Distribution, population dynamics, and production of the suprabenthic mysid Mesopodopsis slabberi in the Mondego estuary, Portugal

The spatial distribution, seasonal abundance, life history pattern, and production of the mysid Mesopodopsis slabberi are described from a warm-temperate estuary on the western coast of Portugal. Mysids were sampled from June 1996 to July 1997, using zooplankton and suprabenthic nets. The higher densities and biomass of M. slabberi were found in upstream areas from the south arm of the estuary. These areas consist of shallow waters with Low circulation rates and high residence times, richer in phytoplankton than the rest of the estuary. Mesopodopsis slabberi was abundant in October, November, and December and again in May, June, and July. No mysids were found in January and February, when salinity decreased to values below 5 parts per thousand and water temperature was below 10 degrees C, and also in August, when oxygen levels declined and the water temperature reached values above 24 degrees C. Morphometric relationships in M. slabberi were determined from freshly caught specimens. Significant positive correlations between total length (TL) and cephalic length (CL) (TL = 2.5 CL + 0.012) and between dry weight (DW) and total length (LnDW = 3.0298 LnTL - 6.0229) were found. All size classes, from 0.95-11.36 mm, were equally represented throughout the year. Moreover, juveniles, immature females, mature ovigerous females carrying eggs or embryos, and mature resting females were found throughout the sampling period. As a whole, this pattern suggests that reproduction is continuous rather than seasonal. This, together with migrations, made identification and tracking of population groups impossible from the analysis of size-frequency diagrams. The secondary annual production of M. slabberi was therefore estimated by the Hynes average cohort method. The annual production was 16.02 mg . m(-3)year(-1), and the P/B ratio was 2.57.