Fish Hatchery Management - fisheries & aquaculture

HATCHERY OPERATIONS8IThe pilot-unit method utilizes a tank or raceway of fish maintained tocorrespond to other tanks or raceways of the same type' The pilot unit issupplied with the same water source and flow, and the fish are fed thesame type and amount of food per unit of body weight. All the fish rearedin the pilot unit are weighed and the gain in weight is used to estimate thefish weight in the other rearing units. This method is more accurate thansample counting for fish up to six inches long.EXTENSIVE CULTUREFish grown in ponds are relatively inaccessible and difficult to inventoryaccurately before they finally are harvested. Pond fish still are sampled frequently,as they are in raceway culture, but the value of such sampling isFIGURE 30. A quarter-sampler can be used to.accurately estimate the number offish per pound or weight per thousand fish. (l) A framed net with four removablepociets in the bo-ttom is designed to fit snugly in a large tub of water. (2)several netfuls of fish are put in the tub and when the frame is removed the fishare divided into four uniform samples. (f) Onty one-quarter of the fish are actuallyused in the sampling. The fish are counted and then weighed. (4) A modifiedframe design has onl of the net pockets closed (arrow) and the other threeopen.Astheframeisliftedoutofthetubthefishintheclosedpocketareretained for counting. It is felt that a sample taken in this manner, from severalnetfuls of fish, reduces bias in sampling' (FWS photos')