Anchovy abundance and assemblage composition in relation with ENSO events

ENSO (El Nino-Southern Oscillation) 是半週期性的大氣海洋作用，通常發生在熱帶太平洋，其包含 El Nino和 La Nina 兩種事件，伴隨ENSO事件的發生週期，太平洋之東部及西部發生氣壓以及海表面溫度的明顯變化。 若干研究顯示，此變化對於海洋生態以及漁業有廣泛及深遠的影響。 在本論文中，我企圖探究台灣周遭重要近海漁業─魩鱙漁業之漁獲組成與ENSO現象變化之間的關聯。 魩鱙漁業的主要漁獲目標為大洋性表層洄游的鯷科和鯡科魚類；在台灣周遭以日本鯷、異葉公鯷以及刺公鯷為主要物種組成。 此三物種之繁殖及洄游路徑各自對應不同洋流，因此三物種之相對組成變化，在探討海洋事件方面可能可以反應其所棲息之洋流改變之情形。 除主要對象外，混獲魚類亦為我的探討目標，尤其在混獲魚類當中有許多魚類其習性不同於鯷科魚類，棲習於不同水層，與洋流之相依程度不一，藉由分析這些混獲魚種，也可以幫助我們釐清ENSO事件發生時其影響是否侷限於海水表層。 本研究ENSO事件方面依據海洋聖嬰指數(Ocean Nino Index)，取2008年春天為La Nina事件，2009年春天為中性狀況，2010年為El Nino事件。 根據台灣魩鱙漁業的漁場分為東北、西北、以及西南三處，取此三地上述時段之漁獲樣本以氣候資料。 在漁獲樣本中，將每一尾魚盡可能鑑定到種的階級。 將魚種分為目標魚種組和混獲魚種組，混獲魚種組中又依其生態習性分為大洋表層魚類與非大洋表層性魚類。 比較不同魚種之豐度變化，以Spearman’s rank correlation analysis 檢驗在同一氣候條件下不同地區間魚種組成的相關性。 並藉由群集分析法檢視各地區在各氣候狀態下魚種組成的相似程度。 最後計算各漁獲樣本之物種多樣性。 研究結果顯示，在西北和西南區域，目標魚種皆有與ENSO相應之變化。 在混獲魚種方面，表層魚類之差異性歸因於ENSO事件的可能性較低；非表層魚類面其物種組成較平均，顯示ENSO現象帶來的影響較小。 在地區間的差異方面，主要反應在目標物種上，中性狀況時東北和西南水域其物種組成有正相關，然而在El Nino時東北和西北組成有正相關，反之在La Nina時西北和西南有正相關，這可能反映了在不同ENSO條件下海流狀況的不同。 群集分析法的結果中，亦只有在表層魚類有較明顯的分群關係。 因此，可以推測ENSO事件對於台灣周遭魚類群集組成的影響主要透過表層海流的變化所影響。 又，許多魚類的相對組成變化，其極端值反而發生在中性條件下，由此點來看，在探討台灣周遭魚類群集組成時，El Nino和La Nina事件可能不完全代表相反的氣候相(meteorological phases)。ENSO (El Nino-Southern Oscillation) is a quasi-periodic climate pattern, which includes two large-scale meteorological events of the El Nino and La Nina. ENSO variations can be seen significantly in the alterations of atmosphere and surface water temperature in the tropical Pacific. The effects from ENSO events on ocean ecology and fisheries have long been noticeable with pretty studied. In this study, I try to estimate the relationship between ENSO and the catch composition of Taiwanese larval fisheries, which are important economic practices in the inshore waters. The main targets of Taiwanese larval fishery are anchovies and herrings, which all live to pelagic life in coastal waters. Japanese anchovy (Engraulis japonica), shorthead anchovy (Encrasicholina heteroloba) and Buccaneer anchovy (Encrasicholina punctifer) are the most commonly species. The migration of these three species are by-and-large associated with 3 different prevailing currents around Taiwan, thus their composition patterns may correspond to the shifts of the prevailing currents. Besides, by-catch is also inevitably interested to illustrate non-pelagic species in response to ENSO variations. In my study, the ENSO event is determined following the Ocean Nino Index (ONI). According to the ONI, I took the spring sample of 2008 stands for La Nina, 2009 as neutral and 2010 as El Nino. My research sites include the northeastern (NE), northwestern (NW) and southwestern (SW) waters of Taiwan. Identified of fish samples were grouped into 3 categories of target, by-catch pelagic and by-catch non-pelagic. I compared the composition of the abundance of the species, using spearman’s rank correlation to test the areal difference in each climatic condition. Cluster analysis was used to the similarity of the categories. The diversity of the samples was also calculated. It turns out that in the ENSO condition, the target species composition reflected a similarity between NW and SW, but the pattern did not reflect the by-catch species. The compositions of non-pelagic species were vague, in response to the ENSO shifts, partly due to the later took place primarily in surface water. The compositions of target species of NE and SW were positively correlated in the neutral condition; but situations shift to NE and NW in the El Nino condition, and NW and SW in the La Nina condition. The explainable reasons for the composition shift may ascribe to the changing pattern of the surface currents. Clustering analyses indicated that the target species presented a clear grouping following prevailing currents, but not non-pelagic species, supports that the ENSO effect took placed in marine surface fish species. Finally, owing to substantial amounts of extreme measurements were taken in the neutral condition, the fish assemblages around Taiwan in these two extreme meteorological conditions may not be simply mirrored to the opposite conditions