Piscicides are described as being what, requiring specific timing for application?

Piscicides are specifically designed to control fish populations and are often sensitive to environmental conditions, which significantly affects their efficacy. When describing piscicides as photochemically degraded, it highlights that their chemical structure and effectiveness can break down when exposed to light. This concept is important because it underscores the necessity for careful timing in their application. For instance, applying them during times of high sunlight may lead to faster degradation and, consequently, a reduced impact on the target fish species.

In contrast, the other options do not accurately reflect the nature and timing requirements of piscicides. For instance, being biochemically stable would indicate they remain effective over time regardless of light, while being thermally activated would suggest they only function under certain temperature conditions, neither of which are accurate for effective piscicide applications. Lastly, being environmentally persistent would imply that the chemicals remain in the environment without breaking down, which is contrary to the degradation that occurs when exposed to light, making it critical to apply the piscicides when conditions are optimal for their effectiveness.