Population Structure And Recruitment Patterns Of The Indigenous Mussel Perna Perna And The Alien Mussel Mytilus Galloprovincialis On The Central Coast Of Namibia

ABSTRACT

Invasive species disrupt ecosystems and may consume resources and eventually replace indigenous species. This has been well documented for the Mediterranean mussel Mytilus galloprovincialis throughout the world. This study assessed the spatial and temporal variation of population structure, recruitment patterns, condition index and abundance of the indigenous mussel Perna Perna and alien mussel M. galloprovincialis at three different sites on the central Namibian coast. Sampling took place during spring tide, once every season along Dolphin Beach, Long Beach, and Shipwreck, near Swakopmund during 2014. At regular intervals of 2 m one 10 x 10 cm quadrat was placed along a line transect from the spring low water level to the spring high water level. Everything was removed within the quadrat and stored at -20 °C. Thereafter, samples were sorted with two sets of sieves with mesh 2 mm and 0.5 mm into different size classes. Mussels were identified to species level. Significantly higher recruitment for P. perna during January and September 2014 was observed (p = 0.006). M. galloprovincialis displayed no significant seasonal difference (p = 0.102). P. perna recruits and adults dominated the low shore zone, while the juveniles dominated the mid shore zone. M. galloprovincialis recruits, juveniles and adults dominated the mid shore zone. The condition index of adults was significantly higher for P. perna in the low shore zone and for M. galloprovincialis in the mid- and high shore zones. No significant difference in the condition index of juveniles between the zones existed for both species. No iii significant seasonal difference was found in the condition index of juveniles and adults for M. galloprovincialis. There was also no significant seasonal difference in the condition index of juveniles for P. perna, however the condition index of adults was significantly higher for P. perna during September compared to the other months. The abundance of recruits of M. galloprovincialis observed during this study might be a sign of possible invasion which needs to be monitored.