Stock structure and resource management of hairtail Trichiurus japonicus based on seasonal broods around the Bungo Channel, Japan

Abstract

responsible for decreased landings. Yield and spawning per recruitment analyses indicate current fishing pressure to be higher than several reference points. We suggest that fishing pressure needs to be reduced by at least 20% of the current level for this fishery to remain sustainable, as the projected stock abundance and catch demonstrate that the current fishing pressure is unsustainable. Analysis of time-series data of recruits per spawning revealed spring-brood recruitment to have been strong in year classes 2003 and 2005. Of various options available for improved management of this fishery, we propose that fishing pressure should be reduced in the years following the appearance of strong year classes to increase future biomasses and landings. Keywords Spawning season · Strong year class · Virtual population analysis · Stock management · Stock abundance IntroductionHairtail Trichiurus japonicas is a widely distributed commercially exploited fish species found around Japan and the waters of the Yellow, Bohai, and East China Seas [1]. Several major fishing grounds exist around Japan, including the East China Sea, the western region of the Sea of Japan, and the waters around the Kii and Bungo Channels (Fig. 1). Though large numbers of hairtail were landed from both the East China Sea and western region of the Sea of Japan [2,3] in the 1960s, the annual catch in these areas has decreased since the 1970s. However, landings of hairtail from waters around the Bungo Channel have increased from the 1970s, increasing the importance of these fishing grounds (Fig. 2). Waters within and around the Bungo Channel provide an important hairtail habitat, where most of the life cycle of this Abstract Declines in landings of the hairtail Trichiurus japonicas indicate the need for more effective management of this species. Hairtail spawning peaks occur twice yearly in the Bungo Channel, in spring and autumn. Relationships between hairtail stock and brood seasonality were examined to determine if an association between either and a decline in landings existed. Stock assessments show that the biomass of both spring and autumn hairtail broods from within and around the Bungo Channel are decreasing, with a rapid reduction in spring-brood stock abundance after 2007 largely . The spring brood comprises individuals spawned during the first peak (May-June), and the autumn brood those spawned during the second peak (September-October). In both broods the annual otolith ring forms from June to August, and thus the radius of the first annual ring in the spring brood is greater than that of the autumn brood. Consequently, spawning season can be determined for each fish, even for older individuals, based on the first otolith ring radius [4,5]. This characteristic has also been observed in hairtail from other areas [1,6,7]. Because of decreased winter growth, an age-length relationship using an extended von Bertalanffy growth model that takes seasonal growth variation into consideration using a periodic function is more suitable than a trad...

A transdisciplinary research of coastal fisheries co-management: the case of the hairtail Trichiurus japonicus trolling line fishery around the Bungo Channel, Japan

“…In other words, this 862 Fish Sci (2017) 83: [853][854][855][856][857][858][859][860][861][862][863][864] scenario is heavily dependent on strong year classes. More than four occurrences of a strong year class in 25 years is a reasonable assumption [27], but it cannot be controlled artificially. Therefore, if a strong year class does not occur for a certain period (e.g.…”

“…As autonomous measures to conserve the hairtail resource, Usuki fishers have introduced various regulations for the fishing season, operation time, total number of baited hooks, and minimum size for landing. However, the stock is shared with purse seiners in the neighbouring prefecture, and the stock biomass (SB) is decreasing [27]. The purse seiners in the neighbouring prefectures have many alternative target species, so their economic dependence on hairtail is minor.…”

“…Consequently, we developed the 'hairtail fishery system operating model' (Hairtail FSOM) which integrates the results from a resource dynamics model [27], a fishing selectivity model [32], economic calculations of the trolling line fishery operation, and hairtail distribution (Fig. 3).…”

“…Hairtail is a rapidly growing fish that has strong year classes periodically [27]. According to the price based on size (Table 1) and the market demand survey, it is sensible to preserve small fish and concentrate fishing pressure (FP) on larger fish.…”

“…Based on the models and data reported in Watari et al [27] and Hirose et al [32], the effects on SB and CV from the combination of (1) replacement of the bait, and (2) decreasing FP, were calculated for 25 years in 4-month steps. As detailed in [32], the type-1 bait represents the status quo (all bait is live sand eel).…”