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 pro duction of the mysid Mesopodopsis slabberi are described from a warm-temper ate estuary on the western coast of Portugal. Mysids were sampled from June 1996 to July 1997, using zooplankton and suprabenthic nets. The higher den sities and biomass of M. slabberi were found in upstream areas from the sou th arm of the estuary. These areas consist of shallow waters with Low circu lation rates and high residence times, richer in phytoplankton than the res t of the estuary. Mesopodopsis slabberi was abundant in October, November, and December and again in May, June, and July. No mysids were found in Janu ary and February, when salinity decreased to values below 5 parts per thous and and water temperature was below 10 degrees C, and also in August, when oxygen levels declined and the water temperature reached values above 24 de grees C. Morphometric relationships in M. slabberi were determined from fre shly caught specimens. Significant positive correlations between total leng th (TL) and cephalic length (CL) (TL = 2.5 CL + 0.012) and between dry weig ht (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 e ggs or embryos, and mature resting females were found throughout the sampli ng period. As a whole, this pattern suggests that reproduction is continuou s rather than seasonal. This, together with migrations, made identification and tracking of population groups impossible from the analysis of size-fre quency diagrams. The secondary annual production of M. slabberi was therefo re 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.