Water uptake in herring (Clupea harengus) and sprat (Sprattus sprattus) as a function of area, salinity and fat content

When pelagic fish is landed, the total weight includes both fish and water. In Sweden pelagic fisheries for herring (Clupea harengus) and sprat (Sprattus sprattus) are allowed to reduce the catch weight with 13% in the Baltic (2003) and 2% in Skagerrak/Kattegat and the North Sea. However, it has also been observed that the weight of individual fish increase due to fish absorb water during storage in the holding tanks onboard fishing vessels. Depending on the magnitude of the water uptake, the catch and landing statistics may be seriously biased. Furthermore, water uptake can also affect our perception of the variations in fish condition related to changes in the environment. Hence in this study, we explore the water uptake in different sized herring and sprat from three areas around the Swedish coast differing in salt concentration. We quantified the water uptake over time in herring and sprat stored fresh in tanks as a function of salinity and fat content. Salinity and fat content was used as covariates since osmotic potential and fat content has been suggested to affect water uptake in fishes. The results indicate that water uptake did not differ between species but did as a function of area and size (length). When exploring the two species separately, water uptake in herring was affected by area and salinity whereas water uptake in sprat by area and size (length). The water uptake difference between the two species was probably related to the stronger relationship between fat content and size (length) in sprat, compared to herring. Overall, the results suggest that the water reduction index used in Sweden as well as other countries should be changed.