Otolith sampling

In scientific demersal trawl surveys, primary sampling units (PSUs) are trawl stations. Biological data, such as otoliths, is usually obtained from a subset from these sampling units. This type of cluster sampling is usually inefficient because it is nearly impossible to create clusters that represent the age-length composition of an entire population (Thompson, 1997). Aanes and Vølstad (2015) used simulation to evaluate different subsampling strategies and found that, for cod, otolith samples from 10 fish (one fish per 5 cm bin per haul or trip) was sufficient enough to provide the ALK. Furthermore, the IBTSWG 2015 (Ralf van Hal) showed that, a reduction of one third to even half of the otolith samples provide an ALK with a similar accuracy.

Two data limited stock (DLS) species turbot (Scophthalmus maximus) and brill (Scophthalmus rhombus) require additional data to enable a proper stock assessment. One of the questions raised was how many otolith samples are needed to provide a reliable ALK for both species. Statistical analyses for the bedrijfssurvey on turbot and brill showed that otolith sample size in 2019 could be reduced to less than half of the otolith sample size from 2018.

Advice on how to adjust otolith sampling schemes

A factor not taken into account yet, complicating this analysis is the large variety in the age-at-length between sexes. Additionally affecting these analyses, are regional differences in ALK known from a variety of species, but not observed in the turbot and brill data. This raises questions about the reliability of age-based data and the validity of the current otolith sampling scheme. This project will extend on the previous analyses done for the IBTSWG and the bedrijfssurvey to design statistical proof for the collection of otoliths. The results will lead to advice on how to adjust otolith sampling schemes to improve data quality for assessments, possibly reduce costs and reduce the number of experimental animals.

Doel van dit project

Twee data limited stock soorten, tarbot en griet, vereisen aanvullende data voor de bestandsschattingen. Dit project heeft als doel het ontwerpen van een statistische onderbouwing voor het verzamelen van otolieten van deze twee soorten om zo de datakwaliteit van de bestandsschattingen te verbeteren, mogelijk de kosten te reduceren en het aantal proefdieren te verminderen. De ontwikkelde methoden zijn toepasbaar op alle WOT surveys om de kwaliteit van verzamelde data en vervolgens de bestandsschattingen te verbeteren.