Abstract
The Zoological Society of London (ZSL), in partnership with Marine Conservation Research (MCR), conducted a visual and acoustic vessel-based survey for the presence and distribution of cetaceans within the Thames Estuary from 22nd to 28th April 2022. The survey incorporated 868 km of effort, of which 629 km included acoustic effort using a towed hydrophone array. There were 31 detections of porpoise groups and 16 sightings; of these encounters, seven were ‘matched’ (i.e. both seen and heard). In addition, five separate porpoise groups were detected at anchor using a hull-mounted array. Group sizes for the visual encounters were estimated to be between one and three individuals (mean 1.4); groups sizes estimated for the acoustic detections were slightly higher (1-10 individuals with a mean of 2.4). There were also 26 seal sightings, of which 10 were ‘definite’ grey seals and four were ‘definite’ common seals. An average encounter rate of 5.57 porpoise groups/100 km surveyed (variance = 1.8) was estimated. The variance inflation factor was above one (2.04), suggesting there was some clustering in the detections, with porpoise presence being highest in the outer Thames Estuary. When compared to a similar survey conducted in 2015, the 2022 survey resulted in more detections and sightings in the western part of the Thames Estuary and fewer in the northern areas of the study area. Overall encounter rates were lower in 2022 than those estimated in 2015 (mean = 8.13 porpoise groups/100 km; variance = 4.2); however, due to the relatively limited survey effort, this should not be interpreted as a significant decline in porpoise density.