The Japanese anchovy Engraulis japonicus is used as live bait in skipjack pole-and-line fisheries. Japanese anchovies are transported to fishing grounds by fishing boats on voyages that can take from 4 to 50 days. Because ammonia excreted by Japanese anchovies might affect their survival during transportation, fishers empirically attempt to maintain them in healthy condition by exchanging water aboard ship holding tanks at a rate of 160–500 % volume h−1. To develop protocols for more efficient and cost-effective transportation of Japanese anchovies, we evaluated their ammonia tolerance and effects of stocking density and variable water exchange on their survival. We estimated median lethal concentrations (with 95 % confidence intervals) of un-ionized ammonia nitrogen (UIAN), at which 50 % of Japanese anchovies had died within 24 h and 48 h, as 0.770 (0.751–0.790) mg l−1 and 0.706 (0.661–0.750) mg l−1 at 15 °C and 0.634 (0.466–0.802) mg l−1 and 0.450 (0.379–0.521) mg l−1 at 25 °C, respectively. While the UIAN concentration increased to lethal levels and severe fish mortality (dependent on stocking density) occurred at 25 °C with no water exchange, the UIAN concentration could be maintained within non-lethal levels at 25 °C and with a one-third water exchange every 12 h. Our results suggest that present-day empirically deduced water exchange rates applied aboard commercial vessels in holding tanks of Japanese anchovies could be reduced, implying a possible cost reduction for the skipjack pole-and-line fisheries.
We conducted field surveys of aquatic animals in seven rivers on the Pacific coast of Atsumi Peninsula, Aichi Prefecture, Japan and collected 30 species of fish, 21 species of decapod crustaceans, and 5 species of gastropods. Fourteen species of them were described in detail based on the specimens, of which two fishes (Kuhlia marginata and Eleotris melanosoma), five freshwater caridean shrimps (Macrobrachium formosense, M. lar, M. latimanus, Caridina typus and C. multidentata), a brackish-water crab (Ptychognathus ishii) and a freshwater snail (Septaria porcellana) represented the first records from Aichi Prefecture. Since all of these species had already been recorded from adjacent areas, lack of surveys and/or reports from small streams in the prefecture may explain why there were so many first-time records. On the other hand, some of them were diadromous species which mainly inhabit more southerly areas, and therefore these species might have come to appear in Aichi Prefecture in recent years due to the effects of global warming.
Photoperiod is a robust seasonal stimulus for phenotypic plasticity in the regulation of reproduction and development. Artemia brine shrimps have a unique reproductive mechanism to alter their reproductive mode between ovoviviparity, with the release of free-swimming nauplii, and oviparity, with the production of diapause cysts, in response to differences in photoperiod. However, the existence of relationships between reproductive modes and other reproduction-related traits (e.g., growing period, number of spawns, and litter size) remains elusive. In this study, we show that long-day conditions increase the proportion of free-swimming nauplii and the number of eggs during a lifetime compared to short-day conditions. Histological analysis revealed structural differences in eggs between ovoviviparity and oviparity, providing insight into the alteration of energy allocation: long-day conditions can produce a large number of eggs because a thin egg shell is formed, whereas short-day conditions decreased the number of eggs, because a thick egg shell is formed. The present study provides new insight into the energy trade-off to develop a phenotypically plastic trait in Artemia.
An inventory of the macrobenthic invertebrate fauna of the temperate tidal flats on southern Izu Peninsula, Pacific coast of Japan is presented. Surveys conducted during from 2014 to 2018 in the Aono and Ogamo River mouths. A total of 74 macrobenthic taxa, including 24 endangered species were recorded in these areas of diverse intertidal habitats including bare tidal flats, salt marshes and semi-mangrove Hibiscus hamabo. The faunal diversity and number of endangered species were comparable to those reported from adjacent regions (e.g., Tokyo Bay and southern Fukushima Prefecture), with 15 of the endangered species specific to the southwest Pacific Japanese coast, suggesting that these populations were nearly their northeastern distribution limits. New records for the gastropod Cerithium coralium and bivalve Isognomon ephippium on southern Izu Peninsula represented the northeastern distribution limits of both species. The inventory will serve as a faunal reference for conservation of biodiversity in the study areas, and aid future monitoring under any environmental changes occurred in the region.