An Approach to Conservation of the Endangered Tokyo Bitterling, Ta Kia Ta Go, in a Tural Habitat in Southeastern Tochigi — Habitat Requirement Assessment and Habitat Restoration for Juveniles

27 Mar An Approach to Conservation of the Endangered Tokyo Bitterling, Ta Kia Ta Go, in a Tural Habitat in Southeastern Tochigi — Habitat Requirement Assessment and Habitat Restoration for Juveniles

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Abstract

国の天然記念物ミヤコタナゴの自然生息地において，ミヤコタナゴ稚魚の生息環境を調査した．生息環境条件を明らかにするために，一般化線形混合モデル (GLMM) を用いたモデル選択を行った結果，流速，水中カバーの有無，二枚貝に産み付けられていたミヤコタナゴの卵数の 3 つの説明変数を含むモデルがベストモデルとして選択された．流速は負の，水中カバーおよび卵数は正の回帰係数を示したことから，稚魚は流速が遅く，水中カバーがあり，卵数の多いところに生息していると考えられた．このモデルに基づいて生息地における稚魚の生息確率を予測したところ，水路上流部で生息確率が低いことが明らかとなった．そこで，上流部で稚魚の個体数増大を図るため，稚魚の生息環境条件に基づいて，水路底の掘削，杭の設置，二枚貝の放流による環境改善を実施した．環境改善の効果を検証するため稚魚の定位状況を調査したところ，稚魚観察数の合計は環境改善を行った調査区で 99 個体，環境改善を行わなかった調査区で 41 個体であった．稚魚数の約 71 ％が環境改善区で確認されたことから，本研究で実施した環境改善によって，ミヤコタナゴ稚魚にとって生息可能な環境を増加させることができたものと考えられる．環境改善区で観察された稚魚が，その後も上流部で定着・繁殖することで，生息地全体の個体数の増大につながることが期待される．本研究で実施した生息環境条件の解析と構築されたモデルは，他のミヤコタナゴ生息地における生息環境の維持・復元を図る際にも，活用することが可能であろう．

English Abstract

The microhabitat characteristics of the Tokyo bitterling, Ta kia ta go, desig ted a tural monument of Japan, were investigated in its tural habitat. To elucidate the habitat requirements of juvenile bitterling, we a lyzed the relationships between the occurrence (presence/absence) of juveniles and microhabitat variables using a generalized linear mixed model (GLMM). The model showed that the occurrence of juveniles was positively correlated with the presence of underwater cover and the number of bitterling eggs deposited in adjacent mussels, and negatively correlated with current velocity. When the habitat was evaluated using this model, it predicted that the occurrence probability of juveniles was low in the upper reach of the ca l. Therefore, the upper reach habitat was restored (i.e., the bottom was excavated, stakes were installed, and mussels were introduced) after considering the habitat requirements of juveniles. To assess the effect of these restorations, we compared the occurrence of juveniles between restored and control (non-restored) stations. Over the duration of the observations, 99 and 41 juveniles were found at restored and control stations, respectively; 71 % of the total number of juveniles was observed in the restored sections. These results suggest that the restoration scheme could increase suitable habitat for juvenile bitterling. Assuming that the juveniles observed in the upper reach of the ca l survive and breed, the size of this population should increase. The habitat-requirement model constructed in this study should be applicable for mainte nce and reconstruction of habitats of this species.