Overcrowding in Fish Farms

Stocking density in ponds is crucial for fish welfare in aquaculture, directly impacting water flow, quality, and feeding. For rainbow trout, overcrowding causes multiple health issues primarily due to stress.

A 2020 BMC Veterinary Research study examined crowding-induced stress effects in rainbow trout. High-density conditions elevate cortisol levels, negatively impacting growth. Chronic stress activates metabolic mechanisms that degrade muscle proteins and reduce protective enzyme activity, causing muscle loss and weight reduction. Immune systems are compromised, increasing disease susceptibility.

Other overcrowding consequences include higher rates of physical injuries like fin erosion (increasing pathogen infection risk), reduced feeding motivation (increasing food waste), and stereotypical movements.

Higher population density worsens water quality. More fish in the same water volume reduces oxygen availability and increases un-ionized ammonia levels due to higher oxygen consumption and waste excretion. A Council of Europe Recommendation notes ammonia accumulation can be prevented partly by reducing stocking density.

Overcrowding also increases aggression, causing fin damage, scale loss, and prolonged stress. Dominant fish often block subordinates from food access, worsening group conflicts and stress.

Sources:

• MisPeces. (2023). Chilean researchers describe effects of overcrowding stress in rainbow trout and muscle weakening.
  Retrieved from https://www.mispeces.com/noticias/Investigadores-chilenos-describen-los-efectos-del-estres-por-hacinamiento-en-trucha-arcoiris-y-sus-efectos-en-el-debilitamiento-muscular/
• Yousefi, M., Hoseinifar, S. H., Soleimani, N., & Van Doan, H. (2020). Stress mitigation by dietary supplements in aquaculture species: Effects on immunity and growth in rainbow trout under high-density farming conditions. BMC Veterinary Research, 16, 1-12.
  Retrieved from https://bmcvetres.biomedcentral.com/articles/10.1186/s12917-020-02518-w
• North B.P., Turnbull J.F., Ellis T., Porter M.J., Miguad H., Bron J. & Bromage N.R., (2006). The impact of stocking density on the welfare of rainbow trout (Oncorhynchus mykiss). Aquaculture 5, 466-479.
• Recommendation concerning farmed fish of the Standing Committee of the European Convention for the protection of animals kept for farming purposes, Article 12.