LAUSR

Abstract Periophthamus modestus is an amphibious fish that inhabits mudflats along the Northwest Pacific, including Vietnam. Knowledge about the length–weight relationship (LWR) and condition factor (K) is important for assessment and management of fish resources, but they are poorly known for P. modestus. The goal of this study was to examine the variation of the LWR and K among sexes, developmental stages, seasons, and habitats of this species. Over the course of two years (2018 − 2020), specimens were collected monthly from the Red River Delta area in north Vietnam. The standard length of the 1037 fish collected ranged from 0.83 to 5.63 cm (mean, 3.33 ± 1.08 cm), and weight ranged from 0.007 to 3.50 g (mean, 0.86 ± 0.70 g). The female to male ratio was 1:0.76. The LWR regression parameter varied significantly between sexes (3.18 for females; 2.98 for males) and developmental stages (2.91 for un-sexed; 3.11 for adult) and among habitats (3.18 for Xuan Thuy National Park; 3.05 for Ba Lat Estuary; 3.02 for Tien Hai Wetland Nature Reserve). Fish growth patterns differed significantly between the dry (3.12) and wet (3.05) seasons. We also found significant impacts of season, developmental stage, season–gender interaction, and habitat–developmental stage interaction on the variation of K. In general, the LWR indicated positive allometric growth, and K was approximately greater than the ideal value (K > 1). These results suggest that P. modestus is well adapted to the environmental conditions in the study area. The positive allometric growth and high K value for adult fish collected from Xuan Thuy National Park suggest the importance of the well-developed mangrove forest that characterizes this locale. The higher variation in the sex ratio, growth pattern and K of the more southern population reflects the biodiversity of this mudskipper across its distributional range.