倉田 修

INTRODUCTION: Temporal elevation of water temperature positively affects immune activity and disease resistance in poikilothermic teleost fish. The ayu, Plecoglossus altivelis, an important fish species for Japanese freshwater fisheries, is usually produced under higher water temperatures than the natural conditions to facilitate rapid growth. However, it has been reported that rearing fish at higher water temperatures inhibits the development of the thymus, suggesting that resistance to infectious diseases is reduced in ayu reared at higher water temperatures. Here, we show that decreased resistance to bacterial cold-water disease and excessive inflammatory responses occurred in ayu reared at 22°C compared with those reared at lower temperatures. METHODS: Ayu larvae were reared at 12°C, 15°C and 22°C for 77 days and fed 3% of their body weight. Thymus index and condition factor was calculated after the fish rearing. Then, ayu reared at the different temperatures were challenged with Flavobacterium psychrophilum and the fish were sampled for histopathology and gene expression analyses. Further, the fish were vaccinated with formalin-killed F. psychrophilum and continuously reared at the three different water temperatures. Serum antibody titer was determined by ELISA and cumulative mortality in each group was recorded after the bacterial challenge. RESULTS: Ayu reared at 22°C showed a significantly lower thymus index and higher condition factor than those reared at lower temperatures. Infiltrated leukocytes and many melanin pigments were frequently observed in the adipose tissues and spleens of ayu reared at 22°C, respectively, but not in those reared at 12°C. The gene expression levels of inflammatory cytokines such as IL-1β, IL-8 and TNFα in the spleen were significantly higher in the 22°C group than in the 12°C group. The cumulative survival rate after challenge with Flavobacterium psychrophilum was 51.7%, 40.0% and 13.3% in the 12°C, 15°C and 22°C groups, respectively. The relative percent survival values of vaccinated fish reared at 15°C and 22°C groups were lower than those reared at 12°C. Moreover, the specific antibody titer of the vaccinated fish was the lowest in the 22°C group and the highest in the 12°C group. DISCUSSION: These results suggest that rearing the fish under high water temperature causes excessive inflammatory responses similar to metabolic inflammation in human obesity, resulting in a decrease of disease resistance. In addition, thymic involution induced by higher water temperature probably leads the poor response to vaccination. The present study provides insights into the physiological and immunological changes of fish under global warming.

無機物の食品添加物で構成されるオキシリンクSPのサケ科卵のミズカビ病に対する防除効果を調べた。室内試験においてSaprolegnia属菌5種に対するオキシリンクSPの殺菌効果を検討した結果，30分間の薬浴では10,000倍以下の希釈で殺菌作用を示した。野外試験では，Saprolegnia属菌以外の菌類が発育したサクラマス卵を除き，ニジマスおよびイワナ卵での菌糸発育抑制効果を確認した。魚卵毒性については，生産上大きな被害はないが，ニジマス卵の発眼率および孵化率に影響を与えた。一方，イワナ卵およびサクラマス卵に対する影響は認められなかった。オキシリンクSPによる薬浴はサケ科卵を対象としたミズカビ病の防除に有用であるが，薬浴条件について検討する必要がある。