Continuous egg separation of the copepod Acartia tonsa. Implications for increasing adult density at an intensive level

Abstract

The concept of continuous egg and adult separation was studied to evaluate the possibility of cultivating the copepod Acartia tonsa at high density (> 2.5 adults mL−1). First experiment: Densities of 5, 10, 15, 30, and 45 adults mL−1 were evaluated. For this, eggs spawned were allowed to settle quickly, harvested, and counted to determine the specific and relative egg production (SEP and REP, eggs female−1 day−1 and eggs mL−1 day−1, respectively). Second experiment: Mature females were spawned on slides to observe the shape of the freshly spawned eggs. Third experiment: respiratory rate under four corporal velocities in the filter system were evaluated (57, 115, 173 and 231 PL min−1; PL, prosome length), corresponding to four water exchange rates (50, 100, 150 and 200 volumes day−1). Fourth experiment: A high-density (7 adults mL−1) prototype based on continuous egg separation was designed and compared with daily egg harvest for siphon (control system). The specific egg production (SEP, egg female−1 day−1) and relative egg production were determined (REP, eggs mL−1 day−1). The SEP decreased with culture density (p < .05), but quickly egg separation allowed a high REP when a density of 10 adults mL−1 was used. Freshly spawned eggs were elongated for the first 7 min, which suggests fragility when freshly spawned. Water velocity increased adult respiratory rate (p < .05), and egg production reached a maximum at 100 volumes per day (p < .05). The above suggested a great potential to increase egg production at a high adult density (>2.5 mL−1), which was confirmed by higher REP in the prototype (100 eggs mL−1 day−1) than in the control system. This study contributes to intensive A. tonsa culture; a superior density of 5 mL−1 is possible with a high water exchange rate and careful egg harvesting.