A three-phase growout process is commonly used in the commercial production of hybrid striped bass in earthen ponds. Phase I fingerlings, 2.2 to 3.3 lb/1000, are stocked into phase II ponds at a density of approximately 12,000/acre. Harvest of fish (2.2 to 4.5/lb) from phase II ponds is followed by size grading that precedes stocking into ponds (3,500/acre) for the final growout phase. Size grading is designed to reduce the degree of variation in harvest weight after the final stage of growout has been completed and may contribute to significant mortality. The prospect of completely bypassing phase II of growout in favor of a direct stocking of graded, phase I juveniles into ponds has been investigated for the past four years. Fish larger than those traditionally stocked into phase II growout ponds would be used in this direct stocking procedure.
The advantages of this management practice can translate into a more efficient enterprise. Mortality that often arises from the inability of smaller fish to transfer from a live to a formulated diet after stocking into phase II ponds would be minimized. Larger juveniles are already feeding on a formulated diet before stocking into the final growout ponds occurs. In addition, by stocking larger fish, the total growout time necessary to produce an acceptable market size would decrease.
Recent experiments at the Eastern Unit of the NWAC have demonstrated that this proposed management practice potentially has application and is economically practical. Juveniles harvested from phase I of growout were graded to produce two relatively uniform individual weight classes, 6.6 lb/1000 (3 g each) and 11.0 lb/ 1000 (5 g each). These fish were then stocked at either 3,500/acre or 4,500/acre into ponds that represented the second (final) stage of growout in the direct stock procedure. After stocking, the fish were fed commercially manufactured > 40% crude protein feeds daily to satiation for 17 months. At harvest, survival of fish in all ponds ranged from 51 to 99%, but was consistently highest in ponds stocked with 5-g juveniles (83% versus 70%). Average production for the 3,500/acre and 4,500/acre stocking densities, 5-g stocking weight, was 4,020 and 4,952 lb/acre, respectively.
A comprehensive economic analysis revealed that the treatment consisting of a 4,500/acre stocking density and 5-g stocking weight yielded the highest net return. Variable costs increase due to the higher cost of purchase of larger juveniles to stock. However, this cost is of little consequence when compared to costs incurred from mortality and the labor to harvest fish from phase II ponds and then grade them for stocking into phase III ponds.
Within the total population of fish harvested from ponds stocked with 5-g juveniles, 18 to 30% weighed more than 1.25 lb, the size generally in highest demand. A 4 to 5 month extension of this final stage of production, equivalent to the total combined time commonly needed for traditional second and third phases of production, would probably have yielded 50% of all harvested fish being market size. For a 17-month experimental growout period, economic analysis indicated that the direct stock, two-phase growout management practice is more profitable than the traditional three-phase growout.
This successful experimental demonstration of the direct stocking procedure for commercial production of hybrid striped bass in earthen lays the foundation for future investigations. For example, using lower stocking densities and a stocking weight even greater than 5 g should reduce the time to market size to approximately 14 or 15 months, rather than the current combined 20 to 24 months for phases II and III. Despite the lower overall production per pond resulting from lower stocking densities, more crops can be harvested per unit of time, and significantly higher returns over time would eventually be realized. In addition, if the time to final harvest is reduced, then the risk of crop loss due to disease or adverse environmental conditions is correspondingly reduced.
With the scaling up the direct stocking procedure from small experimental ponds to commercial ponds, a management problem arises. Fish stocked at low densities into ponds of 5 to 10 acres may not be able to encounter a sufficient amount of formulated diet for maximum growth. The resulting slower and most probably disproportionate growth would probably contribute to a greater variation in the individual size of fish within the pond population. A seemingly practical solution to this problem is to confine the originally stocked fish temporarily (2 to 4 weeks) to a small section of the pond. The reduction in feeding area achieved by this “penning” procedure should serve to enhance encounter to feed, and the higher density within the confined area should also help to stimulate feeding activity.
The continued success and development of the aquaculture industry in the United States require the incorporation of management practices that improve economic return to the producer through an increase in the efficiency of the use of resources. The direct stock management practice currently under investigation for hybrid striped bass production responds to this need.
Presented at the National Aquaculture Extension Conference.