Fish Hatchery Management - fisheries & aquaculture

HATCHERY OPE,RATIONS 79should be made whenever necessary to assure that records provide accuratedata. In any inventory, it is imperative that fish weights be as accurate aspossible.INl'ENSIVE CLILl'UREFish can be weighed either by the wet or /ry method. The zs'el method involvesweighing the fish in a container of water that has been preweighedon the scale. Care must be exercised that water is not added to thepreweighed container, nor should water be splashed from it during weighingof the fish. This method is generally used with small fish. Dry weighingis a popular method of inventorying larger fish. The dip net is hungfrom a hook at the bottom of a suspended dial scale. The scale should beequipped with an adjusting screw on the bottom, so the weight of the netcan be compensated for. Dry weighing eliminates some fish handling and,with a little practice, its accuracy is equal to that of wet weighing.The most common ways to determine inventory weights are the samplecount,total-weight, and pilot-tank methods.In the sample-counting method, the total number of fish is obtained initiallyby counting and weighing the entire lot. In subsequent inventories, asample of fish is counted and weighed and either the number per pound orweight per thousand is calculated (Figr-,.e 29). To calculate the number perpound, divide the number of fish in the sample by the sample weight' Tocalculate the weight per thousand, divide the sample weight by the numberof fish (expressed in thousands). ttre total weight of fish in the lot then isestimated either by dividing the original total number of fish (adjusted forrecorded mortality) by the number per pound or by multiplying it (nowexpressed in thousands) by the weight per thousand. This method can beinaccurate, but often it is the only practical means of estimating the weightof a group of fish. To assure the best possible accuracy the following stepsshould be lollowed:(t) fn" fish should be crowded and sampled while in motion.(Z) Otrc" a sample of fish is taken in the dip net, the entire sampleshould be weighed. This is particularly true if the fish vary in size. Thepractice of weighing an entire net full of fish will obtain more replesentativedata than that of weighing preset amounts (such as 5 or 15 pounds).Light net loads should be taken to prevent injury to the fish or smotheringthem.(g) Wtren a fish is removed from water it retains a surface film of water.For small fish, the weight of the water film makes up a larger part of theobserved weight than it does for larger fish. The netful of fish should becarefully drained and the net bottom wiped several times before the fishare weished.