The effect of freezing on the length and weight measurements of ruffe (Gymnocephalus cernuus)

The ruffe is a percid native to Europe and Asia that has invaded several new areas in the last several decades. For population studies ruffe are often frozen en masse after collection in the field and thawed at a later time for measurements of length and weight. I determined the effect of freezing and thawing on ruffe total length (TL) and weight (W) by making these measurements on freshly caught ruffe and ruffe that had been frozen and thawed. Frozen and thawed fish generally shrank by between 1.25% and 1.63% (95% CI) of their fresh TL and approximately 5–10% of their fresh W. Shrinkage in W   increased as the length of time being frozen increased. The loge(W)loge(W)– loge(TL)loge(TL) relationship differed between fresh and thawed ruffe. Relative underestimation of actual fresh W   using thawed TL and a loge(W)loge(W)– loge(TL)loge(TL) relationship constructed from thawed fish was 7.50%. All observed shrinkage amounts were substantially different than within-technician measurement variability. These results suggest that fresh specimens should be used if it is important to understand the true fresh measurements of ruffe. In the absence of being able to do this, (provided) corrections for errors due to thawing should be used.