Reproduction is variable in the spawning-rearing pond method. Millions of young are produced in some ponds and only relatively few in others. There is considerable size variation among fish because fathead minnows spawn repeatedly over time. Harvested fish must then be graded to sort out saleable minnows from under-sized fish.
In minnow ponds that end up at too high a density, the fish may not grow enough to reach bait size. If the densities are too low, yields will also be low. With such variable yields, there is a tendency to transfer juveniles from heavily-stocked ponds, and spread them out at lower densities for growth. Unwanted fish species, such as mosquitofish, can be accidentally moved in the process.
Is there a better way? Major golden shiner and goldfish producers now tank-hatch eggs in indoor hatcheries and stock the fry into nursery ponds. This provides control over stocking densities, and prevents transmission of most diseases from adults to young. A hatchery system for fathead minnows could provide these same benefits.
The major challenge to an indoor hatchery system is how to efficiently and economically collect sufficient eggs at one time to produce the 1-2 million fry per acre needed to stock a pond. Fathead minnow males are territorial and thus nests are usually spread out over spawning material.
Also, although a female typically will spawn every four days or so, the number of eggs per spawn is relatively low, 300-500 eggs each time. Eggs can either be removed from substrate (1.5 per cent sodium sulfite solution) and jar-hatched or, if suitable compact substrate can be found, it may be possible to leave the eggs attached to the substrate for incubation and hatching. In either case, fathead minnow eggs appear to be very susceptible to fungus, and will require daily treatment during egg incubation.
It appears that rosy red fathead minnow fry can be stocked at 1-2 million per acre or higher, with good survival and high yields, at least in experimental ponds. Obviously, this needs to be tested in larger ponds under commercial conditions. In 2009, hatchery fry were stocked into six, 0.1-acre ponds inside a fenced and netted enclosure. Stocking was done over a two-month period, so the fish densities cannot be directly compared. Starting three weeks after stocking, ponds were aerated nightly (1/2-HP aerators) from 10 p.m. to 8 a.m.
Feed was a 32 per cent crude protein commercial catfish diet (ARKAT Nutrition, Inc.) in the form of a meal (first four weeks), crumbles (weeks 5 and 6) and 1/8 inch extruded pellet (week seven to end of study). Starting at 56 days, fish were fed to satiation once daily, defined as feed consumed within 30 to 50 minutes. Feed was distributed over the surface of the entire pond.
Table 1. Harvest results from stocking fathead minnow fry at 100,000 to 3.22 million per acre in earthen ponds and rearing them for 118-169 days.
|Pond||Stocking Date||Days in Culture||Estimated stocking rate, million fry/acre||Fish harvested, million/acre||Yield lb/acre||Avg wt (g)||Avg wt (lb/1000)||% of weight retained by #13 grader||FCR*|
|57||Aril 28||169||0.100||0.064 + reproduction||398||2.8||6.2||99.6||3.52|
|58||Aril 30||167||0144||0.239 + reproduction||1087||2.1||4.5||97.1||1.95|
|*FCR for low stocking rate ponds was high (poor) because of the experimental protocol – fish were fed at fixed rates for the first 56 days.|
Figure 1. Histograms showing the distributions of fish weights at harvest (based on 50-fish samples) from ponds 64 (left) and 65 (right) with 1.5 and 1.8 million fish/acre, respectively. The majority of fish were between 2 and 3 pounds per 1,000.
At harvest (Table 1), fry that were stocked at densities that resulted in 1.5-1.8 million fish acre, produced yields of 400 pounds per pond, equivalent to 4,000 pounds per acre. Feeding rates during the season exceeded 100 pounds per acre per day in these ponds, so they were similar to channel catfish ponds in intensity. Fish from these ponds were relatively uniform in size.
Most were between 2 and 3 pounds per 1,000 (Figure 1). At low densities (100,000–140,000 fry per acre), the fish grew quickly, but matured early, and reproduction occurred within 3-4 months after stocking fry. The largest fish produced (7 pounds per 1,000) were those stocked at 324,000 per acre, with no reproduction.
Ponds in which fish are fed at high rates require aeration, and while one aerator was sufficient for these small ponds, large commercial ponds may require multiple aerators. It appears unlikely that all the small fat-head minnows in a large commercial pond would congregate in a single location, behind 1-2 large aerators, as catfish do. It may be that multiple aerators located around the pond will be needed, similar to what is done in shrimp ponds.
The additional aerators, though, will increase costs. It is also important to distribute feed over the entire pond surface. Schools of minnows could be seen swimming around, seemingly without directed movement towards available feed, so feed was dropped in front of them. The 1/8-inch pellets used were far bigger than a fathead’s mouth, but even so, fish were able to work on the softening pellets and consume them.
A fathead minnow hatchery could be added to an existing golden shiner or goldfish hatchery. A rough estimate of total costs was developed by Horne et al. (2010), who estimated that, for a 50-million fry hatchery add-on, the costs would be $362 per million fry.