獵物體型大小影響仔魚之豐度

Abstract

仔魚存活率對於魚群入添變異有著重要的影響。過去野外觀察研究發現浮游動物密度與仔魚密度呈現正相關，這意味著獵物密度對仔魚存活率扮演關鍵角色。過去大多數研究皆使用中型浮游生物密度作為仔魚食物量指標。然而，根據仔魚腸胃道內含物之分析，中型浮游動物並非仔魚主要攝取的獵物對象。仔魚胃腸道中發現的獵物體寬大多為 50-200 μm，其主要類群為卵、橈足類幼生。因此，本研究將著重於探討小型 (體寬為 50-200 μm) 浮游動物密度對於仔魚之影響，並將該結果與中型 (橈足類成體) 浮游動物比較。本研究預期小型浮游動物密度將與仔魚密度呈現更顯著之正相關。另外，我們同時探討環境因子是否會對於仔魚密度產生關鍵的影響。我們使用 50 μm 網目之浮游生物網採集小型浮游動物樣本，並將屬於仔魚主要獵物之8個橈足類幼生類群進行辨認、量測其體寬。根據其體寬，我們進一步將小型浮游動物細分為6個大小等級，並計算每個大小等級之密度。此外，我們使用網目為 330 μm 之浮游生物網採集中型浮游動物，並計算其中橈足類成體之密度；仔魚樣本則透過 1000 μm 網目之浮游生物網採集。結果顯示，中型浮游動物與小型浮游動物之密度皆與仔魚密度呈現正相關，但小型浮游動物之結果更為顯著。其中，第一 (50-75 μm) 與第二 (75-100 μm) 大小等級之小型浮游動物密度與仔魚密度的相關性最為顯著。然而，所有本研究調查之環境因子皆沒有與仔魚密度呈現任何明顯相關性。總結來說，本研究結果指出小型浮游動物密度最能解釋東海的仔魚密度。因此，我們認為探究仔魚與浮游動物之營養階層關聯時，應使用小網目 (例如：50 μm) 之浮游生物網來採集獵物樣本，並使用小型浮游動物作為仔魚的食物量指標。

Larval survival is critical to recruitment variability of fish. Some field observations have shown a positive relationship between larval fish and zooplankton abundance, suggesting that prey availability plays an important role in larval survival. In the majority of those studies, mesozooplankton density was used as the index indicating prey availability for larval fish (hereafter, larval prey index). However, mesozooplankton are too large to be the potential prey for larval fish, according to gut content analyses of larval fish. In fact, the most abundant prey in the guts of larval fishes are within the size (body width) of 50-200 μm, which are mainly consisted of nauplii, copepodites, and eggs. Here, we focus on the small-size (50-200 μm) zooplankton; we then compare the result with that of meso-size (adult copepods) zooplankton. We hypothesize that the density of small-size zooplankton exhibits a stronger positive relationship with larval fish density. Also, we analyzed environmental variables to investigate their potential effects on larval density. We collected the small-size zooplankton using the plankton net of 50 μm mesh size. We focused on nauplii and copepodites, which are the main prey for larval fish according to previous gut content analyses. We further categorized the small-size zooplankton individuals into 6 size classes according to the body width. In addition, we used a 330 μm mesh size net to collect the meso-size zooplankton and focused on adult copepods. Larval fish were collected by a plankton net with 1000 μm mesh size. Both meso-size and small-size zooplankton densities showed a positive relationship with larval fish density, but the relationship is stronger for the small-size zooplankton. Furthermore, for the first 2 size classes (50-75 and 75-100 μm), their densities showed the highest positive relationship with larval density. In contrast, none of the examined environmental variables explained larval density. In conclusion, our results showed that larval fish density in the East China Sea is better explained by the density of small-size zooplankton. Our findings suggest that when studying the trophic link between larval fish and zooplankton, it is better to use a plankton net with fine mesh size (e.g. 50 μm) to collect prey samples and focus on the small-size zooplankton as the larval prey index.