基于点格局的中西太平洋金枪鱼围网中鲣的空间格局特征

Spatial distribution characters of the purse-seine skipjack fishery in the western and central Pacific based on point-pattern analysis

中西太平洋海域10°N^10°S是我国金枪鱼围网渔业的主要生产海域,本研究采用点格局分析方法对自由群鲣(Katsuwonus pelamis)和随附群鲣的空间格局特征进行分析。根据我国渔业公司2015年23艘围网渔船的渔捞日志数据,采用点格局分析方法的单变量函数配对相关函数g(r)、交叉相关函数J12(r)和标记相关函数kmm(r)对不同集群(自由群和随附群)的鲣资源的空间分布格局及竞争关系进行了研究。发现围网自由群和随附群点事件的空间分布上都是非均质的,表现为聚集性;自由群在1.9°~2.3°尺度下表现出随机分布格局。表明热带太平洋鲣在生命史的两个不同阶段,空间格局为相互吸引的集聚式分布特征,其原因在于中西太平洋鲣喜好生活于高温低盐的暖池东侧水域,且有永久收敛的表层水团和盐度锋面能够提供鲣群所需的生物饵料;出现随机分布的原因为这个海域的饵料生物分布的斑块状和不可持续性,鲣高度洄游特性能够保证其跟随饵料迁徙。在0°~0.35°尺度下,自由群和随附群的关系为竞争关系;当空间尺度超过0.35°后,随着尺度增大,两者关系为随机性关系。在随附群在空间尺度超过0.8°后,CPUE之间表现为正相关,有相对较弱的聚集热点和冷点区域,其他各尺度上CPUE为随机关系;自由群的CPUE在各尺度上都表现为随机关系。总之,自由群鲣和随附群鲣的空间格局在小尺度下表现为排斥竞争关系,在较大尺度下为独立的随机关系,其竞争关系主要为食物的有限性导致。

Skipjack tuna （Katsuwonus pelamis） is the largest component of the tuna fishery throughout the western and central Pacific Ocean （WCPO） and it is the main targeted species in purse-seine efforts in the region. Purse-seine fleets usually operate in equatorial waters, from 10°N to 10°S, and the purse-seine fishery is classified into two set types, termed associated and unassociated sets. These set types have different spatial distribution characters. We used point-pattern analysis （PPA） to explore the relationship between the set types. PPA is an important area of spatial analysis and it is increasingly being applied to fishery-related studies. PPA can identify patterns indicative of an underlying process, and it can identify inter-point interactions, which are of primary interest in ecological studies. Using 2015 logbook data from 23 vessels belonging to Chinese fishery companies, we applied the point-pattern methods of the pair correlation function g（r）, cross-correlation function J12（r） and mark correlation function kmm（r）, to study the spatial distribution and competing relationship of two types of skipjack schools： free （unassociated） schools and associated schools. Key results are as follows： （1） The spatial distributions of both the unassociated and associated schools of purse-seine skipjack were heterogeneous. Both showed an aggregated distribution over almost all the scales in the study windows, although the spatial patterns of the unassociated school were random at the range of 1.9-2.3 degrees scales. From this we could conclude that at two different lifespans the skipjack in WCPO both are attracted to each other and prefer to aggregate, and hence random characters were much weaker than that of the aggregated. The spatial distribution likely showed aggregated char- acters because skipjack prefer to inhabit the eastern part of warmer and less-salty water, where the surface water masses and salinity fronts permanently converge, thereby supporting plentiful food organisms for skipjack. The random characters were likely attributable to the patchy and random distribution of the food organisms, which could be unsustainable for skipjack; yet, in WCPO, the highly migratory skipjack are able to follow the movements of the food organisms. （2） There was a negative correlation between the unassociated and associated skipjack schools at the range of 0-0.35 degrees scales, indicating that the two types of schools are in competition within smaller scales. With increases in scale, a competitive relationship, as caused by limited food availability, possibly faded and an independent relationship appeared. We conclude that there is probably no prey relationship between the two types of skipjack schools because there were no attraction relationships at any scale. （3） The CPUE marked point patterns showed a positive correlation with the spatial distribution of associated schools at the scale of more than 0.8 degrees, thus revealing windows of ＇hot spots＇ and ＇cold spots＇; meanwhile, only random relationships were identified at the other scales. The CPUE marked point patterns showed a random relationship with the spatial distribution of unassociated schools at any scale.