Estimate of the Intrinsic Rate of Population Increase for the Blue Shark, ＂Prionace glauca＂, in the North Pacific Using a Demographic Method

Abstract

鋸峰齒鮫（Blue shark, Prionace glauca），是一種遠洋的表層性鯊魚，廣泛分布於全球熱帶及溫帶水域。同時也是遠洋鮪延繩釣漁業中重要的混獲魚種（by-catch species），但其資源狀況在北太平洋仍然未知。族群內在增加率（intrinsic rate of population increase, r），是漁業資源評估中的一個重要且關鍵的參數，因此本研究採用過去生物學研究所獲得的生活史參數，包括成長係數、壽命、產仔數、成熟年齡、生殖週期及自然死亡率等，並利用族群統計學分析法來估算北太平洋鋸峰齒鮫的族群內在增加率。族群統計學模式分析的結果顯示，在自然情況下，鋸峰齒鮫的平均族群內在增加率，範圍為0.138-0.513 yr^(-1)，這也表示本種是屬於生產量較高的物種。此外，分析不同的生活史參數輸入值之結果，顯示自然死亡率及生殖週期具有較高的靈敏度。有鑑於此，未來應著重在自然死亡率與生殖週期的研究上，以提高族群統計學參數估計的準確性。本研究的結果可作為北太平洋鋸峰齒鮫貝氏剩餘生產量模式（Bayesian surplus production model）的先驗分布（prior distribution），且此方法可應用於其他種類族群統計學參數的估計上。The blue shark, ＂Prionace glauca＂, is a pelagic species with a circum-global distribution in tropical to temperate waters. It is the top by-catch shark species for the tuna longline fishery in the open seas. However, its population status in the North Pacific Ocean is still unknown. The intrinsic rate of population increase (r) is an important and crucial parameter in fish stock assessment. Hence, the r of the blue shark in the North Pacific Ocean was estimated using a demography analysis in this study. The input life history parameters including the growth coefficient, longevity, fecundity, age at maturity, reproductive cycle, and natural mortality were adopted from the best information available. The results of demographic analysis indicated that the average r of blue shark ranges from 0.138 to 0.513 yr^(-1) in natural condition suggesting that this species is very productive. Furthermore, the results obtained from different cases demonstrated that the natural mortality and reproductive cycle have the highest sensitivity. Future work should focus on accurate estimate of natural mortality and reproductive cycle for this species to improve the demographic estimates. The results derived from this study can be used as the prior of Bayesian surplus production model of blue shark in the North Pacific Ocean. This approach can also be applied to other species on the estimation of demographic parameters.