¿La relación ARN:ADN y el peso en seco a la longitud son indicadores adecuados para determinar el crecimiento de la quisquilla gris (Crangon crangon)?

Growth rates are of fundamental interest for studying population dynamics of species when no age information is available. In these cases in situ growth proxies must be identified and validated, and here we tested whether RNA-DNA ratio (RD) and dry weight condition (DWC) can act as such proxies for the shrimp Crangon crangon. Growth rates (mm d–1) were determined for male and female shrimps (20 to 67 mm) reared at five different temperatures (5°C to 25°C) and caught in two different months (May, July). Due to the potential interactive effects of the factors sex, size, temperature and time, different statistical analysis were applied to test for linear interactions (factorial analysis of mixed data, FAMDs), nonlinear additive effects (generalized additive models, GAMs) and nonlinear interactions (nonlinear regression combined with GAMs). The FAMD indicated positive correlations between length, month and growth rate. A positive correlation between DWC and sex and between RD and temperature was also indicated. RD was not significant in the GAM but a nonlinear link between length, temperature and growth was found. Finally, an extended Bertalanffy growth model was fitted to sex-specific growth rates and in a second step a GAM was fitted to the differences (residuals) between observed and predicted growth. RD again had no significant explanatory power for growth and, although it is a commonly used growth proxy, we concluded that it is not a suitable index for evaluating length growth of C. crangon.En el estudio de las dinámicas poblacionales de las especies, y en particular en las que no se dispone de información sobre la edad de los individuos, es fundamental conocer las tasas de crecimiento. Por lo tanto, es necesario identificar y validar las aproximaciones sobre estimación de crecimiento in situ adecuados. En este estudio se ha analizado si la relación ARN-ADN (RD) y la condición de peso seco (DWC) pueden actuar como indicador para la estimación de crecimiento en el camarón Crangon crangon. Se determinaron las tasas de crecimiento (mm d–1) para los camarones macho y hembra capturados (20 a 67 mm) en dos meses diferentes (mayo, julio) y cultivados a cinco temperaturas diferentes (5 a 25°C). Debido a los posibles efectos interactivos de los factores como sexo, tamaño, temperatura y tiempo, se aplicaron diferentes análisis estadísticos para probar las interacciones lineales (FAMD: análisis factorial de datos mixtos), efectos aditivos no lineales (modelos aditivos generalizados, GAMs) y interacciones no lineales (regresión no lineal combinada con GAMs). La FAMD indicó correlaciones positivas entre la duración, el mes y la tasa de crecimiento. También se observó una correlación positiva entre DWC y sexo, y entre RD y temperatura. La RD no fue significativa en el GAM, pero se encontró un vínculo no lineal entre la longitud, la temperatura y el crecimiento. Finalmente, se adaptó un modelo de crecimiento ampliado de Bertalanffy a las tasas de crecimiento específicas por sexo y, en un segundo paso, se adaptó un GAM a las diferencias (residuales) entre el crecimiento observado y el previsto. De nuevo RD no tenía un poder explicativo significativo para el crecimiento y aunque es un indicador de crecimiento utilizado comúnmente, concluimos que no es un índice adecuado para evaluar el crecimiento de longitud de C. crangon

Depth-dependent nutritional condition of sprat Sprattus sprattus larvae in the central Bornholm Basin, Baltic Sea

Bimodal depth distribution patterns observed for sprat Sprattus sprattus larvae in previous field studies conducted in the deep basins of the Baltic Sea have led researchers to hypothesise that larval sprat condition was depth-dependent. We examined this hypothesis by measuring morphological, biochemical and otolith-based proxies for nutritional condition in sprat larvae collected in discrete 5 m depth intervals from the surface to the bottom in the central Bornholm Basin. Similar to earlier studies, larval sprat were most abundant in 2 depth strata (0 to 10 and 65 to 75 m). Their nutritional condition in surface and deep waters was not uniformly expressed by the different indices. For example, sprat larvae from 0 to 10 m could not be distinguished from conspecifics caught at 65 to 75 m by a long-latency condition proxy (otolith-based growth rates). Similarly, a medium-latency proxy (RNA:DNA) did not suggest differences in condition between the depths. However, short-latency proxies (protein:standard length and DNA:dry weight) supported the depth-dependent condition hypothesis. The lack of correspondence and pitfalls associated with the use and interpretation of multiple condition indices (e.g. the influences of temperature and body size) are discussed and recommendations to strengthen these various metrics are provided