A condition index based on dry weight as a tool to estimate in-situ moult increments of decapod shrimp:Investigating the effects of sex, year and measuring methods in brown shrimp (Crangon crangon)

The determination of in situ growth rates in crustaceans is a major obstacle for stock assessment, especially in short lived small shrimp species such as Crangon crangon. Here a new method is presented, which uses a condition index based on dry weight to predict growth increments. The method is based on a simple equation relating dry weights of shrimp and exuvia directly after moult with length measurements before and after moult. The parameter estimation of the model requires controlled moult experiments in the laboratory. In total 272 individuals moulted successfully within the first 2 days after capture, hence the increments reflected the feeding situation of the prior feeding period in the field. The results confirmed the validity of the method with data from two different years and demonstrated that in C. crangon the model parameters differ significantly between the two sexes. As a side product also moult intervals were estimated as a function of length and temperature. Moult increments were relatively shorter and more variable than previously found.