中西太平洋大型洄游魚類生態風險評估

Abstract

本研究採用生產力與敏感性分析(Productivity and Susceptibility Analysis, PSA)評估中西太平洋主要大型表層洄游魚類之生態風險，包括長鰭鮪、大目鮪、黃鰭鮪、太平洋黑鮪、長腰鮪、正鰹、劍旗魚、紅肉旗魚、黑皮旗魚、白皮旗魚、雨傘旗魚、鬼頭刀及竹節鰆；其中劍旗魚、紅肉旗魚和長鰭鮪分為南、北兩系群予以評估。評估生態風險包含兩大指標：生產力指標及敏感性指標，生產力為受到漁業或自然因子影響時，資源所能復育的能力，其因子包括各項生活史參數，如成長係數、自然死亡率、最大年齡、營養位階、50%成熟年齡與年孕卵數；敏感性用以評估漁業對資源的影響程度，包括區域重疊、垂直重疊、漁獲死亡率與自然死亡率的比值、市場魚價、群游行為及歷年漁獲量趨勢。本研究將生產力與敏感性以高、中、低三種等級估計各魚種之脆弱性(Vulnerability)，以作為生態風險之評估標準。研究結果顯示太平洋黑鮪、白皮旗魚、大目鮪、中西北太平洋紅肉旗魚的生態風險較高，應密切監控並評估其資源現況，尤其白皮旗魚尚未有過完整資源評估，應優先蒐集評估所需的漁業與生物資料；中度生態風險之魚種包括西南太平洋劍旗魚、黑皮旗魚、南北太平洋長鰭鮪、長腰鮪、雨傘旗魚、西南太平洋紅肉旗魚及黃鰭鮪，應持續蒐集評估所需資料；生態風險較低之魚種，如中西北太平洋劍旗魚、正鰹、鬼頭刀及竹節鰆，則應持續監視其漁業的變化。各因子影響分析結果顯示，群游行為和最大年齡對生態風險評估的結果影響較大，營養位階及市場魚價影響程度相對較小。資料品質分析結果建議應加強長腰鮪、鬼頭刀及竹節鰆之漁業生物學(如生殖生物學)及營養位階的研究，另外需進一步蒐集長腰鮪的漁業資料。

Productivity and susceptibility analysis (PSA) was used to assess the ecological risk of large migratory species in the western and central Pacific Ocean (WCPO), including albacore, bigeye tuna, yellowfin tuna, Pacific bluefin tuna, longtail tuna, skipjack tuna, swordfish, striped marlin, blue marlin, black marlin, sailfish, common dolphinfish and wahoo. Information from tuna longline (LL), tuna purse seine (PS) and pole and line (PL) fisheries was also used in the analysis. The risk for swordfish, striped marlin, and albacore was assessed by putative stock boundaries (northern and southern) in the WCPO. The productivity index represents the capacity of the stock to recover and was estimated based on biological parameters of the species, including von Bertalanffy growth coefficient, natural mortality (M), maximum age, trophic level, age at 50% maturity and annual fecundity. Six attributes for susceptibility were calculated including (1) areal overlap of fish distribution and fisheries, (2) vertical overlap (i.e., fish depth distribution and fisheries), (3) ratio of fishing mortality (F) to M, (4) fish price, (5) schooling behavior and (6) annual catch trends that represented factors to measure the potential impact of fishing on stocks. Vulnerability for each stock was calculated based on the ranking of the productivity and susceptibility attributes, which was then used to evaluate the ecological risk of the species. Results showed that the ecological risk for Pacific bluefin tuna, black marlin, bigeye tuna and the northern stock of striped marlin were high, suggesting a critical need for detailed monitoring that would, in turn, allow for stock assessments, with special reference for black marlin that to date, has not been assessed. For species identified with medium risk (such as the southern stock of swordfish, blue marlin, both stocks of albacore, longtail tuna, sailfish, the southern stock of striped marlin and yellowfin tuna), data for stock assessments should be collected continuously. The fishery using resources with lower risk (such as the northern stock of swordfish, skipjack tuna, common dolphinfish and wahoo), should be monitored closely. As indicated by removing one variable each time in the sensitivity analysis, the impacts of schooling behavior and maximum age were highly significant in the model, but were relatively minor for trophic level and fish price. The data quality analysis indentified gaps in knowledge and suggests that further studies are warranted for longtail tuna, common dolphinfish and wahoo to obtain biological parameters, and that fishery information and data should also be collected for longtail tuna.