Effects of Storage Time and Temperature on Subsequent Wet Weight and Percent Dry Weight of Fundulus heteroclitus

Abstract

Fisheries monitoring and research often requires preservation of fish samples for later analysis in the laboratory. Freezing is a common method, but it can affect subsequent fish size and composition, and the interpretation of metrics based on them. We evaluated the effects of freezing on fish wet weight, dry weight, and the percentage dry weight (a measure often used to estimate energy density). Specifically, we quantified changes in the weight of individual mummichogs (Fundulus heteroclitus) frozen at different temperatures (−20°C and −80°C) for 1–3 months. In addition, a temperature change treatment where fish were initially frozen at −80°C for one month before transfer to −20°C for an additional 1–3 months was included to simulate protocols typically used at sea. Individual wet weights were measured before freezing, and after freezing, the wet weights, dry weights, and percentage dry weights were determined. All freezing treatments resulted in minor but significant (p < 0.05) water loss. While the change was measurable and directional, it did not have a significant effect on the percentage dry weight except for samples held at −20°C for 3 months. A similar increase in percent dry weight was not evident in the temperature change group that was also stored at −20°C for 3 months. The least amount of weight loss, and therefore bias, was in the −80°C group, followed by the temperature change group, with the −20°C treatments losing the most water. The results indicate that if specimens for percent dry weight estimation have to be frozen, −20°C is okay for a short period of time (1–2 months), but when longer storage times are needed, −80°C is the best, even if only for the initial freezing period.