Whether or not salmon/trout fry are fed in the classroom prior to release into the stream depends upon how developed they are. The transition from yolk sack dependence to active feeding is directly related to photoperiod (length of day) and water temperature. The fry are ready to feed at "swim-up," when they leave the gravel and enter the water column. At this stage of development the yolk sacs are approximately 75% absorbed. There is a brief period of time, beginning with swim-up, during which the salmon will learn to feed. If food is not offered to the fish until after this window has closed, they will not learn to feed and will eventually die.
It is perfectly acceptable to release non-feeding (or unfed) fry. Indeed, some fishery biologists prefer to. In terms of post-release survival, there appears to be no difference whether fed or unfed fry are released. Keeping incubator water temperatures in the forty degree range right up until several days before the fry release will almost certainly result in stocking unfed fry. This, therefore, is certainly an option if a school so choses.
In order to ensure the best possible chance for success during the feeding phase of the salmon/trout in the classroom experience, the following guidelines are provided for each of the two incubator types:
Using Option A - Rolls Royce Incubator: When the yolk sacks are approximately 75% absorbed OR three or four weeks prior to the scheduled release date (whichever comes first), remove the foam panels and place the incubator in an area of indirect light. Gradually raise the water temperature to 55 degrees Fahrenheit (approximately five degrees per day). The warmer water temperatures will move up the date of "swim-up," (or when the fry first enter the water column). When they do, they are ready to begin feeding. They generally will not feed at temperatures below 50 degrees. The temperature should be kept at 55 until the stocking date. (Note: If stream temperature is being tracked, the incubator temperature should be adjusted to match the stream a day or two before the release date).
Using Option B - Budget Approach Incubator: If a teacher wants students to be able to feed salmon/trout that are being raised in a budget approach incubator, those fish will likely have to be transferred to a larger aquarium with water at room temperature, unless the refrigerator temperature can be increased to 55 degrees Fahrenheit, in which case the same guidelines apply that are outlined for the Rolls Royce incubator. When a refrigerator's temperature cannot be raised to 55 degrees, the salmon should be transferred to a twenty gallon aquarium, with water at room temperature, no sooner than three weeks prior to the scheduled release date. The water temperature should not rise above 70 degrees. The aquarium should be placed in an area with indirect lighting. When "swim-up" occurs, feeding begins.
It should be mentioned that, at room temperature, the salmon will grow quite quickly. Fungus and bacteria will also grow well at warmer temperatures. So, needless to say, water quality testing (pH and ammonia) and tank housekeeping become that much more important.
What's For Dinner?: Salmon/Trout are fed either live, frozen, or dehydrated brine shrimp - in very small amounts. Live brine shrimp can be cultured in the classroom. Frozen or freeze-dried brine shrimp are inexpensive and can be purchased at most aquarium shops. Live brine shrimp should not be purchased at an aquarium shop because of the potential for introducing pathogenic organisms into the incubator. There is no magic amount of food to feed the salmon. It simply requires a careful trial and error approach. The primary rule of thumb is to place no more food in the tank than the fish can consume before the food settles to the bottom. Salmon/Trout fry will not eat food that has reached the bottom. That which settles to the bottom, along with accumulating feces, provides fertile ground for fungus - "public enemy number one" in an incubator.
There's a fungus among us!: Fungus has a filamentous, cotton-like appearance and generally appears at the bottom of the tank, below the gravel. This can make it difficult to detect. Routine gravel vacuuming is the best way to detect and control fungus. Any unusual problems with water quality should be reported to the facilitator immediately.