42 JUNE 2013 • WORLD AQUACULTURE • WWW.WAS.ORG similar to those of other Acartiidae species. Elevated temperature encourages embryogenesis, resulting in satisfying egg hatching success. If lower temperatures are used then more time is needed until the first nauplius appears, since the eggs are in quiescence (Hansen et al. 2010). Effect of Temperature on Acartia Population Growth After 3 days of acclimatization, 12 healthy, actively swimming adults with intact appendages (4 male and 8 female) were transferred to 600-mL beakers filled with 300 mL of seawater without aeration. Previous observations indicate that a complete life cycle is possible in 9-12 days. After 12 days, each beaker was drained through a 55-μm sieve and all retained eggs, nauplii, copepodites and adults were fixed in 4 percent formaldehyde and stored at 4 C. Counting of samples was made using a Sedgwick-Rafter counting cell and microscope. Population growth of A. grani was not significantly affected by temperature. Highest mean final population (663 ± 52) was observed for A. grani cultured at 18 C, followed by 640 ±75 individuals cultured at 28 C. The highest mean final population was observed at 18 C if all stages were considered. However, due to lower metabolic rates at 18 C, the adult population was only 8 ± 1 individuals at 18 C compared to 57 ± 29 adults at 28 C. Implications The historical background of cultivated copepods and therefore the standard reference temperature for eggs of Acartia grani is 18 C (Calbet and Alcaraz 1996, 1997, Guerrero and Rodriguez 1998, Costa and Fernández 2002). Moreover, 18 C seems to be favorable for leaving the dormancy phase (Guerrero and Rodriguez 1998). The expectation was that an increase in temperature (≤ 28 C) would improve hatching rates. Acartia grani eggs hatch when temperature is greater than 18 C (Guerrero and Rodriguez 1998). Within treatments, different levels of activity by hatched nauplii were observed. In first 24 hours, nauplii at 24 and 28 C were very active, whereas this was not seen in nauplii at lower temperatures. High temperatures increase metabolism (Hansen et al. 2010) and the first nauplii that hatch at the high temperatures were actively searching for food. After 35 hours of incubation, copepods at the lowest temperatures became active. Eggs used in this experiment had been stored for three months since collection. Although considerations could be made if fresh eggs were used, Drillet et al. (2006) showed that Acartia tonsa have a hatching rate of 70 ± 2 percent after three months of storage at 23 C. Similarly, in our study, cumulative egg hatching rates of 69 ± 5 percent at 24 C and 64 ± 4 percent at 22 C were obtained. Our results demonstrate that use of Acartia grani eggs stored for three months still present great potential for culture. In aquaculture, it is fundamental that live food is readily available for use as feed to fish larvae. If the time of copepod egg hatching can be reduced by elevating culture temperature, it is one of the many factors that will allow higher production to be obtained in less time. Based upon the results of this study, it is recommended that incubation temperature be greater than or equal to 24 C to obtain improved hatching rate. Notes Bernardo Sumares and Natacha Nogueira, Centro de Maricultura da Calheta, Governo Regional da Madeira, Vila da Calheta, 9370-133 Calheta.email: bernardosumares@gmail.com; phone: (+)351 291820420 Maria Emília Cunha, Estação Piloto de Piscicultura de Olhão, INRB/IPIMAR; Av.5 de Outubro, 8700-305 Olhão, Portugal FIGURE 3. Cumulative hatching rate over time for A. grani eggs stocked into three replicate containers (triangles, squares, circles) at each of four incubation temperatures.
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