On the distribution and population dynamics of the ctenophore Mnemiopsis leidyi in the Belgian part of the North Sea and Westerschelde estuary

- We sampled the Belgian part of the North Sea, 3 ports and Westerschelde estuary.
- Highest densities of Mnemiopsis leidyi were observed in late summer and autumn.
- The enclosed ports and estuary might serve as a source of the non-native M. leidyi.
- Multiple reproduction events were observed in our study area.
- Temperature, oxygen and wave height were related to the observed densities.

The spatio-temporal distribution and population dynamics of the non-indigenous ctenophore Mnemiopsis leidyi A. Agassiz 1865 were investigated through monthly and quarterly surveys in 2011-2012 at several locations in the Belgian part of the North Sea, the main coastal ports and the adjacent Westerschelde estuary. M. leidyi occurred from August to December, but was never found more than 30 km offshore. Densities were generally low (average 0.8 ± SD 2.8 ind m−3) compared to other invaded European systems. Highest densities of M. leidyi were found in the semi-enclosed basin (port of Oostende; 18.4 ind m−3) and Westerschelde estuary (1.9 ind m−3). The presence of larvae and sudden appearance of high numbers across the size distribution in August indicated that ports and estuaries may act as sources, populating the adjacent coastal area. The zero-inflated logistic regression model showed that there is a higher chance of finding M. leidyi (presence) when temperature declines from late summer onwards. Combined with a negative binomial regression, our model suggests that increasing M. leidyi densities are associated with decreasing autumn temperatures, low wave height (low energetic systems) and low dissolved oxygen concentrations Although densities remained relatively low since its first appearance in 2007, a permanent population seems to be established in Belgian waters. As population outbursts may occur with only a small change in environmental parameters, further monitoring of this notorious invasive species is recommended.