Fish Cognition and Behavior

By Culum Brown, Kevin N. Laland, Jens Krause

Of one interest is whether hatchery-reared fish retain the learned recognition of predators for relevant time frames. The fish demonstrated this, after high rates of predation for up to 3 weeks of stocking. They suggest after this period, hatchery-reared fish would have sufficient time to acquire the recognition of predators and/or learn appropriate avoidance behavior. Juvenile rainbow trout retain recognition of predators for at least 21 days, sufficient time for stocked fish to learn and recognize predators and how to avoid them.
That is to say, hatchery fish behave differently than natural fish, in the wild (I'll use the NOAA Fisheries terms for birth type: "wild" is born in the wild, "hatchery" is born in the hatchery, and "natural" is born in the wild of wild parents--hatchery ancestry is possible for all classes). Hatchery fish tend to school more, "hang out" higher in the water column, exert more energy pursuing food, and exert more energy while holding. This is not true if the fish are planted as eggs or very young. They tend to act as wild fish then. Basically, hatchery fish adapt to the hatchery. In studies where wild fish were introduced to hatchery ponds, they were badly out competed by the hatchery stock. Hatchery fish aren't stupid, they just learn how to be trout in an unnatural setting. Having said this, holdover fish can learn to be "real" trout, though will probably never be as good at surviving in nature as wild fish. Also, survivorship of hatchery-released fish is very low. Few will ever live to be holdovers.In terms of being able to tell the difference between hatchery and wild fish while/after you catch them, this is complicated. If the hatchery fish was recently stocked it will not fight like a wild fish (and may never). Hatchery fish are corralled and netted routinely, so they kind of get used to being handled. Also, they were bred to be easy to culture. Further, if you are born in (and live in) a stream you get better muscle development versus being reared in near-standing water in a concrete raceway. If the fish survives to be a holdover, this may become less of an issue. The color of a fish is also complicated. Often the differences in color you see are due to diet. People still don't know what the nutritional requirements of most fish are (as a result they just add fish meal, and assume it will have the necessary nutrients since it is from fish--often anchovettas(sp?) and other bait fish). Many of the striking colors in a Salmonid come from pigments that are ingested. For instance, rainbows don't get pink flesh or stripes if they don't get the proper diet. Hatchery foods can now be purchased with canthaxanthin (a red/orange pigment), but the fish still don't get as bright. Once a hatchery fish lives in nature for some time, it will look like it is wild, due to diet changes. Having said this, there are many strains/races/subspecies of the various Salmonidae, and as a result, two specimens of the same species can look quite different (not to mention individual/genetic color differences). So, if the strain being stocked in your home water was originally acquired from a very different population, you could see color differences between the natural/wild and hatchery fish. If there was a genetic bottleneck in the population's past, this could increase the consistent, unique colorations of the population. So, you may be right about the color differences between stocked/wild fish, or you may not be. Probably, you wouldn't be able to differentiate between wild/hatchery fish based on color after it was in the wild water for some time. Also, if hatchery fish are introduced to a place, and they can breed, they will. I guarantee that the fish of Little J have a fair amount of "hatchery genes" in them. This would lead to intermediate phenotypes, making differentiation even more difficult. In response to wild fish learning from being caught, a recent study showed that statistically you will not catch the same fish more than once or twice. That is not to say you never catch a fish more than once, it just becomes more difficult each time the fish is caught. This is true for hatchery fish as well. However, it will probably take them longer to catch on as they were handled in a hatchery, and as a result are more likely to "accept" such experiences as "normal." Of course, there is some speculation involved in this statement.