Research Article

Commercial vs. survey data in length-based stock assessment: insights from Turkish crayfish fisheries

¹ Isparta University of Applied Sciences, Eğirdir Fisheries Faculty, Isparta, Türkiye
² District Directorate of Provincial Agriculture and Forestry, Eğirdir, Isparta, Türkiye

^* Corresponding author: mehmetcilbiz@isparta.edu.tr

Received: 1 April 2024
Accepted: 9 May 2025

Abstract

In length-based stock assessments for data-poor fisheries, commercial fishing data is often used due to its cost-effectiveness and accessibility. However, factors such as gear selectivity, seasonal closures, minimum conservation reference size (MCRS) regulations, and market-driven harvesting practices can render commercial catch data unrepresentative of the true population structure. Reliance on such data without correction or complementary sampling may lead to biased stock assessments, undermining effective fisheries management. This study investigates how length data from commercial fisheries and experimental sampling influence the estimation of biological reference points (BRPs) and explores differences in catch compositions between these data sources. Between June 2021 and May 2022, a study was conducted at Lake Eğirdir (Isparta, Türkiye), involving 10 different stations. A total of 400 fyke nets were deployed, consisting of 200 experimental and 200 commercial ones. The experimental fyke nets had a stretched mesh size of 18 mm, while the commercial ones had a mesh size of 34 mm. These nets were evenly distributed, with 40 nets at each station. TropFishR package was used for estimating of the life history parameters and stock assessment. The comparison of catch composition was performed using “Length-dependent catch comparison” and “catch ratio” analyses through the SELNET program. As a result, it has been determined that, due to the potential size selectivity feature, the commercial fyke net is unsuccessful in catching small-sized individuals (<4 cm carapace length), while the experimental fyke net is unsuccessful in catching advanced-sized crayfish (>6.5 cm carapace length). The estimated BRPs showed significant differences depending on the data source, and there were also differences of up to 56% in the recommended total allowable catch (TAC) amounts. As a result, it is considered that relying solely on commercial or experimental fyke nets for crayfish may be misleading, and it would be more appropriate to use both for successful sampling and stock assessment.