与随机捕捞的银河鱼(中)相比,拿走大鱼(下)和拿走小鱼(上)都将对种群个体的平均大小产生深远的影响。(图片提供:Stephan Munch)
渔夫都爱钓大鱼,久而久之,这种做法会使鱼类种群在遗传层面上保持较小的个体。然而一项新的研究表明,这种向下的螺线是可以逆转的。如果渔夫能够随机地撒网,那么大鱼的储备就会出现一个反弹。
大多数水产业都只把一个种群中的大鱼当做目标。拖网使得最小的鱼能够逃脱,而只把钓鱼当做消遣的人们通常会把那些小家伙扔回大海。在这种情况下,如今因大量捕捞而在上世纪80年代面临灭顶之灾的加拿大鳕鱼的个体要远远小于历史记录的水平。这样的趋势让渔业科学家们感到无比担忧,因为与小鱼相比,大鱼往往更健康并且繁殖力更强。
研究人员一直想搞清,他们在渔船甲板上看到的鱼类个体平均尺寸的下降是否源于环境因素,抑或鱼类种群的遗传变化。2002年,美国纽约州立大学石溪分校的海洋生态学家David Conover和他的同事发现,遗传因素是造成这一局面的部分原因。在实验室分析中,Conover的研究小组在4年或4代的时间里,从两组银河鱼(Menidia menidia,生活在美国东北海域的一种具有商业价值的鱼类)中拿走了90%的大鱼。结果显示,随着体型大、生长快的银河鱼从种群中消失,那些在遗传上生长速度较慢的银河鱼逐渐在种群中占据了统治地位,并导致银河鱼平均尺寸的下降。
Conover于是便想搞清这一过程是否有逆转的可能性。研究人员这一次还是用缩小的银河鱼作为研究对象。他们从每组中拿走了90%的银河鱼,但这一次研究人员并不是专拣大的捞,而是随机选择。经过6代的时间,这些鱼又重新增加了50%的体长。研究小组在最近出版的英国《皇家学会学报B》上报告了这一研究成果。
据Conover分析,这一结果意味着“如果我们停止捕捞那些已经变得很小的鱼类……那么就可以期待大鱼的回归”。以银河鱼为例,他推算这种情况会在12年或12代之后发生。Conover表示,对包括大麻哈鱼在内的其他经济鱼类而言,由于每一代的时间间隔更长,因此实现同样的过程大概需要几十年。
挪威卑尔根大学的进化生态学家Christian Jorgensen指出:“这一积极的信息告诉我们逆转是可能的,并且相当快。”然而芬兰赫尔辛基大学的生物统计学家Anna Kuparinen却指出,在实际捕捞过程中,渔夫们并不能像在实验室中那样高效地识别每条鱼的大小,因此种群所面临的选择压力并没有那么高。Kuparinen表示,正是基于这种原因,你很难确定一种野生种群是否会在捕捞过程中产生类似的变化。(生物谷Bioon.com)
生物谷推荐原始出处:
Proc. R. Soc. B March 4, 2009, doi: 10.1098/rspb.2009.0003
Reversal of evolutionary downsizing caused by selective harvest of large fish
David O Conover1,*, Stephan B Munch1 and Stephen A Arnott1,2
1School of Marine and Atmospheric Sciences, Marine Sciences Research Center, Stony Brook University Stony Brook, New York, NY 11794-5000, USA
2South Carolina Department of Natural Resources, Marine Resources Research Institute PO Box 12559, Charleston, SC 29422, USA
Evolutionary responses to the long-term exploitation of individuals from a population may include reduced growth rate, age at maturation, body size and productivity. Theoretical models suggest that these genetic changes may be slow or impossible to reverse but rigorous empirical evidence is lacking. Here, we provide the first empirical demonstration of a genetically based reversal of fishing-induced evolution. We subjected six populations of silverside fish (Menidia menidia) to three forms of size-selective fishing for five generations, thereby generating twofold differences among populations in mean weight and yield (biomass) at harvest. This was followed by an additional five generations during which size-selective harvest was halted. We found that evolutionary changes were reversible. Populations evolving smaller body size when subjected to size-selective fishing displayed a slow but significant increase in size when fishing ceased. Neither phenotypic variance in size nor juvenile survival was reduced by the initial period of selective fishing, suggesting that sufficient genetic variation remained to allow recovery. By linear extrapolation, we predict full recovery in about 12 generations, although the rate of recovery may taper off near convergence. The recovery rate in any given wild population will also depend on other agents of selection determined by the specifics of life history and environment. By contrast, populations that in the first five generations evolved larger size and yield showed little evidence of reversal. These results show that populations have an intrinsic capacity to recover genetically from harmful evolutionary changes caused by fishing, even without extrinsic factors that reverse the selection gradient. However, harvested species typically have generation times of 3–7 years, so recovery may take decades. Hence, the need to account for evolution in managing fisheries remains.