AMMONIA TOXICITY IN CULTURED LARVAE OF THE AMERICAN LOBSTER (Homarus americanus) 1

Ammonia is the chief nitrogenous excretory product of lobsters and is known to be toxic to these crustaceans at relatively low concentrations. The primary purpose of this study was to determine tolerance limits of larval Homarus americanus to ammonia in order to optimize design of the seawater circulation and waste treatment components of a lobster mariculture system. This is often a critical problem in culture systems in which a major part of the seawater is recycled. The incipient median lethal concentration (LC 50 ) and tentatively safe ammonia concentration were determined for fourth stage larvae of H. americanus to be 1.4 and 0.14 mg/liter un‐ionized ammonia‐nitrogen (NH 3 ‐N), respectively. The effects of pH, temperature, and salinity on ammonia toxicity also were evaluated. Increasing pH from 7.4 to 8.4 increases toxicity markedly. Increasing temperature from 21°C, near the optimum for growth, to 25°C increases toxicity only slightly. Changes in salinity from 31 to 36 ppt produce no appreciable effect. These increase in pH or temperature shift the NH 3 /NH+ 4 equilibrium unfavorably toward the more toxic NH 3 negligible effect on the equilibrium. A simplified steady‐state equation was developed for predicting the optimum biomass loading for a lobster culture system, based on the ammonia toxicity determined for fourth stage larval lobsters. This equation relates the ammonia tolerance limit, ammonia excretion rate, ambient ammonia concentration of the water source, water flow rate, degree of water reuse, and efficiency of ammonia removal. The time lag required for the culture system to attain steady‐state conditions also is considered.