A theoretical analysis was carried out in order to examine the economic effects of a reduction of the number of fishing boats with compensation shared by the remaining fishermen, using a generalized stock production model. The results showed: 1) the reduction improves the profitability of fishery if the stock size at the time of the reduction is less than that at the optimal level of the dynamic Maximum net Economic Yield, 2) a 5-20% reduction sufficiently improves profitability if the stock size at the time of the reduction is similar to that at open access equilibrium, and 3) the extent of improvement of the 5-20% reduction will be maximized when the stock size at open access equilibrium is around 10-30% of the carrying capacity. Issues to be resolved such as a lack of incentive to reduce costs in fisheries were discussed.
Yellow goosefish Lophius litulon is an economically valuable fish species around Japan. The aim of the present study was to evaluate a suitable technique for age determination of yellow goosefish using the illicium. Specimens of yellow goosefish were collected by commercial boats and research vessels from the Sea of Japan, Tsugaru Strait and the Pacific Ocean off Aomori Prefecture from November 2013 to March 2016 using bottom gill, set and trawl nets. Each specimen was measured and sexed, and the illicium and 8th vertebra were removed in the laboratory. The epidermis of each illicium was removed after boiling, and the illicia were dried, whereas each of the 8th vertebra was boiled and sectioned. We found that the best ageing method was a count of opaque zones in the illicia. This was facilitated using the cross-sectioned illicia that had been etched with 1 mol/L HCl for 30 s, stained with methylene blue for 4 h and viewed under transmitted and incident light. Opaque zones form twice per year (mainly during June and November-December) in the illicia. Age determination using opaque zones in the illicia was more accurate than that using opaque zones in the vertebral centra. Because the illicia of yellow goosefish are easy to collect and manipulate for age determination and the growth rate estimated using the illicia and tagging experiments is similar, this method of age determination may be useful for these fish in this region.
Age and growth rates of the invasive exotic catfish Ictalurus punctatus were assessed by examination of otolith sections of 937 specimens (3.8-57.2 cm in standard length, SL) collected in Lake Kitaura, Ibaraki Prefecture, Japan. An opaque zone on the otolith margin, formed once a year from May to June, coincided with the peak spawning season. Von Bertalanffy growth equations differed significantly between sexes, being SLt=63.5{1−exp[−0.120(t+0.416)]} for males and SLt=56.5{1−exp[−0.134(t+0.437)]} for females [SLt is SL (cm) at t (year)]. The estimated maximum ages for males and females were 14 and 13 years, respectively. An age-length key for converting length frequencies into age frequencies was established, and is a useful tool for assessing the population status of the species and the development of effective management strategies.
The species composition of the genus Skeletonema (Bacillariophyceae) was investigated in five Japanese coastal waters by analyzing large subunit rDNA D1-D3 sequences and the fine morphological structures of vegetative and germinated cells. The predominant species were S. grevillei from Main Okinawa Island Midwaters in the subtropical zone and S. dohrnii from the Ariake Sound, Toyama Bay, and Onagawa Bay in the temperate zone, and Funka Bay in the subarctic zone. S. japonicum was found with S. dohrnii in the latter four bodies of water. During the warm season, S. costatum, which can acclimate to low salinity, was found predominantly in the Ariake Sound and Toyama Bay, where the salinity was low, despite the fact that S. costatum is able to germinate from sediments over the temperature range of 10-30°C. The absence of S. costatum vegetative cells in the winter was due to lower specific growth rates of S. costatum than those of S. dohrnii and S. japonicum. During warm periods, S. grevillei and S. pseudocostatum occurred as the dominant species in Onagawa Bay and Funka Bay respectively, where salinities were usually >30. The species composition and seasonal succession of Skeletonema were closely related to temperature and salinity.
Diet components of the cutlassfish Trichiurus japonicus around Hiuchi-Nada in the central Seto Inland Sea, Japan were investigated by monthly samples during 2011-2013. The Japanese anchovy Engraulis japonicus, the western sandlance Ammodytes japonicus and the pasiphaeid shrimp Leptochela gracilis showed conspicuously high values in the index of relative importance percentage (%IRI). Engraulis japonicus and L. gracilis appeared in the diets of T. japonicus for most of the year. The %IRI of fish increased as T. japonicus became bigger, while that of crustacean decreased. Frequent feeding on A. japonicus occurred during spring, which seems to be a specific characteristic of T. japonicus around Hiuchi-Nada.
Mass mortality of cultured bluefin tuna Thunnus orientalis, with the loss of approximately 1,800 individuals, occurred in the coastal area of Tsushima Island, Nagasaki in September 2015. A red tide of harmful unarmored dinoflagellate Cochlodinium polykrikoides and turbid water were observed simultaneously with the fish mortality. Since bluefin tuna farming has not long been established, the impacts of red tide and water environment on cultured bluefin tuna are not fully understood. To understand the cause of bluefin tuna mortality, we examined cell densities of C. polykrikoides and environmental factors at Asou Bay and offshore from west Tsushima Island. At the time of the mortality of cultured bluefin tuna, C. polykrikoides were 172-795 cells mL−1 around the cages. The cell density of C. polykrikoides red tide was lower than that of previous cases of mortality of different fish species. Moreover, mortalities of bluefin tuna were obvious at the cages exposed to turbid water caused by river water inflow due to heavy rain, 3.8-7.6 FTU. These facts suggest that cultured bluefin tuna is rather sensitive to C. polykrikoides red tide and turbid water.
North Pacific krill Euphausia pacifica has very low market value due to its small size and quick quality degradation. To add high value to the krill, the effect of isoelectric solubilization/precipitation using food additives to recover the krill protein was investigated. The krill protein could be solubilized by adding alkaline additives (K2CO3, Na2CO3, K3PO4 or Na3PO4) and precipitated by adding organic acids (citric acid, dl-malic acid, l-tartaric acid or acetic acid). The protein recovery yield was the highest when solubilized at 10.0 and precipitated at pH 4.0. In particular, the treatment using K2CO3 and citric acid showed the highest protein recovery yield (58.8%) which was higher than that of the control (NaOH and HCl treatment; 40.9%). SDS-PAGE pattern, proximate composition, mineral composition and amino acid composition of the protein recovered by K2CO3 and citric acid treatment were comparable to those of the control. From the above, it can be concluded that the isoelectric solubilization/precipitation using K2CO3 and citric acid is effective to recover proteins from the krill.
This study examines the relationship of recognition between companies, which want to start new businesses in aquaculture, and local fishermen in Kamisaki-ura district in Mie Prefecture, as an example. Two different styles of questionnaires were provided to the local fishermen, the first with a five-point scale and the second as a choice experiment, to evaluate the conditions under which they allow companies to start businesses in their region. The first questionnaire was analyzed by factor analysis, and clarified that fishermen accept newly starting companies in aquaculture under six conditions. With these six conditions, the second questionnaire was presented to the fishermen as a choice experiment. The results showed that the fishermen essentially assume an opposite position toward the new entry of the companies. It is also found that they place importance on acceptable conditions in the following order: priority on hiring local people, restriction on use of raw food, hometown of the company president or staff, kind of fish in aquaculture and size of the company.