Age estimation of brown shrimp Crangon crangon: comparison of two approaches applied to populations at the biogeographic edges

In this study, 2 methods for age estimation of Crangon crangon were compared: one based on total length, the other based on the number of segments in the antennules, as suggested by Tiews' findings (1954: Ber Deut Wiss Kommiss 13:235-269). Shrimps from populations near the species' geographic edges, from Valosen Estuary (Norway) in the north and the Minho Estuary (Portugal) in the south, were used. These showed great individual variability in shrimp size growth and in antennule segment number increment, due to the presence of fast- and slow-growing shrimps in the 2 populations, implying that shrimps of different ages may have similar sizes, as well as similar segment numbers. Additionally, the origin of shrimp population, sex, length and segment number, and water temperature were found to influence growth, both in the length- and the segment-increment models, often being interacting parameters. Both methods resulted in growth curves similar to a typical von Bertalanffy growth curve. The growth rate was higher for females than for males and positively related to temperature; the maximum size was also larger for females. In contrast, maximum segment number was larger for males than for females, as has also been found by Tiews (1954). The 2 models resulted in considerably different age estimates, particularly for older shrimps (i.e. shrimps of larger size or with more antennule segments). The models also suggest site-specific growth rates. Any age estimation approach will therefore need to be validated for each population.