Fish larvae produced in commercial hatcheries are typically not considered for welfare purposes due to assumptions about early nervous system development, but emerging research raises questions about this assumption.
The welfare status of fish larvae is genuinely uncertain. The development of nociceptive pathways in fish larvae occurs at specific developmental stages, and the question of when these become functional for pain experience is active research. Current hatchery practice assumes larvae are not welfare-relevant, but this assumption is not definitively supported by neuroscience. High-density larval rearing may cause physical injury from conspecific interactions. Nutritional deficiencies in larval fish cause skeletal malformations that impose lifelong welfare costs on survivors. Handling for grading and water quality stress cause physiological responses in larval fish consistent with stress, whether or not they involve conscious experience.