Using a method similar to that used to estimate wild finfish numbers, we estimate that 390-1,000 (midpoint 700) billion1 molluscs were slaughtered in recorded global aquaculture production in 2022 as follows:
- 360-900 billion bivalves
- 31-95 billion unnamed molluscs
- 4.1-8.8 billion gastropods.
Details of the estimate are available by species and by country. The most numerous species include the Japanese carpet shell (Ruditapes philippinarum). Most farmed molluscs were reared in China. EU27 countries combined accounted for 13-25 billion1 molluscs, of which 85% were sea mussels (Mytilidae).
Note that these estimates exclude on-farm mortalities. A review by Lupo et al. (2021) found that mortality risk in blue mussel (Mytilus edulis) and Mediterranean mussel (Mytilus galloprovincialis) varied across the seasons, increased with an elevated seawater temperature above a thermal threshold of 20 and 24°C respectively, decreased with protecting mussels from predation, and was associated with the presence of pathogens in blue mussel. Climate change foretells negative effects on mussel production, including through rising sea temperatures (Soliño & Figueras 2025), potentially increasing mortality numbers. These estimates also exclude mortalities of young mussels, spat, occurring during collection from the wild and in hatcheries.
These estimates also exclude farmed cephalopod molluscs, i.e. octopus, the farming of which is currently experimental and raises huge new animal welfare concerns (Lara 2021, Schnell et al. 2022), since no farmed cephalopod production tonnages are reported by FAO for 2022 (FAO 2025).
Implications for animal welfare
The rearing of bivalves like mussels and oysters is arguably more welfare-friendly than finfish and decapod aquaculture (Jacquet 2017), particularly when cultured on ropes suspended under the water rather than on the seabed (which involves dredging), due to their being generally sessile (semi-permanently attached) and filter-feeders. However, after ‘harvesting’ they are eventually killed by boiling or cooking (FAO 2009) which, if bivalves experience pain, may cause intense suffering.
In the wild, mussels attach themselves to rocks or other mussels using strong byssal threads produced from a gland located within their foot (ASC 2010-2025), which is also used for occasional locomotion (Deep Look 2023). In farming, mussel spats attach themselves to ropes, which would appear to offer a natural life that allows motivated normal behaviours.
Unlike farmed finfishes and decapods, which consume large numbers of inhumanely caught wild fishes as feed (Mood & Brooke 2024), bivalves do not require feed inputs (Naylor et al. 2021) and feed entirely off very small, naturally occurring, organisms in the water. In addition to phytoplankton, these include zooplankton and other tiny animals (Davenport et al. 2000), possibly numbering quadrillions annually2.
While there is sufficient scientific evidence to conclude that finfishes (Sneddon et al. 2014, Sneddon & Leach 2016), decapod crustaceans (Birch et al. 2021, Crump et al. 2022), cephalopod molluscs (Birch et al. 2021) and certain groups of insects (Gibbons et al. 2022) are sentient, there are almost no studies on bivalve sentience (de Souza Valente 2024). Scallops (free swimming bivalves) and clams have simple eyes and chemosensory organs, and they initiate escape swimming if a threat is detected, thus some integration of information and basic decision making occurs (Crook et al. 2011). Queen scallops (Aequipecten opercularis) have demonstrated increased cardiac activity when threatened by a predator (starfish) if they are unable to find refuge compared to when a refuge was available or the predator was absent (Kamenos et al. 2005). Mussel defensive responses to the presence of predators include producing more byssal threads to reduce risk of dislodgement by dexterous crabs (Garner & Litvaitis 2013), and trapping dogwhelks with byssal threads, leaving them immobilised (Petraitis 1987, Farrell & Crowe 2007). There is some evidence that mussels can ‘learn to fear’ their parasites, from a study by Selbach et al. (2022). In this study, blue mussels that had previously experienced parasitic load reduced their filtration rate when parasites later reappeared, compared to parasite-naïve conspecifics.
Given the large numbers used for food, research into the sentience of bivalves should be given a high priority (AAC 2024). Meanwhile, on a precautionary basis, since they may have the capacity to suffer, and since their numbers are very large, there is an ethical need to develop welfare codes for mollusc farming and ‘harvesting’, to provide for welfare as far as possible.
Welfare requirements during rearing are likely to include the availability of clean, unpolluted and plankton-rich water and keeping stocking density at a level that avoids undue competition for food, as well as mitigating mortality.
Since many bivalves are sold live direct to the consumer, welfare during transport and killing may prove more challenging. It may be possible to develop methods of humane killing (e.g. with food-grade anaesthetics), especially for the production of pre-cooked mussels and other bivalves targeted at welfare-conscious consumers.
A. Mood and P. Brooke. 6th October 2025.
Notes
1. rounded to 2 significant figures.
2. Davenport et al. (2000) estimated that a medium sized (24 to 47 mm shell length) blue mussel inhales 120 small animals per day in spring. Assuming this is typical for a farmed mussel’s lifespan, and that none survive even if they are not digested, then based on this and an estimated lifespan of 6-24 months (ASC 2010-2025), each mussel kills between 22 and 88 thousand animals. If this consumption is typical for all farmed bivalve molluscs, then our estimated 360-900 billion1 bivalves ‘harvested’ globally in 2022 consumed between 7.8 and 79 quadrillion tiny creatures i.e. 7.8 x 1015 to 7.9 x 1016 individuals.
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