ON THE RATIONAL NORMS FOR THE MESH SIZE OF NETTINGS AND FOR THE INTERSTICE OF FENCES INTENDED FOR PREVENTING FISH-ESCAPING

The rather low recapture rate of the implanted fish in many sizable lakes andreservoirs at present is due,for one thing,to the fleeing of the implanted fingerlings.In order to establish a set of standards on the nettings and fences to be installedfor sake of preventing the fingerlings from escaping,the total length (L) and themaximal girth (MG) of the body,as well as the width of head (WH) and the widthof the paired parietal bones of cranium (WP) of 2227 fingerlings of four“domestic”fish (Hypophthalmichthys,Aristichthys,Ctenopharyngodon and Mylopharyngodon),ranging from 3 to 16 cm in total length,have been measured,and therefrom theregressions of MG,WH and WP on L,together with their 95% confidence intervals,have been worked out.It is demonstrated,experimently,that under lotic conditions,fingerlings againstthe current are capable of“squeezing”through meshes of which the circumference issmaller than the MG,or through interstices of which the width is smaller than the WH.Such capability can be measured in terms of what the authors call as“penetrant coef-ficient”,designated by either Kg or Kp.For Hypophthalmichthys fingerlings,Kg was determined as 1.5,and Kp as 1.2,based upon the data obtained from some sewage-containing inlets of Lake Tung-Huduring March and April,1973.The authors propose that under ordinary still-water conditions,nettings to beinstalled for preventing escaping should have their mesh-length not longer than 1/2the lower limit of the 95% confidence interval for the MG of the fingerlings to bestocked,and fences of all sorts should have their interstices not wider than the lowerlimit of the 95% confidence interval for the WH of the fingerlings to be implanted.Under running-water conditions,moreover,allowance should be made for the penetrantcoefficient.As an appendix,numerical values of the MG,WH,WP,corresponding to the Lof the fingerlings for each of the four species are tabulated,and values of the upperand lower limits of their 95% confidence intervals are also given for reference.