溫度與橈足類豐度對亞熱帶仔稚魚成長率的影響： 以日本帶魚為例

Abstract

仔稚魚的成長率攸關存活率以及族群動態，然而亞熱帶魚種的相關研究仍然較少。 許多亞熱帶的魚種產卵季節較長，因此在仔稚魚時期可能經歷不同的成長環境 (如：溫度、食物量)。為了增進對亞熱帶魚種生活史早期的了解，我們以日本帶 魚作為研究物種。日本帶魚是重要的經濟魚種，在台灣沿海(約21.8 – 25.4⁰N, 119.2 – 122.1 ⁰E)全年有產卵的紀錄。而台灣沿岸環境受到季風以及海流的影響， 形成多樣的溫度與生產力關係。我們提出假說認為，溫度與食物量的變化會造成 日本帶魚仔稚魚成長率的變異。我們分析台灣西北、東北與西南沿岸，日本帶魚 仔稚魚成長率、橈足類豐度，以及水溫資料得到以下結論：橈足類豐度在西北部 與溫度呈正相關，但在東北及西南岸無此關係(相關係數r 分別為0.44, 0.15, -0.25)。 日本帶魚仔稚魚(日齡2 至60 天)成長率在西北部也與溫度呈正相關，在東北及 西南沿岸則無明顯溫度趨勢。我們建立生物能量學模型，模擬溫度與橈足類豐度 對帶魚成長的影響，並比較現今溫度下與全球暖化後帶魚的成長率。結果顯示溫 度對帶魚成長率造成的影響在區域間不同，成長率在西北海域隨溫度增加，東北 及西南則無明顯變化，模擬結果與觀察到的平均成長率數值不同但空間趨勢一致。 暖化造成的成長率改變趨勢在區域間也不一致，會增加西北岸成長率，而減少東 北及西南岸成長率。本研究整合實測與模擬的資料，在較小的空間尺度下，印證 了環境因子對日本帶魚生活史早期成長率的不同影響。氣候變遷造成溫度與生產 力的改變，可能影響亞熱帶仔稚魚的成長與入添量，因此也需要在漁業管理中納 入考量。

Early life growth rates provide insight on larval survival and determine future population dynamics. Such knowledge for subtropical fishes, however, is understudied. Subtropical fishes often display prolonged spawning seasons, thus, their larvae may experience differential growing conditions (e.g., temperatures and food availability). To advance current knowledge on early life growth rates for subtropical fishes, we carried out a study based on an important fisheries species, the cutlassfish Trichiurus japonicus, which spawn all year round throughout the coastal water of Taiwan (around 21.8 – 25.4⁰N, 119.2 – 122.1⁰ E). The coastal environments of Taiwan are influenced by differential monsoons and currents, interacting to form variable patterns in temperatures and productivity. We hypothesized that such variable temperature and food abundance could lead to variation in early life growth rates for cutlassfish. We compared growth of larval and early juvenile cutlassfish, copepod abundance (i.e., an index of food availability), and temperature data among the three coasts: NW, NE, and SW coasts, 2000-2015. Copepod abundance was positively correlated with temperature at the NW but not NE or SW coasts (r = 0.44, 0.15, and -0.25, at NW, NE, and SW respectively). Similarly, daily growth rates of cutlassfish (at age = 2-60 days) were positively correlated with temperatures at the NW but not NE or SW coasts. To elucidate the effects of temperature and copepod abundance on fish growth, we constructed a bioenergetics model and compared simulations with the temperature and copepod abundance among three coasts in present-day vs. global warming scenarios. Simulation outputs reflected differential temperature effects on cutlassfish early life growth rates, with similar pattern as our empirical data, but the magnitudes did not match the mean growth rates among coasts. The impacts of climate change on growth also varied among coasts: simulated growth increase at the NW but decrease at NE and SW coasts. Integrating field data and bioenergetics modeling, our study demonstrates diverse environmental effects on early life growth for cutlassfish in subtropical ocean within small spatial scale. Our study provides implications for population dynamics under climate change, which induces anomaly in temperature and food availability, influencing growth and recruitment of fish. Such effects should be considered for fishery management of subtropical marine fishes.