Objective  Herbicides applied to control Spartina alterniflora with minimum collateral damage to fish and mangroves were tested for aquacultural application.

Method Field control efficacy on S. alterniflora of 8 herbicides were determined by absolute value investigation method. Effective herbicide was selected with spray concentration determined which were used subsequently in a semi-static toxicity test on large yellow croakers (Larimichthys crocea) and zebrafish (Danio rerio) as well as in a pot experiment to monitor the photosynthetic and growth of mangroves (Kandelia obovata).

Result The 25% imazapyr aqueous solution sprayed at the rate of 18 750–26 250 mL·hm⁻² was found most effective for the weeding, with the reductions of 88.43%–92.62% on weed count and 87.98%–93.62% on fresh weed weight in 45 d. The 96 h median lethal concentration LC₅₀ of the active herbicide ingredient on L. crocea was 1.32 mg·L⁻¹, and 6.78 mg·L⁻¹ on D. rerio. An inhibition rate ranging from 25.38% to 49.50% on the leaf photosynthetic rate, plant height, aboveground fresh weight, belowground fresh weight, chlorophyll content, and root activity was shown on mangrove plants 28 d after herbicide spraying. The field control efficacy of the other seven herbicides against S. alterniflora was significantly lower than that of imazapyr.

Conclusion Spraying 25% imazapyr aqueous solution at a rate of 18 750–26 250 mL·hm⁻² could effectively control the spread of S. alterniflora on a farming field. The application was moderately toxic to L. crocea and D. rerio and inhibitory to the photosynthesis and growth of K. obovata. Hence, in practice, it needed to be cautiously implemented with proper supervision.