Non-native species (NNS) are those which have been spread, either intentionally or accidentally, beyond their natural geographical range as a consequence of human activity. Invasive non-native species (INNS) can pose major threats to native biodiversity, human health and ecosystem services.
Harbours, marinas and aquaculture facilities are frequently located in estuaries and are potential entry points for marine NNS. There is little systematic information on the relative tolerance of different NNS to estuarine conditions and their penetration landwards along estuarine gradients.
In 2017 The Marine Biological Association undertook a series of surveys to map the presence of INNS along the estuary of the River Itchen, Hampshire. Natural England commissioned this further project in 2022 to determine whether there has subsequently been any onward colonisation of INNS, and in particular the Red-ripple bryozoan, (Watersipora subatra), an invasive bryozoan which has been shown to be spreading widely in the Solent (and elsewhere) and was previously recorded at only one site at the mouth of the River Itchen. The Itchen estuary is part of the Lee-on-The Solent to Itchen Estuary SSSI.
Re-survey of the Itchen confirmed the general pattern of succession of NNS in the estuary and added four NNS to the list of those recorded in 2017. The failure of Watersipora subatra to encroach further on the estuary over a period of five years was documented, and a remarkable increase in NNS at one site was detected.
This information provides updated NNS species records as well as helping to inform biosecurity planning.