Hiroyuki Kurota

Developing a management procedure robust to uncertainty for southern bluefin tuna: a somewhat frustrating struggle to bridge the gap between ideals and reality

Fisheries management is conducted to achieve sustainable use of fishery resources, mainly through regulation of fishing activities. For almost a decade, the Commission for the Conservation of Southern Bluefin Tuna (CCSBT) struggled to reach agreement on a total allowable catch (TAC) for southern bluefin tuna (SBT) because of stock assessment uncertainties. To address this, in 2002 the CCSBT commenced development of a management procedure (MP), a pre-agreed set of rules to determine how the TAC will be adjusted as new monitoring data become available. The CCSBT Scientific Committee tested various candidate MPs using operating models which simulate fish population and fishery dynamics as well as incorporate process, observation, and model uncertainties. Candidate MPs were evaluated using performance measures related to the following management objectives: maximize catches, avoid stock collapse, and minimize interannual catch variation. Of the MPs explored, some relied solely on empirical data [i.e., adjusted TAC based on catch per unit effort (CPUE) trends], whereas others were more complicated, based on population models. In 2005, the CCSBT adopted a model-based MP that realized a moderate catch with low variability and avoided stock collapse. This MP struck a compromise between the risk-prone and risk-averse standpoints of the different stakeholders. However, despite this concerted scientific effort, the MP was not implemented because, shortly after its adoption, it became evident that historical catches may have been substantially underreported. This complication necessitates returning to near the beginning of the development process. MP approaches have various advantages and challenges to be explored further. However, it is essential to lessen human-introduced uncertainty (such as catch misreporting) by enhanced enforcement, and to increase management robustness to biological uncertainties by implementing MPs.

Geographical Variation in the Seasonal Population Dynamics of Bruchidius dorsalis (Coleoptera: Bruchidae): Constraints of Temperature and Host Plant Phenology

Abstract We investigated the seasonal population dynamics of the bruchid beetle, Bruchidius dorsalis Fahraeus, in relation to the phenology of its host plant Gleditsia japonica Miq. (Fabaceae: Caesalpinoidea) in two geographically distinct areas of Japan. We compared life cycles to clarify influences of seasonal variation in temperature and host plant availability on insect life cycles. Results from a 4-yr study indicated that in Sagamihara (warmer climate): (1) B. dorsalis has a trivoltine life cycle, (2) it cannot reproduce during a period of more than 1 mo until new seeds become available in early or mid-August, and (3) it overwinters in several different developmental stages (young and old instars, and adults). However, studies over 3 yr in Tatsuno (cooler climate) indicated the following: (1) B. dorsalis has a bivoltine life cycle, (2) G. japonica phenology lags several weeks behind that in Sagamihara, and (3) B. dorsalis overwinters as diapausing larvae (late fourth instar) or as adults. Thus, the life cycles of B. dorsalis are more constrained in cooler areas, not only because of the direct effects of lower temperature on insect development, but also because of the phenological delay in host plant development.

Photoperiod- and temperature-dependent induction of larval diapause in a multivoltine bruchid, Bruchidius dorsalis

The wild bruchid beetle, Bruchidius dorsalis Fahraeus (Coleoptera: Bruchidae), has a multivoltine life cycle and overwinters in several developmental stages in the middle part of Japan. We investigated the incidence of diapause under different conditions of photoperiod (from L8:D16 to L16:D8) and temperature (at 20 °C and 24 °C). Our experiments revealed the following results: (1) B. dorsalis entered diapause at the final (late fourth) instar larva under short photoperiods, (2) the larval diapause incidence was dependent on temperature (critical photoperiods were 12.5 h at 20 °C and 12 h at 24 °C), (3) some individuals did not enter diapause under short‐photoperiod conditions at 24 °C, and (4) the sensitive stages to the photoperiod were from the late egg stage to the early first instar larva. Based on these results, we discuss not only the evolution of a complex overwintering strategy inB. dorsalis but also the domestication process of stored‐bean pests.

Geographical variation in photoperiodic induction of larval diapause in the bruchid beetle, Bruchidius dorsalis: polymorphism in overwintering stages

The bruchid beetle, Bruchidius dorsalis Fahraeus (Coleoptera: Bruchidae), has a multivoltine life cycle and shows geographical variation of overwintering stages in Japan. Our previous study found that B. dorsalis enters larval diapause in the final instar under short photoperiods. In cooler areas, we observed that most individuals overwinter in the final larval stage in diapause, whereas beetles at different developmental stages (non‐diapausing young instars, diapausing instars, and adults) were overwintering in warmer areas. In this study, we investigated geographical variation in the photoperiodic response for induction of larval diapause at 20 °C (three populations) and 24 °C (two populations) to clarify the overwintering strategy of B. dorsalis. We observed that (1) diapause incidence at 20 °C changed sharply from ca. 100% to 0% with a change in photoperiod in all the populations, (2) critical photoperiod was longer at 20 °C in populations from cooler areas, and (3) critical photoperiod at 24 °C was shorter than at 20 °C and a fraction of the larvae did not enter diapause, even under short photoperiods. Overwintering stages estimated from these results were consistent with those actually observed in the field. This study indicates that the geographical variation of overwintering stages is likely to reflect adaptive diapause induction in each local environment.

Photoperiod‐dependent adult diapause within a geographical cline in the multivoltine bruchid Bruchidius dorsalis

Abstract The bruchid beetle Bruchidius dorsalis Fahraeus (Coleoptera: Bruchidae) has been known to undergo larval diapause during the final instar under short photoperiods ( Kurota & Shimada, 2001 ). This species has a multivoltine life cycle and the overwintering stages show a geographical variation across Japan ( Kurota & Shimada, 2002 ). In cooler areas, overwintering occurs during the final instar, whereas in warmer climates overwintering can occur during several developmental stages: non‐diapausing young instars, diapausing instars, and adults. In this study, we investigated the adult reproductive diapause in three populations from different geographical regions to clarify the role of geographical variation on overwintering strategies. We found that: (1) B. dorsalis entered reproductive diapause in addition to larval diapause under short photoperiods, (2) diapause propensity was higher and the critical photoperiod was longer in populations from cooler regions, and (3) the sensitive photoperiod range was the first 5 days after emergence. Predictions of the overwintering stage, derived from critical photoperiods, were consistent with actual overwintering stages observed in each population. The geographical variation in diapause induction is likely to reflect the adaptive overwintering strategy in each local environment.

Regular distribution of larvae and resource monopolization in the seed beetle Bruchidius dorsalis infesting seeds of the Japanese honey locust Gleditsia japonica

Abstract Larvae of the wild multivoltine bruchid seed beetle Bruchidius dorsalis infest dry mature seeds of the Japanese honey locust Gleditsia japonica. The frequency distribution of B. dorsalis larvae per seed was examined in the laboratory. First, preliminary observation showed that the first-instar B. dorsalis larva crawled from an eggshell and traveled in excess of 25 cm before boring into a seed. Larvae prefer boring into noninfested rather than infested seeds (indicated by a previous emergence hole). When a larva occasionally bored into a previously infested seed, it could emerge as an adult in spite of decreased body weight. Second, we conducted experiment 1 to investigate the frequency distributions of the number of boring holes per seed; results showed significantly regular distributions (a smaller variance than Poisson) in which most seeds had been bored by one larva. Third, we performed experiment 2 to examine the relationship between the initial number of larvae boring into one seed and the number of emerged adults and their body weight from a given seed. When we increased the ratio of egg density to seed abundance by experimental manipulation, multiple larvae (up to ten) could bore into one seed. However, only one adult emerged from each of about 85% seeds with multiple borings. When one adult emerged, the adult body weight was constant, independent of the initial number of larvae boring into the seed. Two adults occasionally emerged from one seed with a large difference in their body sizes. Even in this case, the larger adult was as heavy as that in a single emergence, although the other was significantly smaller (about 60%) than the normal size. These results show severe contest-type competition among B. dorsalis larvae within one seed. The winner often monopolizes almost all the resources even though a part of the seed resource still remains.

Overwintering Strategies Depending on High Cold Hardiness in Nondiapause Stages in Bruchidius dorsalis (Coleoptera: Bruchidae)

Abstract Bruchidius dorsalis Fahraeus (Coleoptera: Bruchidae) enters winter diapause in either the fourth (final) instar or during the adult stage (Kurota and Shimada 2001, 2003b). In the middle part of Japan, it overwinters in several developmental stages: nondiapausing young instars, diapausing instars, and adults (Kurota and Shimada 2002). To understand the relationship between the complex overwintering stages and the adaptive diapause strategy of B. dorsalis, I investigated (1) overwintering survivorship in developmental stages observed in the field and (2) cold hardiness at all stages including nonoverwintering ones such as eggs and pupae after enduring a controlled chilling experiment at 5°C for 100 d. The following results were obtained: (1) survivorship of nondiapausing larvae under natural conditions was almost as high as that of diapausing larvae and adults, and (2) cold hardiness in eggs and pupae was much lower than the other developmental stages in terms of survivorship and reproductive ability after the chilling treatment. From these results, I concluded that larval and adult diapause of this species would be induced at the most appropriate time to avoid overwintering in developmental stages with low cold hardiness. The complex overwintering stages in species that inhabit the middle part of Japan are likely to result from the adaptive timing of diapause induction at the two stages.

Over-wintering stage polymorphism of a bruchine beetle: geographical variation in optimal diapause strategy

Abstract 1. Most temperate insects undergo diapause at a specific developmental stage to overcome severe winters. The bruchine beetle Bruchidius dorsalis in a warmer region in Japan, however, has diverse over‐wintering stages – non‐diapausing larvae, diapausing larvae, and diapausing adults, whereas in a cooler region, it over‐winters only as the final instar larva or adult in diapause.