40 SEPTEMBER 2014 • WORLD AQUACULTURE • WWW.WAS.ORG Mezirici). Minimum size at hatching of perch differed significantly by population (Duncan’s test, p<0.05). Growth rate analysis - 2012. At 14 dph, differences in larva weight were significant (ANOVA p<0.001). Mean weight of all larvae at 14 dph was 5.3 mg (Fig. 4), ranging from 3.8 mg in a Polish population (Zator) to 6.7 mg) in the Slovak population (Strbske Pleso Lake). At the conclusion of the rearing period, weight differences among populations were significant (Duncan’s test, p<0.05). The mean weight of larvae at 56 dph was 331 mg, ranging from 238 mg in the Polish population (Zator) to 465 mg in the population from Finland (Lake Vajkjarvi). In 2012, there was a positive correlation of individual egg weight with larva weight. The minimum egg weight (3.4 mg) was in the Polish population (Zator) and the maximum egg weight (8.6 mg) was in the Finnish population (Lake Haukajarvi). However, this effect cannot be confirmed definitively because eggs were obtained from natural sources and the time of spawning was not known with certainty. It is possible that water absorbed by fertilized eggs contributed to their weight. Nonetheless, these data should be taken into consideration for selection of eggs for perch rearing. Older and larger females have greater fertility but produce smaller eggs. The diameter of perch eggs is in the range 2-2.5 mm, with a minimum of 1 mm (Reshetnikov et al. 2003). The use of small eggs for aquaculture should be avoided. Swim bladder inflation. The average proportion of the larval population with non-inflated swim bladders at 14 dph was 63 percent in 2011 (Fig. 5) and 67 percent in 2012 (Fig. 6). At 56 dph, the average proportion of the larval population with non-inflated swim bladders fell to 11 percent in 2011 and 28 percent in 2012. The proportion of the larval population with non-inflated swim bladders in 2011 was greatest for two populations from Poland (Masurian Lakes, Olsztyn Lake District) and was associated with slower growth (Fig. 5). The proportion of the larval population with non-inflated swim bladders in 2012 was also greatest in the Polish population (Zator) (Fig. 6). These populations differed from other groups in the minimum larval size at hatching. This supports the mechanism that small larvae are unable to overcome surface tension of water. The maximum decrease in the proportion of larvae with swim bladder non-inflation was observed between 28 and 42 dph. Therefore, it is possible that swim bladder inflation is a longer duration process than reported by Summerfelt (1996). The process could also be affected by cannibalism, which starts during this period. Conclusion The proportion of larval populations with swim bladder inflation is associated with larval size. A greater proportion of larvae with inflated swim bladders occurred in populations with larger larvae (Figs. 5 and 6). Recommendations to prevent swim bladder non-inflation include: • Select larger eggs for use in perch culture, with a preference for those from mid-age and mid-size females. • Use a water surface spray to clear the water surface of oil and debris or provide additional water flow to larval rearing tanks. • Sort larvae using anesthetics and a salt solution to produce “clean” larval populations without sinker syndrome, which increases feeding efficiency and improves productivity. In our laboratory, we have received groups of fish with noninflated swim bladders that were grown to market size. The growth rates and physiology of such fish are topics for further study. Notes Aleksey Pimakhin , University of South Bohemia in Ceske Budejovice, Faculty of Fisheries and Protection of Waters, South Bohemian Research Center of Aquaculture and Biodiversity of Hydrocenoses, Zatisi 728/II, 389 25 Vodnany, Czech Republic; Jakub Žák, Charles University in Prague, Faculty of Science, Department of Zoology, Viničná 7, 128 44 Praha 2, Czech Republic 1 OceanNutrition Artemia Cysts 2 Gendex Expert LEFT, FIGURE 3. Mean weight of Eurasian perch larvae, 2011. Population designation provided in Table 1. RIGHT, FIGURE 4. Mean weight of Eurasian perch larvae, 2012. Population designation provided in Table 1.
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