It starts with the brooder

Authors: Torrans, Eugene; Ott, Brian

Submitted to: Aquaculture America
Publication Type: Abstract Only
Publication Acceptance Date: 2/21/2019
Publication Date: N/A
Citation: N/A

Interpretive Summary: A large size variation in stocked fingerlings in single-batch production ponds may lead to many "out of size" fish (too large and too small) at harvest. This can be minimized by a partial harvest of food-size fish in the summer and/or by grading the fingerlings before stocking. We conducted this study to determine when the size variation begins. It turns out that although troughs of sac fry and swim-up fry look to be identical in size, there is great variation in all stages. The age of the female has a great influence, producing larger sac fry as they age from 2-, to 3-, and to 4-years old. The age of the swim-up fry (2, 4, or 6-days post-swim-up) also has a great impact on fry size and fry size variation. Stocking fry from different batches in the same pond (fry produced by different-aged females, or swim-up fry of slightly different age) will magnify the differences. We believe that this initial fry size variation if magnified if stocked in a pond with a mixed-size zooplankton population which will allow the larger fry to forage on larger zooplankton.

Technical Abstract: It is not unusual to have fish ranging in size from 0.5 lbs to 5 lbs from a single-batch hybrid catfish production pond at the end of a growing season. Farmers may be docked for fish that are either larger or smaller than the processor’s preferred size range of 1-3 pounds. The graph at right shows the size distribution of a sample of ungraded fingerlings. While the average weight of fish in this sample was 0.05 lbs (5.4”), individual fish ranged in weight from 0.01 lbs to 0.23 lbs (coefficient of variation, CV = 78%). The size range may be even greater with fingerlings of a larger average weight. This initial size variation expands as the fish grow and results in the large size range observed at harvest. Currently the best management practice to minimize small and large food fish is to stock graded fingerlings. We believed that the size range seen in ungraded fingerlings begins at an early age, perhaps even with the female brooder. We hatched eggs from ten spawns each of 2-, 3- and 4-year-old female channel catfish brooders and individually weighed 100 sac fry and fry at 2-, 4-, and 6-days post-swim-up from each spawn to determine mean weight (mg) and uniformity (CV, %). There was considerable variation within spawns in sac fry and fry weight at all ages post swim-up (see table at right). The weight of sac fry and swim-up fry at all ages was less for offspring of two-year-old females, and higher and generally similar for offspring of three- and four-year-old females. The CV of weight for all ages of fry was greater for offspring of two-year-old females; the CV in weight increased with fry age among all female-age groups. Mixing eggs or sac fry from different age females or mixing fry of different ages post swim-up will increase fry size variation. Any initial variation in size of fry will likely be magnified when fry are stocked in ponds with mixed-size zooplankton populations. Presumably, larger fry will be able to consume larger zooplankton and thus feed more efficiently and grow faster than the smaller fry, further increasing the size range of the population. It may be possible to reduce catfish fingerling size range by stocking only fry of the same age and from the same age female brooder in each fry pond. This may reduce the need to grade fingerlings before sale.