Body-Size Boundaries for Sexual-State Classification in a Protogynous Freshwater Fish: Evidence from Monopterus albus in Thailand

Monopterus albus (Asian swamp eel) is a commercially important, air-breathing freshwater fish harvested throughout East and Southeast Asia. As a protogynous species, it begins life as a female, passes through a transitional phase, and ultimately becomes male, making sexual-state classification difficult and potentially affecting management in capture fisheries and aquaculture. We examined sex–size relationships in market-purchased, reportedly wild-caught M. albus from northern and northeastern Thailand using gonadal histology to determine sexual state, together with body length (BL), body mass (BM), and a separate otolith-aged sample. Among 199 histologically examined individuals, no females were observed above approximately 70 cm BL or 270 g BM. Across much of the intermediate size range, female, transitional, and male individuals overlapped in size. Transitional individuals reached 78 cm and 424 g, showing that the upper values represent female-exclusion boundaries rather than thresholds identifying males. No male was smaller than 55 cm BL or 200 g BM, although sampling below these values was sparse. Otolith-derived age estimates from a separate sample of 70 eels provided local size-at-age context but could not establish age at sex transition because sexual state was not determined in those individuals. Comparisons with published studies showed substantial regional variation in size–sex relationships, indicating that size-based criteria developed in one region may not transfer reliably elsewhere. The boundaries identified here provide practical guidance for fisheries monitoring and broodstock selection in northern and northeastern Thailand while emphasizing the need for locally validated criteria elsewhere.