魚類族群之空間變異會隨著年齡截切及豐度變動而提升

空間結構對於魚群的穩定度扮演重要的角色，例如魚群可以透過分布在不同區域的子魚群（subpopulations）來分散環境變異對特定區域造成衝擊的風險，使魚群得以在變動的環境中存活下去。過去研究指出魚群的空間結構會受到生物因子及環境因子所影響。就生物因子而言，族群的豐度及體長結構是主要的影響因子。首先，當族群的豐度改變時，個體會依據族群數量的多寡以及棲地的優劣來選擇合適的居住地，以調和種內的競爭壓力，並提升族群繁衍的配適度。其次，個體因其體長不同而具備不同的環境適應能力與移動能力，故在空間上呈現不同的分布。而就環境因子而言，環境變異會改變棲地的物理條件，迫使族群遷徙，進而改變族群在空間上的分布及聚集結構。　　本研究利用1991至2015年間北海的底拖網調查資料，檢驗九種受捕撈物種的空間結構是否受到族群的豐度、體長結構及環境變異所影響。在過去文獻上已有發展出許多量化空間結構的指標，大多數的指標都是基於衡量族群在空間上分布的歧異度。依據相同概念，本文利用變異係數（coefficient of variation）衡量族群的空間結構，並觀察其如何受生物及環境因子所影響。我們利用經驗動態模型（empirical dynamic modeling）分析空間變異和豐度、體長結構及環境波動間的因果關係。結果顯示，魚群的空間變異主要受到豐度及體長結構所影響，而環境變異的影響效果則相對不明顯。此外，當豐度下降或平均體長變小時，族群的空間變異將會提升；但環境變異對於空間變異的作用方向則無法確定。根據本研究結果，我們認為漁撈行為不只是過去所認為會減少魚群的豐度及造成年齡截斷現象，還會進一步提升魚群的空間變異，破壞子魚群間的連結關係，因而削弱魚群抵抗環境變異的能力。

關鍵字

空間結構；分散風險；豐度；大小結構；環境變異；經驗動態模型

並列摘要

Spatial structure of a population provides bet-hedging capacity to maintain population stability, which has become an important management concern of exploited marine species. Population spatial structure has been suggested to be affected by both biological processes and environmental changes. For biological processes, two most discussed issues are the abundance and size structure of populations. First, population abundance can critically determine their occupied areas, as explained by the density-dependent habitat selection theory. Second, size structure influences population spatial structure because individuals of different sizes inhabit different areas due to size-specific requirements and mobile capabilities. In addition, environmental changes can force populations shift their distribution and thus alter spatial structure. Here, we examined how the spatial structure (measured by coefficient of variation) of nine exploited fish species in the North Sea responded to changes in abundance, size structure, and environment, using data from the International Bottom Trawl Survey from 1991-2015. Applying empirical dynamic modeling approaches, we found that population spatial variation responded more to changes in abundance and size structure than to changes in the environment. More specifically, population spatial variation increased in response to decreasing abundance and mean age. However, environment exhibited no consistent effect on population spatial variation. Our results suggest that size-selective fishing may enhance spatial population variation through reducing abundance and/or truncating size structure, and thus exert more pressure on exploited populations than previously thought.

並列關鍵字

population spatial structure ； bet-hedging capacity ； abundance ； size structure ； environmental changes ； empirical dynamic modeling

參考文獻

Anderson, C. N. K., Hsieh, C., Sandin, S. a, Hewitt, R., Hollowed, A., Beddington, J., May, R. M., et al. 2008. Why fishing magnifies fluctuations in fish abundance. Nature, 452: 835–839.

Atkinson, D. 1994. Temperature and organism size—A biological law for ectotherms? Advances in Ecological Research, 25: 1–58.

Bailey, K. M., Abookire, A. A., and Duffy-Anderson, J. T. 2008. Ocean transport paths for the early life history stages of offshore-spawningflatfihes: a case study in the Gulf of Alaska. Fish and Fisheries, 9: 44–66.

Baudron, A. R., Needle, C. L., Rijnsdorp, A. D., and Tara Marshall, C. 2014. Warming temperatures and smaller body sizes: Synchronous changes in growth of North Sea fishes. Global Change Biology, 20: 1023–1031.

Berkeley, S. A., Chapman, C., and Sogard, S. M. 2004a. Maternal age as a determinant of larval growth and survival in a marine fish, Sebastes melanops. Ecology, 85: 1258–1264.

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魚類族群之空間變異會隨著年齡截切及豐度變動而提升

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