“Reasonable and Prudent Measures” include maintaining flows sufficient to allow the out-migration of juvenile steelhead from the creek to the ocean. The City has implemented a five-year study to determine the timing of out-migration so that flows in the creek can be maintained at a sufficient level to facilitate steelhead trout passage during critical periods.

About 6 to 8 weeks after fertilization, the embryos hatch and become alevins, or sac fry. Over the next 3 to 4 weeks the alevins remain within the gravel, living on the rich nutrients contained in their large yolk sac. Once the yolk sac is fully absorbed, the inch-long fish emerge from the gravel and are called fry. They are fully formed, free-swimming and begin feeding on tiny insects and drifting plankton.

When fry attain a length of about 3 inches, they are referred to as fingerlings or parr. They have a camouflage pattern consisting of distinctive dark vertical stripes on their sides, called parr marks. Parr feed primarily on aquatic and flying insects, although small fish become an increasingly important part of their diet as they grow. Predators of juvenile steelhead trout in freshwater include raccoons, mergansers (“fish ducks”), herons, kingfishers, garter snakes, larger fish, and humans.

When parr feel the instinctive urge to migrate downstream to the ocean, they become smolts. They go through many changes to prepare for the critical transition from fresh to salt water. The scales of smolts turn very silvery, masking the parr marks. The scales become delicate and very loosely attached to the skin. Extended residence in the estuary allows essential physiological adaptations to occur gradually, thereby increasing their chances of survival.

Resident rainbow trout are the non-migratory form of steelhead, opting to remain in freshwater for their entire life. Relatively little is known about their specific habits and environmental requirements. Because of the limited food supply in freshwater streams, resident rainbow grow slowly and a 12-inch long fish may be 5 years old. Populations of resident rainbow trout tend to occur above impassable barriers such as debris jams and waterfalls, although in some areas the distribution of the resident and anadromous forms overlap. Many "trout" that people see during the summer in San Luis Obispo Creek are actually juvenile steelhead that have not yet gone to the ocean.

The life histories of steelhead and resident rainbows differ primarily in the extremes of wanderlust, growth rate and fish size. In fact, the two forms are genetically indistinguishable. Interestingly, a small fraction of the surviving progeny produced by a pair of steelhead might not develop the urge to migrate seaward, but rather may remain as resident fish. Apparently the reverse scenario can also occur, with a few offspring of resident rainbow parents developing into smolts and becoming anadromous steelhead.

Very little is known about the oceanic distribution of steelhead. Smolts are thought to stay close to the continental shelf in shallower water. Their first year of life in the ocean is the most critical, as the smolts are highly susceptible to predation by larger fish, seals and sea lions, and a variety of birds. Gradually, the fish venture further out to sea, growing rapidly as they feed voraciously on small fish (e.g., herring anchovies, needlefish, etc.), squid, and crustaceans such as shrimp and krill. Unlike salmon, which often travel in large schools within 200-300 miles of shore, steelhead are solitary and may roam far out into the open ocean.

In the spring and summer of their return year, maturing fish begin migrating back toward their “home streams”. Scientists speculate that the uncanny precision of homing is achieved through a combination of celestial navigation, orientation to the earth’s magnetic fields, and a very highly developed sense of smell. In the fall, early-run steelhead congregate off shore, waiting for water levels to raise enough to allow the fish to swim upstream. At this point the adults are particularly vulnerable to predation by sea lions, harbor seals and human poachers.

Once the steelhead enter the stream they swim towards the headwaters. When the female finds a good place to lay her eggs, she flips on her side and flaps her tail against the gravel bottom of the stream, creating a pit, 8 to 16 inches deep. The male has been hovering close-by defending the nest (or “redd”) from competing males. When there is a pit dug the mating pair position themselves side by side and she lays a few hundred eggs into the pit, then he releases his “milt” to fertilize the eggs. The female moves slightly upstream and digs another hole, thereby covering the previous “egg pocket” with gravel – and the dance begins again. This process can last for several days, as the fish extends the redd upstream.

Unlike salmon, steelhead may return to the ocean after spawning and live to spawn again. Some steelhead spawn as many as 4 or 5 times, though twice is most common.

Trap Operation

The trap is installed in SLO creek from March to December each year, it is removed during the winter to allow free upstream passage to adult steelhead returning to the creek to spawn. The object of the study is to identify the approximate timing of the smolt out migration so that critical flow levels can be maintained during these periods. The trap is constructed in the form of a weir which allows water to pass through it while channeling fish into a pipe located at its center. The pipe is connected to a ‘live box’ in which entrained fish are held until the trap is emptied.  Occasionally, researchers will mark the underside of a group of smolts with a spot of blue dye. The marked smolts are then released upstream of the weir and the efficiency of the trap can be calculated from the number of marked smolts that are later re-captured.

The smolts caught in the live box are placed in a little carbonated water which has a calming effect, making the fish easier to handle, they are then measured and assigned a ‘smolt index’. Most smolts migrate during rainy periods when the flows in the creek are high, they tend to move downstream during the hours of darkness when temperatures drop and the fish are less easily seen by predators