still need a SEAP/Mapa licence and an environmental permit (LO), though.

What is the penalty for sub-mesh nets? Fines open at R$ 700 per kilogram of illegal fish. On top of that you lose the net, and you may face criminal charges under Lei 9.605/1998.