利用雙性別模式進行西北太平洋海域灰鯖鮫之資源評估與管理

Abstract

灰鯖鮫（Isurus oxyrinchus）是台灣沿近海延繩釣漁業常見的漁獲物種，同時也是遠洋鮪延繩釣漁業中主要的混獲魚種。由於灰鯖鮫具有體型大、成長慢、產仔數少及性成熟晚等生活史特性，導致本種很容易有過度開發的風險，但有關其資源狀況的研究至今仍十分有限。單位加入量模式（Per-recruit models）與族群統計學分析法（Demographic analyses），現今已廣泛被應用來評估鯊魚的資源現況。然而單位加入量模式與族群統計學的估計值，例如生物參考點（biological reference points）或族群成長率（population growth rates），通常會受到生活史參數的不確定性影響。此外，大多數的族群統計學方法都是單性別模式，而且假設雌雄具有相同的生活史參數，或者雌性是決定族群動態的主要因子。這些傳統的族群統計學方法，主要由雌性的產仔數與死亡率所建構，不考慮生殖過程的重要性，也因此忽略了交配機制（mating mechanism）這個關鍵因子的影響。然而許多魚種包括灰鯖鮫都具有性別差異（sexual dimorphism）的特性，這也顯示了發展雙性別模式的必要性。因此，本研究建構了雙性別的加入量模式（sex-specific per-recruit model）及雙性別生活史階段別模式（two-sex stage-structured model）來估計灰鯖鮫的生物參考點及族群統計學參數，並使用蒙地卡羅模擬（Monte Carlo simulations）來評估參數的不確定性對生物參考點與族群成長率估計值的影響。 決定性模式（deterministic models）分析的部份，單位加入漁獲量模式（yield-per-recruit model）分析的結果顯示，不論雌、雄魚，當前的漁獲壓力幾乎都高於所相對應的管理參考點 F0.1，除了 Fmax 外。而單位加入親魚量模式（spawning-per-recruit model）分析顯示，目前的親魚量比例（spawning-potential ratio; SPR%）則遠低於一般鯊魚類常用的目標參考點 SPR35% 與門檻參考點 SPR30%。儘管此結果可能會受到不確定性影響，然而即使考量不確定性的影響，目前的親魚量比例仍是低於門檻參考點。這也暗指西北太平洋灰鯖鮫在現今的漁獲壓力下已經處於過度開發的情況。此外，比較族群統計學方法與單位加入量模式所估得的生物參考點顯示，以漁獲死亡率為基礎的參考點，可能不適合做為灰鯖鮫的管理指標（例如 F0.1 與 Fmax）；以生物量為基礎的參考點分析則建議，較為保守的參考點更適合於這個物種的管理（例如設定 SPR40％ 為目標參考點， SPR35％ 為門檻參考點及 SPR30％ 為限制參考點）。另一方面，考量交配機制與性別差異的族群統計學雙性別模式評估結果，同樣顯示灰鯖鮫的族群量在西北太平洋海域已經有開始萎縮的情形，但是如果施行體長限制（size limits）仍有可能維持其族群的穩定。本研究模擬不同管理策略（management measures）的結果指出，基於單性別模式所提出的管理策略，將會低估族群減少的風險（50% population decline risk）。這個結果也表明，應該針對雌、雄魚分別制定合適的管理策略。雌、雄魚全長分別大於 267 公分及 152 公分的體長限制，應可確保本種資源的永續利用。儘管如此，仍有一些不確定性因素未被考量在本研究中（例如灰鯖鮫的洄游型式及不同交配機制是否共存）。未來更多著重在資源現況評估、移動路徑、交配機制與自然死亡率估計方法的研究，將有助於更了解西北太平洋海域灰鯖鮫的族群動態。

Shortfin mako shark, Isurus oxyrinchus, is one of the most commonly species caught in the Taiwanese commercial offshore longline fishery and is the major by-catch of tuna longline fisheries in the far seas. As a large size, slow growth, low fecundity and relatively late-maturing pelagic shark species, shortfin mako is particularly susceptible to exploitation owing to its life-history characteristics, but very little is known on its stock status. Per-recruit models and demographic methods have become popular for assessing the stock status of sharks. However, per-recruit models and demographic estimates such as biological reference points (BRPs) and population growth rates may be influenced by uncertainties in life-history parameters. In addition, most fisheries demographic models are single-sex, and assume that both sexes have the same vital rates or that the dynamics of the population are determined by only females. Thus, these traditional models ignore important aspects of the reproduction process, in particular, because they are based on female fertility and death rates but neglect variability in the mating mechanisms. Nevertheless, many species, including the shortfin mako, are sexually dimorphic in vital rates, which suggest the need for two-sex models. In this study, a sex-specific per-recruit model and two-sex stage-structured models were therefore constructed to estimate shortfin mako BRPs and population dynamics. Monte Carlo simulations were used to evaluate the impact of uncertainties in the estimation process. For each sex, deterministic yield-per-recruit analyses showed that the current fishing mortality was much higher than the corresponding BRPs of F0.1, except for Fmax. However, results of the spawning-per-recruit (SPR) analyses indicated that the current SPR (SPRcur) was substantially lower than target (SPR35%) and threshold (SPR30%) levels. Although these results may be affected by uncertainty, the estimates of SPRcur were never larger than the threshold level of SPR30% when uncertainties were taken into account. This finding implies that the shortfin mako stock in the Northwest Pacific Ocean is over-exploited under current fishing conditions. Furthermore, by comparing the BRPs derived from demographic models with those from per-recruit models showed that fishing mortality based BRPs (e.g. Fmax and F0.1) were not an appropriate BRP candidate for the management of shortfin mako shark. The analysis of biomass based BRPs implied that more conservative BRPs are recommended for this species (e.g. setting target level of SPR40%, threshold level of SPR35% and limit level of SPR30%). The two-sex model assessments that account for mating mechanism and sexual dimorphism indicate that the stock status of shortfin mako population in the Northwest Pacific is shrinking but might stabilize if size-limit management measures are implemented. The analyses also demonstrated that proposed management measures based on single-sex models would underestimate the probability of population decline risk. The findings reported herein imply that sex-specific management decisions should be implemented for this species. Size limits equivalent to approximately > 267 cm TL for females and > 152 cm TL for males could be implemented to ensure sustainable utilization of the stock. There are still some uncertainties which may not have been considered in this study such as migration patterns and co-exist mating mechanisms for shortfin mako. Additional research focused on estimates of abundance, movement routes, mating mechanism, and direct measurements of natural mortality would advance and refine the understanding of population dynamics of shortfin mako in the Northwest Pacific.