Effect of Preservation in Formalin and Alcohol on the Growth Rate Estimates of Larval Northern Pike

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 GR s 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.