Seasonal variations of RNA/DNA ratios and growth rates of the Alboran Sea sardine larvae (Sardina pilchardus)

Abstract

RNA/DNA ratios and larval growth of winter and spring-spawned sardine larvae (Sardina pilchardus), from the southern coast of Spain were studied in 1997 and 1998. Seasonal variations of larval growth and RNA/DNA ratios were analysed in relation to environmental variables. Oceanographic features in the area of study are strongly influenced by the flow of nutrient-poor Atlantic water, which enters the Alboran Sea through the Gibraltar Strait. In 1997, the influence of Atlantic water was stronger in spring than in winter, leading to lower salinity in spring 1997. The predominance of Atlantic waters could cause lower chlorophyll-a concentration in spring 1997 than expected for this season. In addition, lower microzooplankton biomass was observed in spring 1997 than in winter 1997. In concordance with microzooplankton biomass, the RNA/DNA ratio of sardine larvae was higher in winter 1997 than in spring 1997. No seasonal differences in somatic or otolith growth rates were observed in 1997, which could be due to a time lag in the response of larval growth to changing environmental conditions. Unlike the previous year, during 1998, no seasonal changes were observed in the amount of incoming Atlantic waters, as evidenced by salinity values. In 1998, temperature and microzooplankton biomass were higher in spring than in winter, causing higher somatic and otolith growth rates in spring 1998. The higher temperatures observed in spring 1998 compared to winter 1998 could be responsible of a decline in the RNA/DNA ratio of spring-spawned sardine larvae in comparison to winter-spawned larvae. Larval growth and RNA/DNA ratios may show a different response to changes in the environment, particularly temperature. It its known that temperature has a positive effect on growth rates but it has a negative effect on the RNA/DNA ratio, then it is recommended to bear in mind these effects, particularly when joint analysis of larval growth and RNA/DNA ratios are carried out.