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Greers Ferry National Fish Hatchery raises rainbow trout year round. The outdoor fish production cycle takes place in the long rectangular concrete pools we call “raceways”.
Visitors to the outdoor fish rearing area of the hatchery will see lots of rainbow trout swimming in raceways. Fish are grouped in a raceway by length or size. Occasionally, one may notice that fish in a raceway are not all the same size and wonder why? It’s because the more aggressive fish are outcompeting the smaller ones for food and that causes size variations in the group.
The hatchery's feeding strategy for juvenile trout (one to five inch fish) is to increase the feed pellet size incrementally as the fish grow. Smaller fish in the group not able to eat the larger pellet will begin to lag behind.
To even out the playing field, hatchery personnel place a bar grader in the raceway to separate the small fish from the larger ones. The bar grader is similar to how a vertical venetian blind works. The grader has adjustable bars and a manual dial at the top corner. The dial is graduated in fractions of an inch and is used to open or close the space between each bar. The bar space openings are set according to the size of fish we need to separate out.
The grading process involves crowding fish toward the bar grader and leaving them concentrated there for a period of time. The smaller fish can swim through the openings of the bars and escape being crowded while the larger trout remain behind.
The grading process is time consuming but yields more uniform sized fish, improves the feeding strategy, and provides better overall fish growth. That's why good grades do matter at Greers Ferry National Fish Hatchery.
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