Overview of annual Shetland Inshore Fish Survey (SIFS) data (2011 - 2022): catch rates and size compositions of commercial demersal fish species

Shaun Fraser, Chevonne Angus, Mia McAllister, Angharad Powell, Sarah Ayres, Leander Harlow

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Abstract

Data from commercial fisheries are key components of stock assessments for many locally
important species. However, the behaviour of individual fishing vessels targeting specific
species and using different gear configurations can bias commercial data. Such effects must
be considered when interpreting fisheries dependent data. Therefore, independent data from
scientific fisheries surveys are an important additional source of information and annual
surveys can be used to build valuable timeseries for investigating the dynamics of fish
communities and for informing the management of fish stocks.
An annual fish survey has been undertaken in the waters around Shetland by the NAFC Marine
Centre and now UHI Shetland since 2011. This survey was originally initiated in response to
fishermen reporting high abundances of small cod on inshore fishing grounds (inside 12
nautical miles and approximately 50 - 150 m depth). Standardised scientific trawling gear and
fishing methods have been used to provide an independent index of the nearshore
distribution and relative abundance of demersal fish species. By repeating the survey each
year, the resulting data have become increasingly valuable for determining the inter-annual
variability of nearshore fish catch rates. Since 2017, these data have been further enhanced
by an extended survey design which targets potential nursery grounds in shallow areas
(approximately 20 - 50 m) to collect additional information on juvenile fish (those smaller and
younger fish yet to reach sexual maturity).
The catch from each haul provides information on which species are present at that location
and in what quantities, as well as the size compositions of key species. Size information is
used to infer population structure and to indicate the strength of particular year-classes
(those fish born in any one year) which can reveal variations in recruitment (the number of
fish surviving to enter the commercial fishery). Young individuals yet to be recruited to the
fishery can be captured using scientific trawl gear utilising small mesh sizes. Consequently,
scientific trawl data provide important information on juvenile abundances which is not
available from commercial landings due to the commercial restrictions on landing sizes and
gear design.
Original languageEnglish
Number of pages24
Publication statusPublished - 29 Nov 2022

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