Integrating fish size, condition, and population demography in the estimation of Atlantic sardine annual fecundity

Spatial and temporal variability in reproductive parameters was studied in combination with population demography to estimate sardine (Sardina pilchardus) population annual fecundity, as an indicator of its reproductive potential. Samples collected from commercial landings were used to determine the monthly proportion of active females and the duration of the spawning season, while data obtained during the 2002, 2005, and 2008 Daily Egg Production Method surveys were used to analyze the relationship between reproductive parameters (batch fecundity, spawning fraction) and individual biological characteristics (length, gutted weight, condition) for the west and south coasts of Portugal by generalized linear models. Population abundances and biological characteristics obtained per length class during acoustic surveys were subsequently used to integrate individual egg production to population demography. At the individual level, results indicated that the duration of the reproductive season was size-dependent, being shorter for smaller sardines. Batch fecundity was positively related to body size and differed significantly between the years and areas studied, with higher values on the west coast and an increasing trend towards the 2007/2008 spawning season. Mean values of spawning fraction varied significantly with space and time but the poor explanatory relationship obtained by the modelling of the spawning fraction against the individual biological characteristics did not allow the clarification of the factors affecting sardine spawning activity. At the population level, the differences obtained in egg production were found to be related to changes in population abundances as well as to the demographic structure, as larger females contributed proportionally more to the population annual fecundity. These results are in accordance with numerous recent studies indicating that the population reproductive potential may not always be proportional to the spawning biomass. Se estudio la variabilidad temporal y espacial de los parametros reproductivos junto con la demografia poblacional para estimar la fecundidad anual de la poblacion de sardina (Sardina pilchardus), como un indicador de su potencial reproductivo. Muestras de desembarques comerciales fueron utilizadas para determinar la proporcion mensual de hembras activas y la duracion del periodo de desove. Con los datos obtenidos durante las campanas del Metodo de la Produccion Diaria de Huevos en 2002, 2005 y 2008, se analizo la relacion entre los parametros reproductivos (fecundidad parcial, fraccion de desove) y las caracteristicas biologicas individuales (talla, peso eviscerado, condicion) para las costas oeste y sur de Portugal mediante modelos lineales generalizados. Las abundancias poblacionales y caracteristicas biologicas obtenidas por clase de talla durante muestreos acusticos fueron utilizadas para extrapolar la produccion individual de huevos a la demografia poblacional. A nivel individual, los resultados indicaron que la duracion de la epoca reproductiva es dependiente de la talla, siendo mas corta para sardinas mas pequenas. La fecundidad parcial esta positivamente relacionada con la talla y difiere significativamente entre anos y areas, con valores superiores en la costa oeste y en la temporada de desove de 2007/2008. Los valores medios de la fraccion de desove difirieron significativamente en tiempo y espacio, pero la escasa relacion explicativa obtenida al modelar la fraccion de desove contra las caracteristicas biologicas individuales no permitio determinar los factores que afectan la actividad de desove de la sardina. A nivel poblacional, las diferencias obtenidas en la produccion de huevos estuvieron relacionadas con los cambios en abundancias poblacionales y con la estructura demografica, ya que las hembras de mayor tamano contribuyeron proporcionalmente mas a la fecundidad anual poblacional. Estos resultados concuerdan con estudios recientes que sugieren que el potencial reproductivo poblacional no es necesariamente proporcional a la biomasa desovante.