Growth, Feeding, and Age of the Inshore Hagfish, Eptatretus burgeri

Understanding the growth of organisms is of fundamental importance in achieving the sustainable use of bioresources and/or the conservation of ecosystems. Hagfishes are oceanic scavengers that typically inhabit deep water and play an indispensable role in marine nutrient cycling. However, as the conventional methods for age determination are not applicable, the growth and lifespan of hagfishes are completely unknown. In the present study, the growth and feeding of the inshore hagfish (Eptatretus burgeri) in the laboratory were continuously monitored for an unprecedentedly long period (more than 1.4 years), with feeding once every 4-5 weeks. The animals displayed either positive/negative/zero growth across the experimental period. Eptatretus burgeri typically takes 8-10 days to digest food, and excretes all the gut contents at once as huge feces. Based on the growth, feeding and excretion data, it seems plausible that the wild populations feed at less than 2-week intervals. In addition, by adopting a recently developed technique to estimate fish age from the relative growth data, the age-size relationship in E. burgeri was explored. The generated growth curve predicted that the gonads start to develop only 4 years after hatching at the earliest. Eptatretus burgeri can live nearly 50 years, and the animals targeted in commercial hagfishery are mainly 6-9 years old. Moreover, additional insights into the oviposition cycle were obtained. The present study provides essential information for understanding the ecology of hagfishes and consequently highlights the putative vulnerability of wild hagfish populations to fishing and environmental stresses.