Fry to Fingerlings
The specific details in the life cycle of fish can vary among species, but fish generally progress through 6 stages. Those don’t necessarily always match up with aquaculture care stages, though, so we’ll just talk about aquaculture’s early stages of larvae, fry, and fingerlings (small juveniles).
- Fertilized eggs that manage to hatch are referred to as larvae. This stage may last only a few days or a few weeks while they live off a yolk sac attached to their bodies. Once the yolk sac is fully absorbed, the young fish are called fry, and they’re ready to be moved to the nursery pond.
- In the nursery pond, fry are ready to start eating on their own and will continue there, typically until they’re fingerlings, which refers more to a size than a developmental stage. The actual transition to juvenile stage, which includes the growth of scales and working fins, occurs during this growth period. Juvenile fish are maintained in nursery ponds until they’ve reached fingerling size primarily because it improves survival rate by taking them outside of the “tasty snack” stage for larger fish.
- Once juvenile fish are roughly a finger length in size, they’re moved from the highly controlled environment of the nursery pond to larger growing ponds where they have more room to swim and larger food sources to keep them growing.
Time
The time young fish will spend in the nursery pond depends on growing conditions and species of fish, of course. It’s fairly safe to say that cold water fish like trout and carp will grow more slowly than their warm water cousins, and you may even want to risk saying that larger fish species tend to grow more slowly than small ones, but not always. In wild Atlantic Salmon, for example, growth rates are affected by ambient temperature and can range from 20 to 120 days. Under ideal conditions, including the right amount of food and minimization of stress, farmed fish will mature faster and at much more uniform rates. You can get estimates from your egg supplier, but it will likely take some experience with your own aquaculture setup and variations in your local climate to get a farmer’s instinct on timing.
Size
As a general rule, once young fish have reached the length of your finger, they’re officially juveniles, with fully formed scales and working fins. Their mouths are also big enough to handle larger food, and they need more space to swim and grow in. Time to reach this size can vary, of course, and some aquarium fish (neon tetras, for example) may never reach this size. Do some research about the life cycle of your fish species so you can decide whether to measure with your pinky finger or your second one!
Survival rate
The fry stage of fish development is not the most robust. Just the process of moving to the nursery pond can trigger significant loss if it’s not handled well. Youngsters can be stressed by handling, transportation and acclimation, to start with. In a carefully managed nursery pond, it’s possible to minimize or even eliminate other typical stressors, so once your fish are in the nursery, plan on keeping them there until they’re less vulnerable to stress. Depending on your fish type, you can use the fingerling rule or average weight to determine whether they’re ready to move on to growing tanks.
