Scottish sea fisheries statistics: technical manual

Data quality

As National Statistics, this Sea Fisheries Statistics publication has been assessed by the UK Statistics Authority as being of the highest standards of trustworthiness, quality and value. Data quality is assured throughout the publication process.

The sea fisheries data in this publication come from administrative databases where the primary purpose of collecting the data is for sea fisheries protection. The statistics team then uses the data for the secondary purpose of producing this statistical publication. As administrative data gathered under legislation for law enforcement, the data form a census (a complete dataset) that records every voyage and landing by UK commercial fishing vessels or by any vessel landing into the UK.

Illegal, unreported and undeclared fishing

Marine Directorate Compliance assess Illegal, Unreported and Undeclared (IUU) fishing (sometimes called “black landings”) as being very low with minimal impact on the trends shown in the data.

Data entry validation

To minimise data entry error, fishers and agents choose from drop-down menus when completing forms and there are automatic checks for logical inconsistencies. This includes things such as wrong combinations of ICES rectangle and sea area.

There are more automatic checks that ensure the data remain correct when moved between databases and systems.

Compliance checks

As Marine Directorate Compliance use the data throughout the year for sea fisheries protection, the data are checked on an ongoing basis. If Compliance find errors in the data, they request that fishers or agents supply correct data and amend the records.

Marine Directorate fishery officers are experts on data reporting and the fishers in their port district, ensuring data quality by finding and amending any data errors.

Expert user checks

The statistics team also use the data throughout the year for purposes listed in the Outputs and publications section. Expert users of the data such as the Fish Producer Organisations give regular feedback on consistency between our reports and their own internal records, providing a robust external check on data accuracy.

The Marine Management Organisation use UK fisheries data for their own work throughout the year, providing another expert check on data quality.

Fishers, Producer Organisations and agents also check their own data to ensure they are fairly and accurately represented.

Statistical quality assurance

The statistics team undertake multiple checks on the data before publication. The data are checked against the provisional publication for the same year and against the final statistics for the previous year. Any large changes or inconsistencies are investigated further.

The process of querying the database and building the statistical tables has been automated to ensure quality and consistency.

The Marine Analytical Unit has also developed code which is used to identify issues. The program looks for and identifies the following issues in the data:

• non-Scottish vessels with a Scottish licence – this may be because they have recently moved and all the relevant datasets have not yet been updated
• duplicate voyages
• duplicate sales
• inactive vessels with landings
• multiple RSS numbers for the same vessel – the RSS number is supposed to be a unique identifier assigned to vessels registered with the Registry of Shipping and Seamen
• price check – identifies sales greater than one tonne in weight where the price per kilogram is too high or too low based on the average price per killogram. This is any price that is more than three standard deviations away from the average price
• large landings by under 10 metre vessels
• large landings with no value
• Scottish under 10 metre vessels landing into foreign ports
• foreign vessels landing abroad – data about foreign vessels landings abroad should not be included inthe data, this program extracts those records
• species by presentation – the program extracts species and presentation combinations to allow for checking of invalid combinations
• species by gear - species and gear combinations to check for invalid combinations
• multiple gears during a voyage by an over 12m Scottish vessels
• voyages with activities before departures
• voyages with activities after return to port
• landing date before return to port
• discrepancy between voyage days and voyage length
• long voyages (over 30 days) - voyages for Scottish vessels who are at sea for more than 30 days and the catch is not frozen
• price check on catch - voyages landing more than 1 tonne of an individual species with a price per tonne over £20,000
• above average weight in pots
• large number of pots - voyages hauling an above threshold number of pots in a single voyage

When inconsistencies are identified, they are sent to Compliance or Fisheries Officers in the relevant port district for correction. The statistics team quality assures the results before Marine Scotland policy colleagues provide further sense checks based on their expert knowledge of the industry.

Limitations of the data

Due to the legal basis for collecting sea fisheries data, the quality of the data is high and forms a census, or complete record. Some limitations of the data include:

• timeliness
• lack of data on foreign vessels in UK waters
• lack of data on Scottish vessels operating far from Europe
• limits on the precision of marine geography

Information on landings of fish species not subject to quota, or fish landed by UK vessels abroad, can often take months to be supplied. This is why the provisional statistics are not published until 4-5 months after each year end and the final statistics are published in September.

The UK fisheries data warehouse is a complete record of all UK vessels voyages and landings as well as all foreign vessels landings into the UK. However, the UK does not have a system for recording foreign vessels catches from UK waters if they land their catch abroad. This is because the legal responsibility for recording details of that vessel and its activities lies with its own country.

Sales notes are not always supplied for Scottish-registered vessels landing abroad as there is no legal requirement to provide this. This is a bigger issue following EU exit as previously sales information for landings into the EU would be shared automatically. Where sales notes are not available estimates based on other landings are calculated instead. 

A small number of records will be identified as erroneous but will not be amended immediately. This is because they serve as a legal record of what fishers and agents supplied to Compliance under sea fisheries protection law. If the Marine Directorate takes enforcement acton, then known errors may not be able to be amended until the case is resolved. This affects a small proportion of records so the impact on published figures and trends is minimal.

Some data is presented by a specified geography – such as by local authority or ICES Area of fish capture. This is limited by the complexity of marine geography. Data on area of capture are not available below the level of an ICES rectangle which is a 30 nautical mile by 30 nautical mile area. The data do not report whether fishing occurred in inshore or offshore areas.

Manual corrections applied to published data

In 2023, it was discovered that a larger than expected number of erroneous records for previous years were being held in the compliance system without being fixed. This mainly affected landings by Scottish vessels abroad or landings by foreign vessels into Scotland. These records had missing or inconsistent information which prevented the compliance system from marking the voyage as complete and supplying the record to the statistical database. As there is no legal requirement for foreign vessels to provide voyage information even when landing in the UK, this information is unlikely to become available at a later date. Likewise, for landings abroad by Scottish vessels, there is no legal requirement to provide sale notes. There is also a specific issue with landings into Norway where vessels land into multiple ports after a single voyage.

In order to make the data as complete and accurate as possible, the available information on the erroneous records for the last five years was extracted from the compliance system. These records were then added to the publication data prior to publication. Any missing information (for existing and added records) was estimated based on available information. The value of landings for records with missing sales information was calculated using average prices. This did not affect the overall trends but resulted in large changes to the tonnage and value of Mackerel landings into Norway in 2018 and the value of Mackerel landings into Denmark in 2020.

Changes from the provisional statistics

The time lag between sales and the provision of sales notes can cause changes between provisional and final statistics for a given year.

Value of landings information is derived from sales notes, which are submitted by Registered Buyers and Sellers. This information is matched up with the landing declaration submitted by the vessel to obtain values for each landing. If there is no sales note, for example when a UK vessel lands into a non-EU country, the value of the landing is estimated.

The most accurate estimation is based on the average price for the same species in the same quarter of the previous year. If that method is not available, the database uses the next most accurate method based on the price for the same species in the previous quarter, and then opens up to the same species type, and so on. When a sales note is supplied for that landing, the estimates are replaced by the true figure from the sales note.

Revisions to previous years’ final statistics

The landings tables present data for the past five years. Previous years’ data are updated to ensure users have access to the latest data available. Revisions are typically small and affect landings information where data comes from logbooks, landing declarations and sales notes that can be amended for small changes due to late data entry or rectification of errors.

As these do not affect the main trends presented, they are not highlighted and not footnoted. Data older than five years is not updated.

The list of the main species displayed in the landings tables may change each year as the criterion for a species being a ‘main species’ is that the total value landed into the UK or by UK vessels is £2 million or more.

On the rare occasions that revisions are required in the previously published figures that do affect the main trends presented, the revised figures are marked "(r)" and suitably footnoted to explain the reason for the revision. The statistics are routinely revised between the provisional statistics and final statistics – see above. However, provisional statistics are always explicitly identified as such. Vessel numbers and employment remained unchanged as these data are collected once for the final version of each edition of the publication.

Comparability with other countries’ statistics

The main difference between information collated in Scotland and that collated in the rest of the UK is that, Scotland obtains equivalent information on a full coverage basis for vessels of 10 metres and under. In the rest of the UK, this information is provided on a sample basis only. However, the vast majority of fish are caught by vessels over 10 metres, so the information is effectively comparable for all UK countries for most species.

The method for counting Scottish registered vessels is by totalling the number of active vessels registered in Scottish ports on 31st December of each year. In comparison, in the rest of the UK vessels are counted if they are registered (regardless of whether they are classed as active or not). This can lead to slight discrepancies between figures published at the Scottish level and those published at the UK level.