WWW.WAS.ORG • WORLD AQUACULTURE • SEPTEMBER 2014 39 (Fig. 2) and images were enhanced for sharpness, brightness, and contrast using Microsoft Office Picture Manager. X-ray examinations indicated the effectiveness of sorting larvae with saline solution (Table 3). By preserving larvae with a 4 percent saturated formalin solution, the swim bladder cavity was preserved or deformation was minimal and did not affect the validity of sorting using salt solution. Results and Discussion Larval mortality. Analysis of postembryonic development identified three major peaks in larval mortality. The first occurred at the transition to exogenous feeding, approximately 10 dph, at mean body lengths of 7-12 mm. Mortality was as great as 99 percent, primarily the result of a lack of feeding and starvation, especially in larvae fromVelke Mezirici, Czech Republic in 2012. The second period of high mortality occurred at swim bladder inflation (14-42 dph) at mean body lengths of 10-25 mm. Up to 86 percent of larvae did not inflate their swim bladder. Maximum mortality was 17-25 percent in populations with a high number of larvae with noninflated swim bladders (populations C, D, F, I, J, K). The third peak in mortality occurred with the emergence of cannibalism at 6-7 weeks post-hatching, at mean body lengths of 20-25 mm. Direct mortality was 20-50 percent and indirect mortality, resulting from damage caused by mutual attacks, was 10-20 percent. Larvae were sorted after 4-5 weeks to reduce size heterogeneity and to select and rear separately larvae without a functional swim bladder and those prone to cannibalism. As a result, the difference in survival among different populations was minimal, confirming the efficacy of the sorting methods. Growth rate analysis - 2011. Larvae at 14 dph had an overall mean weight of 5.5 mg (Fig. 3), ranging from 4.2 mg in the population from Poland (Masurian Lakes) to 6.6 mg in the population from Velke Mezirici, Czech Republic. The overall mean weight of 56 day-old larvae was 319 mg, ranging from 258 mg in a Polish population (Olzstyn) to 405 mg in a Czech population (Velke (CONTINUED ON PAGE 40) FIGURE 2. Radiovisiographic examination of sorted larvae using Gendex Expert. TABLE 3. Radiographic images of perch larvae (photo: Pimakhin). SBN = swim bladder not inflated; SBI = swim bladder inflated. SBN SBI SBN SBI 14 days post-hatching 42 days post-hatching 28 days post-hatching 56 days post-hatching
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