Fish Hatchery Management - fisheries & aquaculture

HATCHERY OPERATTONShowever, fish over ll inches long can outrun the pulled seine, and are likelyto escape, leaving a nonrepresentative sample. This problem can be partiallyovercome by setting the net across, or pulling it into, a corner of thepond instead of pulling it to a straight shore. The uniformity of fish distributionis the most difficult aspect to determine. Many species form aggregationsfor one reason or another. A seine might net such a cluster or therelatively empty space between them. It helps to sample several areas ofthe pond and to average the results, although this is time-consuming, andseines rarely reach the pond center in any case.Fish can be concentrated for sampling if the pond is drawn down. Thiswastes time-it can take two or three days to empty a pond of severalacres-and a lot of water. It also can waste a lot of natural food productionin the pond. Unless fish have to be concentrated for some other purpose,such as for the application of disease-control chemicals, ponds should notbe drawn down for sampling purposes.In summary, pond fish should be sampled regularly, but the resulting informationshould be used for production calculations only with caution.Fish GradingFish grading-sorting by fish length-makes possible the stocking of uniformlysized fish if this is necessary for fishery management programs. Also,it reduces cannibalism in certain species of fish; some, such as striped bassand northern pike, must be graded as often as every three weeks to preventcannibalism. Grading also permits more accurate sample counting and inventoryestimates by eliminating some of the variation in fish size. An additionalreason for grading salmon and steelhead is to separate smaller fishfor special treatment so that more of the fish can be raised to smolt size bya specified time for management purposes (Flgure 32).In trout culture, good feeding procedure that provides access to food byless aggressive fish can minimize the need for grading. However, grading offish to increase hatchery production by allowing the smaller fish to increasetheir growth rate is questionable. Only a few studies have demonstratedthat dominance hierarches suppress growth of some fish; in most cases,segregation of small fish has not induced faster grorvth or better food utilization.In any fish population there are fish that are small because of theirgenetic background and they will remain smaller regardless of opportunitiesgiven them to grow faster.In warmwater culture-and extensive culture generally-fish usuallycannot be graded until they are harvested. Pond-grown fish can vary greatlyin size, and they should be graded into inch-groups before they are distributed.Products of warmwater culture often are sold in small lots to