Assessing the stock status of two commercially important flounder species in the East Sea of Korea using multiple length-based methods

Abstract

The blackfin flounder (Glyptocephalus stelleri) and pointhead floundeer (Cleisthenes pinetroum) are the commercially important flounder species in the Korean East Sea. In this study, we utilitzed three length-based methods known as TropFishR, length-based spawning potential ratio (LBSPR), and length-based Bayesian biomass (LBB) to assess the stock status these two flounder species. Length and weight data for a total of 4715 blackfin flounder from August 2011 to December 2023 and 1947 pointhead flounder from march 2015 to December 2023 were collected from two landing sites (Hupo and Ayajin) in the East Sea of Korea. Both flounder species exhibit postive allometric growth patterns based on the length-weight relationship with b (slope) values of 3.39 and 3.40 for G.stelleri and C.pinetorum, respectively. The estimated growth parameters, growth co-efficient (k), and asymptotic length (L∞) using the ELEFAN-GA method were 42.27 cm and 0.16 year-1 for for G.stelleri, and 43.44 cm and 0.20 year-1 for C.pinetorum. The estimated total mortality based on the length-converted catch curve (LCC) was Z=0.92 year-1 and 1.19 year-1 for G.stelleri and C.pinetorum, respectively. The exploitation rates (E) for both of the species were higher than the optimum rate of 0.5, i.e. 0.66 year-1 for G.stelleri and 0.69 year-1 for C.pinetorum. This indicates that both species are in an overexploited condition. The results of the LBSPR showed that both flounder species are experiencing recruitment overfishing due to very low spawning potential ratio: 0.23 for G.stelleri and 0.19 for C.pinetorum, this is because the majority of the samples were below the size at first maturity (Lm50) of 24.09 cm for G.stelleri and 24.67 cm for C.pinetorum. The fishing mortality to the natural mortality ratio (F/M) for G.stelleri was estimated at 2.25, while for C.pinetorum it was 2.84, both excedding the reference point of 1. The current biomass with respect to virgin biomass (B/Bo) was 0.15% and 0.12% for G.stelleri and C.pinetorum. For both species, the current biomass was lower than the reference point of 0.50. The current fishing mortality was 36% and 61.44% higher than the reference point of F0.4 for G.stelleri and C.pinetorum, respectively.