Pre-slaughter mortality of farmed shrimp (Rethink Priorities)

Executive summary

• Mortality rates are high in shrimp aquaculture, implying welfare threats are common.
  • It is typical for ~50% of shrimp to die before reaching slaughter age.
  • This equates to around 1.2 billion premature deaths a day on average.
• Mortality varies among species; prioritizing interventions should take this into account.
  • Because of high pre-slaughter mortality (~81%), Macrobrachium shrimp represent a larger share of farmed shrimp than slaughtered shrimp.
  • Most individual deaths are P. vannamei, despite having the lowest mortality rate.
• More larvae die than any other life stage, but this does not necessarily mean efforts should focus on them.
  • Uncertainty remains about whether larval shrimp are sentient—they are planktonic, so do not make autonomous decisions.
  • Minimizing larval deaths could cause compensatory deaths in later life stages (e.g., ensuring weaker larvae survive, who then die from later harsh conditions).
  • Interventions should likely concentrate on the ongrowing stage (postlarval and juvenile–subadult shrimp), where there are still tens of billions of deaths.
• There are several causes of mortality and differences between farm types.
  • Most causes are likely a combination of intrinsic shrimp traits (e.g., young shrimp are sensitive to environmental fluctuations), farming practices, and diseases.
  • Disease is a main cause throughout life, but it is often a downstream effect of issues that farmers have some control over (e.g., poor water quality).
  • Variation among reported figures, especially that more intensive farms have fewer deaths, suggests many factors are at play and that some are controllable.
• The effects of reducing early mortality on industry trajectory are uncertain.
  • The number of shrimp born may decrease if farmers must produce fixed output. But shrimp would live longer, increasing the chances for negative experiences.
  • However, consumer demand has historically outstripped supply, so the industry may grow if it had better control of conditions causing mortality.
  • If reduced deaths come from intensification of practices, more shrimp may be reared in conditions that can harm welfare (e.g., high stocking densities).
• Pre-slaughter mortality cannot depict total welfare because it misses nonfatal effects.
  • Mortality is only a lower-bound proxy of how many shrimp suffer negative welfare.
  • Premature mortality is more appropriate as one indicator among many that a welfare reform was successful, rather than an end in itself.
• Pre-slaughter mortality data is limited and non-uniform.
  • Reports should clarify whether mass die-off events were excluded from mortality estimates and if rates are based on intuition from experience or empirical studies.