2004 Volume 20 Issue 1 Pages 39-52
The Land Improvement Low, as amended in 2002, stipulates that agricultural practices should not damage the surrounding environment; hence, there is a growing need for quantitative measures of the impact on lake ecosystems of changes in use of agricultural land. In particular, paddy fields, which function directly as areas where fish breed and spawn, may also be having an indirectly adverse effect on fish ecology through water pollution that results from the use of fertilizers and agricultural chemicals. To investigate this possibility, the present research focuses on the Lake Kasumigaura catchment basin. We applied path analysis (a type of graphical modeling effective in the analysis of cause-and-effect relationships featuring both indirect and direct effects), constructed statistical models of the lake ecosystem centering on fish species, and quantitatively clarified the impact on the lake ecosystem attributable to changes in the use of agricultural land. We first carried out, by means of factor analysis,verification of conceptual models based on a qualitative rationale; then constructed a path diagram relating to the lake ecosystem and carried out path analysis; and lastly, identified which models fitted best.The results obtained show that changes in the area of paddy fields cause a standardized overall eccect of 0.331 on loaches and 0.079 on crucian carp, indicating quantitatively that loaches are more susceptible than crucian carp to the impact of changes in agricultural land use. The present research has also shown the potential for applying graphical modeling to the assessment of the impact on lake ecosystems of changes in the use of agricultural land.
