Age composition and growth of the yellowfin goby Acanthogobius flavimanus downstream of the Tonegawa Estuary Barrage determined by an angling method

Abstract

The yellowfin goby Acanthogobius flavimanus inhabits the inner bays and estuaries of Japan and has contributed to both recreational and commercial fisheries. However, recently, the catch has drastically declined and it has become an expensive fish. Although there are detailed reports on the spawning ecology, early development, and morphology of larvae and juveniles of this species, few reports compare annual changes in abundance. In this study, long-term changes in population density were investigated by conducting a fixed-point survey using an angling method with constant effort for 10 years. Throughout 122 surveys conducted between 2009 and 2019, with a total fishing effort of 244 hours, 847 individuals were captured. The monthly catch per unit effort (CPUE, individuals per person per two hours) for this species ranged from 0 to 31, with no interannual variation. On the other hand, the seasonal variation in CPUE was distinct throughout the year, with values remaining near zero from February to May, gradually increasing from June onward, and peaking at high values of 8 to 31 between August and November. For some individuals, age was assessed by observing scale ridges. Scales from fish of 93 mm standard length (SL) had one set of broad and narrow bands, indicating that the fish was in the 0-year old class. Scales from fish of 143 mm SL had two sets of broad and narrow bands, indicating that the fish was 1-year old. It was confirmed that two age classes inhabit the study site. Based on the monthly body length composition, the occurrence patterns of each age class were estimated. For the 0-year age class, individuals measuring approximately 80 mm began to be caught around August. Over time, they continued to grow, reaching approximately 120 mm between October and December. For the 1-year age class, individuals measuring approximately 120–130 mm in August grew to 140–190 mm between October and November. This study demonstrated that scrupulous catch records maintained by recreational anglers are effective for fish monitoring, as they provided insights into the long-term and seasonal variations in CPUE, as well as the occurrence and growth dynamics of year classes.