Mobile marine species, including cetaceans, offer a considerable challenge when attempting to assesses abundance and distribution. Cetaceans can be cryptic and visual survey methods must take into account the limited opportunities for sightings, i.e. when the animal surfaces to breathe. There are several well establish approaches and methods used to for gathering data and monitoring cetacean populations using visual sightings such as transect sampling, photo identification and records of live and dead animals. These methods can be costly in terms of the resources needed to run effective field surveys, but offer a wealth of valuable data which underpins population abundance estimates and distribution patterns (Reid, Evans and Northridge, 2003; Waggitt et al., 2020; Hammond et al., 2021). However, there are additional methods developed and used for collecting data on cetacean populations, including acoustic monitoring with mobile or static devices, recording and sampling from strandings or biopsies from live animals, remote monitoring using satellite imagery or telemetry tracking using specialised tags. These methods are outside of the scope of this report and the JCDP. The JCDP currently focuses on data collected following transect methodologies based on either vessel or aerial platforms, and these methods are the focus of this document. Transect methods are based on the principle of traveling along set transects within a study area to count all individuals of a population encountered along the transects, then the total population can be calculated, for example using Distance modelling approaches (Miller, 2021) The Joint Cetacean Data Programme was developed between 2019 and 2022, funded by the Department of Environment, Food and Rural Affairs (Defra). The project was managed by JNCC with development and hosting of the Data Portal by ICES. Contributors Published Refereed Current 14.a 14.5 Marine turtles, birds, mammals abundance and distribution Mature Organisational International Method Specification of criteria