Workshop 2 on the identification of clupeid larvae (WKIDCLUP2)

  

The Workshop 2 on the Identification of Clupeoid larvae (WKIDCLUP2) is part of a series of workshops that aim to calibrate fish larvae identification. Different clupeoid (herring, sprat, sardine and anchovy) larvae surveys are carried out on the Northeast Atlantic Shelf and provide essential data for the assessment of fish, herring in particular, stocks in the North Sea, Irish Sea and the Baltic. In recent years, other clupeoids besides herring are occurring in the survey samples in increasing numbers. Since clupeoid larvae can easily be mixed up, effective quality control and proper larvae identification is essential to reliable survey results. These identification workshops are repeated regularly in order to maintain consistency of identification across the community as well as to train and improve the skills of new survey participants. As part of this workshop, the WebApp SmartDots was adapted to be utilized for ichthyoplankton identification based on images. 

Identification agreement generally varies from 56-94% depending on the species and participation cohort. Based on 60 different fish larvae evaluated in 2020, overall agreement in identifying clupeid and discriminating them from other, non-clupeid larvae among all participants was 81.7%. Agreement for herring larvae was 86%, for sprat 80%, for sardine 86% and for anchovy 71%. For 2 reading rounds in 2021, 120 fish larvae were used for each. Overall agreement during the first round was 72.5%, 81% for herring larvae, for sprat 56 %, for sardine 67 % and for anchovy 72 %. During the second round, overall agreement increased to 81.7%, 85% in herring, 83% in sprat, 69% in sardine, 82% in anchovy and 94% in non-clupeoids.

Subsequent analysis myotome counts, which was facilitated through the SmartDots WebApp during all larvae reading events, showed that particularly in those specimens that showed low agreement in correct identification, variation of counts was high. Consequently, techniques for their correct determination were discussed and existing information on clupeoid larvae identification updated accordingly. New, previously unpublished information on specific pigmentation discriminating between sprat and sardine was added to existing larval descriptions.

The only species for which larvae abundance indices are currently used in the assessment is herring. Two of those surveys are conducted in the North Sea, the third in the Baltic. Based on the reassuring results of the workshop identification trials, the potential error caused by misidentification of the larvae can be considered as low or negligible for all of these surveys. 

Though the agreement in correctly identifying larvae of clupeoid fish increased considerably in comparison to the first WKIDCLUP in 2014, the results of this workshop underscored the importance of carrying out such events regularly. Increasing spatial and temporal overlaps in distribution of larval clupeoids due to warming oceans demonstrates the need to have ongoing discussion and training for consistency in identification. Through adaptation of the SmartDots WebApp to ichthyoplankton identification trials, such workshop should also become more feasible as online events and, thereby, receive higher attendance.