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Body length measurements and growth rate (GR) analysis are important steps in research on the ecology of early development stages of fish. In order to obtain accurate results, the lengths of field‐collected and preserved larvae must be corrected for shrinkage. Northern Pike Esox lucius larvae were kept in the laboratory in freshwater conditions (300‐L tank; temperature = 16°C), where they were fed six times a day ad libitum with aquaculture feed. Every 3 d over a period of 30 d, a subsample of approximately 30 larvae was collected, measured, and placed in separate vials in 96% alcohol and 4% formalin. All of the larvae (8.8–34 mm SL; n = 276) were measured again after 90 d of preservation. The shrinkage ranged from 0% to 11.6%, averaging 3.42% in alcohol (n = 134) and 3.74% in formalin (n = 139). The difference was not statistically significant. In both preservatives, the relative shrinkage (%) was higher for small fish than for large fish. The length correction for 8–34‐mm SL fish in both alcohol and formalin should be done by adding 0.7 mm to the length measured after preservation. Shrinkage did not affect fish GRs estimated based on linear regression fitted to size‐at‐age data (0.645 mm/d for unpreserved larvae and 0.650 mm/d for preserved larvae). In the opposite case, when growth was estimated as mean GR between hatching and sampling, the shrinkage effect was significant: the mean GR was 0.604 mm/d for unpreserved larvae and 0.558 mm/d for preserved larvae. The presented shrinkage correction formula allows for obtaining accurate length and GR data for larval Northern Pike that should lead to conclusions that credibly support undertaking appropriate management strategies.

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Cannibalism can be a significant factor limiting the survival of pike, Esox lucius, early life stages in both natural and aquaculture conditions. In the present research, type I cannibalism was studied in pike larvae (size range: 9-32.5 mm standard length, SL) in controlled conditions for 45 days post-hatching (dph). For this purpose, the occurrence of cannibalistic pairs, i.e. prey caught by a cannibal inside the cannibal's mouth, was monitored for 15 min periods by two observers six times daily. Cannibalism was not observed in fish that still had yolk-sacs, but it did occur as soon as the yolk-sac was absorbed on 5 dph at a body size of 13 mm at 22°C and 10 dph at a body size of 13.4 mm at 15°C. The ratio of prey to predator size was on average 87%, but the smallest observed size difference was only 1.3%. The intensity of cannibalistic attacks depended on water temperature with more intense cannibalism at 22°C than at 15°C, and no cannibalism at 10°C. Fluctuating asymmetry (FA) in all three sagittal otolith size parameters (area, width, and length) was higher in prey than in cannibals, the differences were not statistically significant.