印度洋長鰭鮪系群之體長別單位加入生產量分析

Abstract

印度洋長鰭鮪為高迴游性的溫帶鮪類。自1980年到2002年間，漁獲量約維持在10,000噸到41,000噸，主要漁具為鮪延繩釣，台灣歷年來漁獲量占60%以上，為主要利用國。鑑於1980年後資源量指標減低，年漁獲量由1998-2001年約40,000噸遞減為2002年約33,000噸與2003年約23,000噸，以最新年度的漁獲資料重新評估刻不容緩。因此，本研究目的在利用1980年至2002年間台灣鮪延繩釣漁船在印度洋捕獲長鰭鮪抽樣體長頻度及漁獲量資料，經區域性及時間序列上的體長頻度資料取代以及漁獲量修正後，分別採用黃等人(1990年)及李與劉(1992年)兩組成長方程式，應用體長別單位加入生產量模式，分析系群資源狀況。 利用體長轉換漁獲曲線所估計的全死亡率(Z)分別為0.468 year-1及0.533 year-1，庖利經驗式所估得的自然死亡率(M)分別為0.323 year-1及0.223 year-1，而漁獲死亡率(F)分別為0.145 year-1及0.310 year-1。由體長轉換漁獲曲線估計漁獲選擇率參數，估計50%選擇率年齡分別為4.79歲及4.27歲。模擬自然死亡率為0.2、0.25、0.3、0.35 year-1情形，各年間的漁獲死亡率及 在單位加入生產量等深線圖上顯示1990、1991、1996及2002年的單位加入生產量接近最大值。由單位加入生產量曲線來看，現今的漁獲死亡率應介於F0.1與Fmax 之間，表示系群處於完全開發狀態。在自然死亡率0.2-0.25 year-1之間，估計產卵潛能比例(%SPR)為8%-34%，雖然採用不同的成長參數及最小捕獲體長對系群評估結果造成差異，但多數情況下皆顯示本系群有加入型過漁的現象，建議目前管理的政策應減少漁獲努力量，使漁獲死亡率在 的水準，同時使現今的產卵親魚量維持在20%的水準之上，以確保永續資源量的補充。

The Indian albacore, Thunnus alalunga, is typically a highly migratory and temperate tuna. The annual total catches of the Indian albacore ranged from 10,000 to 41,000 tones, most of which were caught by longliners between 1980 and 2002. Taiwan is the major fishing country which accounted for about 60% of the total Indian albacore catch. According to the decreasing trend of abundance indices since 1980 and the drop of the annual catch from about 40,000 tones level during 1998 to 2001 to about 33,000 tones in 2002 and 23,000 tones in 2003, the reevaluation of this stock status is urgently needed. Therefore, the objective of this study was to evaluate the stock status of Thunnus alalunga in the Indian Ocean by length-based yield per recruit analysis based on the corrected Taiwanese longline fisheries data from 1980 to 2002. The fisheries data were corrected for their length distributions and catches as spatial-temporal bases. The growth parameters of Huang et al. (1990) and Lee and Liu (1992) were used for age determination, estimation of mortalities and biological reference points. The total mortalities (Z) were estimated as 0.468 year-1 and 0.533 year-1 by the length-converted catch curves. The natural mortalities (M) were estimated as 0.323 year-1 and 0.223 year-1 with the Pauly’s empirical equation. Furthermore, the fishing mortalities (F) were estimated as 0.145 year-1 and 0.310 year-1. The age at 50% selectivity (t50%) was estimated as 4.79 year and 4.27 year, respectively. Given M as 0.2, 0.25, 0.3 and 0.35, the isopleth diagrams of yield per recruit showed F and tc made the yield per recruit close to the maximum in the year of 1990, 1991, 1996 and 2002. According to the yield per recruit curves, the prevailing (Fcur) was in the middle of F0.1 and Fmax, suggesting that the stock status was full exploited. For given M as 0.2 or 0.25, the spawning potential ratio (%SPR) was about 8%-34%. Though the values of %SPR were different due to the growth parameters used, the recruitment overfishing occurred in most cases. The management policy should decrease fishing effort in order to keep the level of %SPR above 20% for the recruitment replenishment of sustainable fisheries resource.