太平洋黑皮旗魚空間性別年齡結構模式及資源評估

Abstract

黑皮旗魚廣泛分布於太平洋熱帶與溫帶海域，其對於特定棲息環境喜好可能影響其被捕獲的機率，本研究藉由泛加成模式(GAMs)分析延繩釣漁業黑皮旗魚漁獲資料，探討黑皮旗魚資源時空分布與環境因子間的關係，結果顯示黑皮旗魚資源分布隨著海表層水溫(SST)而有季節性變動，然而先前資源評估研究卻忽略季節性分布變動可能造成的影響以及黑皮旗魚在成長上之性別差異，有鑑於此，本研究建構空間性別年齡結構族群動態模式，以進行太平洋黑皮旗魚資源評估。模式套適的資料包括日本與台灣遠洋鮪延繩釣漁業黑皮旗魚釣獲率與體長頻度資料及GAM所推估黑皮旗魚族群密度之空間分布，考慮季節性洄游與性別差異之評估結果指出，太平洋黑皮旗魚資源現況良好，目前產卵親魚量(S2006)維持在未開發程度時相當高的比例(S2006/S0 = 0.81)，亦高於維持最大可持續生產量(MSY)的水準(S2006/SMSY = 1.66)，而目前漁獲死亡率(F2006)低於達到MSY的水準(F2006/FMSY = 0.51)，風險分析顯示目前之漁業利用率可使黑皮旗魚資源維持相當穩定。利用蒙地卡羅模擬法(Monte Carlo simulation)，評估此資源評估模式對於參數估計之表現，模擬結果顯示資源評估結果因為自然死亡率與加入量與親魚量關係式之曲度(steepness)設定值不同產生差異，透過非漁業性(fishery-independent)資源調查以及標識放流試驗，則可以降低此兩參數的未確定性。當評估模式忽略季節性分布變動與性別差異的影響時，會對資源量、MSY與漁業利用率之估計產生偏差，而目前產卵親魚量對未開發產卵親魚量之比值，則為管理參數中最為穩健之估計量，本研究建議太平洋黑皮旗魚資源評估模式必須考慮季節性洄游與性別結構所造成的影響，以降低參數估計之不確定性，並能改善資源評估結果的準確性。

Blue marlin are widely distributed throughout tropical and temperate waters in the Pacific Ocean. However, the preference of this species for particular habitats may impact its vulnerability to being caught. The relationship between spatio-temporal patterns of blue marlin abundance and environmental factors is examined using generalized additive models (GAMs) fitted to catch and effort data from longline fisheries. Spatial distributions of relative density of blue marlin indicate that there is seasonal variation in the distribution of blue marlin, which appears to be related to shifts in sea surface temperature (SST). However, previous stock assessments for blue marlin in the Pacific Ocean have ignored the possible impacts of seasonal movement and sexual dimorphism in growth. A spatially sex-specific age-structured population dynamics model was therefore constructed and fitted to the catch-rate and length- frequency data for the Japanese and Taiwanese pelagic tuna longline fisheries, along with information on the relative density of the population over space derived from the GAM. Assessments that account for seasonal movement and sexual dimorphism indicate that the blue marlin population in the Pacific Ocean is in a healthy condition, with the current spawning stock biomass (S2006) at a fairly high fraction of its unfished level (S2006/S0 = 0.81) and larger than that corresponding to maximum sustainable yield (MSY) (S2006/SMSY = 1.66) and the current fishing mortality (F2006) less than that needed to achieve MSY (F2006/FMSY = 0.51). A risk analysis suggests that the status of this population will remain optimistic if exploitation rates are maintained at the current level. The estimation performance of the assessment model is examined using Monte Carlo simulation. This shows that the outcomes of the assessment are sensitive to the values assumed for natural mortality and stock-recruitment steepness. The uncertainty related to these two parameters could be reduced through fishery-independent abundance surveys and tagging studies. Although estimates of biomass, MSY, and fishing intensity are substantially biased when the assessment method ignores seasonal movement and sexual dimorphism, the relative biomass measure S2006/S0 appears to be the most robust among the quantities considered. Therefore, assessment methods for blue marlin in the Pacific Ocean need to take seasonal migration and sex-structure into account to reduce the uncertainty of parameter estimates, and thus improve the accuracy of stock assessments.