From Ocean to Table: Everything You Need to Know About Mussels

September 25, 2024

From Ocean to Table: How Mussels Are Farmed, Harvested, and Delivered

Mussels are consistently rated as one of the most sustainable animal protein sources available — the Monterey Bay Aquarium’s Seafood Watch rates farmed mussels as a "Best Choice," and a 2021 analysis in Nature Food identified bivalves (mussels, oysters, clams) as among the most climate-friendly animal proteins. But what does mussel farming actually involve? How do mussels go from a microscopic larva to the shellfish on your plate? This guide covers the complete mussel farming process — from spat collection through harvest — and explains why the farming method is responsible for mussels’ exceptional sustainability profile.

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The Mussel Life Cycle

Understanding mussel farming starts with the mussel’s natural life cycle:

• Spawning: adult mussels release eggs and sperm into the water column; fertilization occurs in the water; spawning is triggered by water temperature changes and typically occurs in spring and fall
• Larval stage (2–3 weeks): fertilized eggs develop into free-swimming larvae (called veligers) that drift in the water column; at this stage they are microscopic and feed on phytoplankton
• Settlement (spat stage): after 2–3 weeks, the larvae settle onto a surface and attach using their byssal threads (the "beard"); at this stage they are called spat; they are approximately 1mm in size
• Juvenile to adult (12–18 months): mussels grow from spat to market size (typically 2–3 inches) over 12–18 months, depending on water temperature, food availability, and species; in warmer waters, growth is faster; in colder waters, growth is slower but the resulting mussel is often plumper and more flavorful
• Harvest: mussels are harvested at market size; the timing is managed to avoid the spawning season (when mussels are thin and watery) and to coincide with peak condition (fall through spring)

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How Mussel Farms Work: The Longline System

The dominant mussel farming method in the US, Canada, Europe, and New Zealand is the suspended longline system. Here is how it works:

1. Spat Collection

Mussel spat (newly settled larvae) are collected in one of two ways: (1) Wild spat collection: collectors (typically rough-textured ropes or mesh tubes) are suspended in the water during the spawning season; wild mussel larvae settle onto the collectors naturally; this is the most common method and requires no hatchery; (2) Hatchery-produced spat: some farms use hatchery-produced spat for more consistent supply and timing; adult mussels are induced to spawn in controlled conditions; the larvae are raised in tanks until they reach the spat stage, then transferred to the farm.

2. Seeding the Grow-Out Ropes

Once spat are collected, they are transferred to grow-out ropes — long ropes (typically 10–30 feet) suspended vertically from a horizontal mainline anchored at the surface. The spat attach to the grow-out ropes using their byssal threads. The ropes are then suspended in the water column at the optimal depth for food availability and temperature.

3. Growing Out

The mussels grow on the suspended ropes for 12–18 months. During this time:

• No feed is required: mussels are filter feeders; they feed by drawing water through their bodies and filtering out phytoplankton and organic particles; the farm provides no feed, fertilizer, or chemicals — the mussels eat what the ocean provides
• Thinning: as the mussels grow, the ropes may become overcrowded; farmers periodically thin the ropes by removing some mussels and reseeding them onto new ropes; this gives the remaining mussels more space and food access and produces larger, better-conditioned shellfish
• Monitoring: farmers monitor water quality, mussel condition, and growth rate throughout the grow-out period; water quality testing is required by regulatory agencies in most jurisdictions

4. Harvest

When the mussels reach market size (typically 2–3 inches), they are harvested by pulling the grow-out ropes from the water. The mussels are stripped from the ropes mechanically or by hand, cleaned, sorted by size, and packed in mesh bags or boxes. The entire process from rope to packing typically takes a few hours. Mussels are then shipped live to distributors and retailers.

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Why Mussel Farming Is Exceptionally Sustainable

The longline farming method is responsible for mussels’ exceptional sustainability profile:

• No feed inputs: mussels require no fishmeal, fish oil, grain, or any other feed; they eat phytoplankton filtered from the surrounding water; this is in stark contrast to farmed salmon or shrimp, which require significant feed inputs (often including wild-caught fish)
• No freshwater use: mussel farming requires no freshwater inputs; most terrestrial animal agriculture requires large amounts of freshwater for drinking, cleaning, and crop irrigation
• No land use: mussel farms are located in coastal waters; they require no agricultural land
• Water quality improvement: mussels actively improve water quality by removing excess phytoplankton, nitrogen, phosphorus, and suspended particles from the water column; mussel farms have been shown to reduce eutrophication (algal blooms caused by excess nutrients) in surrounding waters
• No habitat destruction: the longline system has minimal impact on the seafloor; no dredging or trawling is involved; the ropes are suspended in the water column and do not contact the bottom
• Low carbon footprint: mussels have one of the lowest greenhouse gas emissions per gram of protein of any animal food; the 2021 Nature Food analysis found that bivalve aquaculture produces approximately 0.6 kg CO₂ equivalent per 100g of protein — compared to approximately 25 kg for beef and 6 kg for farmed salmon
• Biodiversity support: mussel farm structures (ropes, buoys, anchors) provide habitat for other marine organisms; mussel farms often have higher local biodiversity than surrounding open water

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From Farm to Your Table: The Supply Chain

After harvest, mussels move through the supply chain quickly — they are live animals and must be kept alive until cooking:

• Harvest to packing: same day; mussels are cleaned, sorted, and packed in mesh bags or boxes immediately after harvest
• Packing to distributor: 1–2 days; mussels are shipped live in refrigerated trucks or containers; they are kept moist but not submerged (they need to breathe)
• Distributor to retailer: 1–2 days; mussels arrive at fish markets, grocery stores, and seafood suppliers
• Retailer to consumer: mussels should be cooked within 1–2 days of purchase; the total time from harvest to your table is typically 3–5 days for domestic mussels

This is why freshness matters: a mussel that was harvested 5 days ago and has been in transit is at the end of its viable window; a mussel harvested 1 day ago is at its peak. When buying mussels, ask your fishmonger when they were harvested or received.

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For buying, storing, and cleaning mussels: Mussel Preparation Guide

🎥 Watch mussel preparation guides on the Global Seafoods YouTube Channel

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FAQs: Mussel Farming and Sustainability

Are farmed mussels better than wild mussels, and what is the difference?

For most consumers, farmed mussels are the better choice for several reasons: (1) Consistency: farmed mussels are grown in controlled conditions and are more consistent in size, condition, and cleanliness than wild mussels; (2) Cleanliness: farmed mussels grown on suspended ropes are not in contact with the seafloor and accumulate less grit and debris than wild mussels that attach to rocks and pilings; (3) Sustainability: farmed mussels are rated "Best Choice" by Seafood Watch; wild mussel harvesting varies in sustainability depending on the method and location; (4) Availability: farmed mussels are available year-round from consistent sources; wild mussels are seasonal and location-dependent. Wild mussels can be excellent — particularly in areas with very clean, cold water — but they require more cleaning and are less consistent. For home cooking, farmed mussels are the practical and preferred choice.

How do mussels filter water, and does this mean they accumulate pollutants?

Mussels feed by drawing water through their bodies using cilia (tiny hair-like structures) and filtering out phytoplankton and organic particles. An adult mussel can filter approximately 1–2 gallons of water per hour. This filtering process removes excess nutrients, algae, and suspended particles from the water — which is why mussel farms improve water quality in surrounding areas. However, the same filtering process means that mussels can accumulate pollutants (heavy metals, bacteria, biotoxins) if they are grown in contaminated water. This is why: (1) commercial mussel farms are located in clean, monitored coastal waters; (2) regulatory agencies test commercial mussels for bacterial contamination, heavy metals, and paralytic shellfish toxins (PSP) before they reach the market; (3) commercial mussels from reputable suppliers are safe to eat; (4) wild mussels harvested from unmonitored areas (harbors, industrial coastlines) should be avoided. Always buy commercial mussels from reputable suppliers — they are tested and certified safe.

What is the carbon footprint of mussels compared to other protein sources, and how does mussel farming affect the environment?

Mussels have one of the lowest carbon footprints of any animal protein. The 2021 Nature Food analysis found that bivalve aquaculture (mussels, oysters, clams) produces approximately 0.6 kg CO₂ equivalent per 100g of protein — compared to approximately 25 kg for beef, 6 kg for farmed salmon, 3.5 kg for chicken, and 1.5 kg for pork. The reasons: no feed inputs (no land, water, or energy required to grow feed); no freshwater use; no land use; low energy requirements for farming and processing. Environmental effects of mussel farming: (1) Positive: water quality improvement (nutrient removal), biodiversity support (farm structures provide habitat), no seafloor disturbance; (2) Potential negative: very high-density farms can locally deplete phytoplankton, which may affect other filter feeders in the immediate vicinity; this is managed through farm siting and density regulations. Overall, mussel farming is one of the most environmentally beneficial forms of animal food production available.

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