Project

Improved age determination of pikeperch and bream for stock assessment Ijsselmeer

In the WOT IJsselmeer data collection program, specimens of pikeperch and bream are collected routinely from both the annual field survey as well as the market (fish auctions). Age reading as part of this data collection program is currently based on reading scales (bream, pikeperch) and/or fin rays (pikeperch). However, age reading based on scales for fish older than 6-7 years is difficult or impossible since growth is strongly retarded at higher ages. This may result in significant underestimation of age (as recently shown in samples collected from Dutch rivers) and is therefore judged to be inappropriate.

Using otoliths for age estimation is a reliable alternative, and is also common practice for pikeperch and bream in other countries. Models to support fisheries advice in IJsselmeer have recently been improved. At the same time the fisheries management is currently undergoing considerable reconstructions. Along with this development there is a rapidly growing demand for more accurate fish stock data for cohort analysis. In particular, age readings are required to be highly reliable, as the relative occurrence of larger and older fish is one of the main fisheries management indices. Therefore, there is an urgent need to improve the accuracy of age reading of freshwater species, especially pikeperch and bream. Otolith reading for freshwater species would be easy to implement on the basis of current knowledge, quality development and routines (the otolith-line) used for marine fish species. A higher accuracy and better quality of age readings of freshwater species would directly result in improved scientific fishery advice, to the satisfaction of commissioning authorities as well as the community.

Leeftijdsbepalingen

Leeftijdsbepalingen vormen een belangrijke basis voor visserijmodellen die gebaseerd zijn op het volgen van cohorten over de tijd. Onlangs zijn nieuwe visserijmodellen voor het IJsselmeer ontwikkeld waarbij nauwkeurige schattingen van de leeftijdsopbouw van de bestanden van snoekbaars en brasem van groot belang zijn. De huidige wijze van leeftijdsbepalingen in het WOT-programma heeft het risico leeftijden van met name oudere vissen aanzienlijk te onderschatten. In dit project worden daarom routines ontwikkelt om de leeftijdsbepaling op basis van otolieten te kunnen doen, een internationaal bewezen methode om ook van (zeer) oude vissen de leeftijd nauwkeurig te kunnen bepalen.

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