Identification of herring in Shetland waters

Project Details

Description of project aims

There are several populations of herring (Clupea harengus) in the northeast Atlantic. Around the Shetland Islands, North Sea autumn-spawning herring are present, for which there is a valuable commercial fishery. Those herring undergo sexual maturation between March and August, ready to spawn in September. In recent years there have been observations in Shetland waters of herring undergoing maturation in December, which could potentially be spring-spawning herring (maturing between May and January, to spawn in February/March). The presence of these fish is of interest to both science and industry, with the possibility of a stock distribution which is currently unknown and unaccounted for, and which could have implications for stock management.

To provide information on the identity of herring in Shetland waters, this study worked with local fishers to collect specimens of herring for biological sampling and analyses. Between December 2020 and April 2021, and January 2022 and March 2022, 656 herring were collected by local fishers for sampling. Length, sex and maturity stage were recorded for all fish, with a tissue sample taken for genetic analysis, and a gonad sample taken for histological examination (providing microscopic maturity stage).

The genetic analysis showed a baseline sample of the Shetland collected herring to be distinct from other herring populations of 6.a.S; North Sea and 6.a.N autumn spawning; 6.a.N spring spawning; and Downs winter spawning herring. And while the Shetland baseline herring were shown to be different, there was indication from the genetic analysis that these fish were most similar to spring spawning 6.a.N fish. These results provide evidence of spring spawning herring in Shetland waters; with the potential for a previously unidentified Shetland spring spawning population component (in the context of this study and the available data). However, the identity and origin of this population is not known, and Norwegian or Western Baltic spring spawning herring cannot be discounted as the source. Across all the herring samples collected in Shetland, there was evidence of the presence and potential mixing of multiple herring population components (including autumn spawning North Sea and 6.a.N, and spring spawning 6.a.N and the potential Shetland component), with overlap likely occurring at various stages of spawning, feeding and over-wintering.

The application of maturity stage classification alongside genetic assignment further substantiated the genetic results. Using gonad samples to identify the maturity stage of fish caught between December and March, herring were classified as either autumn spawners (if immature, spent, recovering/resting) or spring spawners (if early maturing, maturing, spawning prepared and spawning). The majority of fish classified by maturity stage as spring spawners were genetically assigned as the potential Shetland component, while the majority of fish classified by maturity stage as autumn spawners were assigned
as North Sea autumn spawners.

The study had some limitations. No genetic baseline data were available for Norwegian or Western Baltic spring spawning herring. Owing to the small sample sizes involved and limits to the genetic analyses, the findings are intended as exploratory indicators to genetic identification only. Nonetheless, this study has gathered valuable new information to provide insight into the identity of herring populations around Shetland, with clear evidence of autumn spawning and spring spawning herring. New connections with other academic institutions and staff working on similar research have been established, with further collaboration planned (e.g. genetic comparisons with other herring populations such as Norwegian and Western Baltic spring spawning), to provide more detailed information and to contribute information to a wider body of work.

This study was funded by the European Maritime and Fisheries Fund (reference SCO2721).
Effective start/end date1/12/201/12/22


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