Evaluation of otolith collection : generic evaluation method and case study BTS plaice

During many seagoing activities otoliths from fish are collected. These bony structures are used to identify the age of fish, as age of fish cannot be determined externally. Otoliths are collected according to a stratified scheme that may vary depending on survey, species, and length class. The origin of the number of otoliths to be collected is often unknown. Given that otolith collection is costly and labour intensive, and each fish is counted as an experimental animal, it is important to evaluate the otolith sampling scheme. In this study we developed a generic methodology to (a) define the required number of otoliths per length group for age-length keys used in stock assessment, and (b) evaluate the current data collection by species and propose modifications to the sampling plans. The generic methodology was applied to the plaice (Pleuronectes platessa) data from the Dutch beam trawl survey (BTS) in the North Sea. One of the important findings was the need for a spatial split based on longitudinal ranges to ensure comprehensive coverage of age groups. An updated sampling scheme, without over-all otolith reduction, but with a re-division of otolith collection over the sampling area, and an aggregation by length groups instead of centimeter classes was implemented during BTS 2024. From autumn 2024 to mid-2025, the new BTS plaice sampling scheme will be evaluated, and the generic methodology will be extended to other species in BTS, and in the Sole net survey and Demersal young fish survey conducted by the Netherlands.