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Wednesday 6th May, 2009

Last time I pulled some sciency stuff out of the archives it sparked an interesting discussion over on the board. Hopefully this will do the same. Regardless of how you feel about hatchery fish, they are a fact of life in many areas. Here's a bit of discussion on a related topic that you might not have thought of in the past.


Hatchery rearing of fish in the “traditional” manner results in behavioral problems and, as a result, poor survival rates among released fish. According to Brown and Layland (2001), “hatchery-reared fish show deficits in virtually all aspects of behavior due to the impoverished conditions in which they are raised.” It is known that fish behavior, although frequently resulting from innate characteristics, is often acquired through active learning. Impoverished hatchery environments either do not promote such learning, or teach fish behavioral skills which are ineffective outside of the hatchery environment. Some of the potential behavioral problems associated with “traditional” hatchery rearing techniques include deficiencies in foraging, anti-predator, habitat selection, and migratory behavior.

As I mentioned earlier, it is known that fish behavior, although frequently resulting from innate characteristics, is often acquired through active learning. In fact, we have found that learning ability in many species of fish is far greater than originally believed. Not only do fish learn by trial and error and association, but also through social learning processes. Investigators have found that these mechanisms for social learning can be implemented in hatcheries as an effective means to train fish before release, thereby increasing survival rates in the wild. A discussion of the behavioral deficiencies of hatchery reared fish and possible solutions to the problem are discussed below.

In the “traditional” hatchery environment, fish are accustomed to feeding on food pellets, and often learn to associate the presence of a human with feeding time. As a result, many hatchery reared fish do not immediately recognize and utilize natural food sources upon release into the wild. Often, as in the case of some stocked trout, they will equate the presence of an angler with feeding time and expose themselves to unnecessary predatory risks. It has been shown that when hatchery salmonids are released they consume less food and fewer prey types and take longer to recognize the presence of new prey types that wild salmonids (Sosiak et al., 1979; Ersbak & Hasse, 1983). The result is lower growth rates and poor survival rates.

It is widely believed that exposing fish to a variety of live, natural foods while still in the hatchery environment would result in “increased foraging efficiency and attraction to novel food items” (Brown and Laland, 2001) upon release. Furthermore, the results of exposure to live food items in the hatchery environment could be enhance by social learning (by raising hatchery fish in the presence of individual fish that readily accept live food). Also, by changing feeding techniques from the standard “bucket toss” method to methods where timed automated release systems are used the tendency of fish to associate humans with food would be eliminated.

Poor predator avoidance behavior is one of the main reasons why hatchery fish have such low survival rates. This is especially true within the first few days after release into the wild. When the hatchery fish are released they are subjected to a new and different environment which may be highly variable. They will also be exposed to predation for the first time. A stocked fish that encounters a predator species for the first time may not recognize it as such or may not respond appropriately in time to escape and learn from the experience. I have witnessed this first hand on the Mohawk River in central New York State. Resident northern pike have an easy meal when the state stocks hundreds of naïve rainbow trout into areas where the pike are present. The trout simply do not respond to the approaching pike until it is far too late.

In fishes, although some anti-predator behavior is innate, much is dependent on learning from experience. It is known that naïve fish can rapidly improve their anti-predatory behavior with experience, and many believe that social learning may play a vital role in the development of predator avoidance behavior of hatchery fish. Several novel methods have been suggested as ways to improve anti-predator behavior in hatchery fish. According to Brown and Laland, certain individuals have “found that control of [fishes] alarm reaction could be transferred to previously neutral stimuli via paired conditioning and could provide a mechanism whereby naïve animals learn to recognize predators without ever coming into contact with them.” These methods center around training hatchery fish to become alarmed when they recognize chemical cues that are emitted by predator fish species. Other methods focus on teaching predator avoidance by exposing hatchery fish to predation inside the hatchery environment. These methods include pairing negative stimuli such as electric currents with the appearance of a model predator, placing fry in a tank with a live predator and allowing them to interact for a short period of time (Olla’s “Fear of Cod” experiments), and placing fry in a situation where they observe and react to a predator which is located behind a clear partition. These methods are often most effective when a component of social learning is involved in the experiments. Individual fish that come to recognize the actions/reactions of conspecifics as evidence of a predatory threat have a marked advantage over naïve individuals.

Poor habitat selection behavior exhibited by fish reared under “traditional” hatchery conditions also results in poor survival rates. Hatchery fish often choose inappropriate habitats when released into the wild resulting in poor feeding efficiency and/or increased exposure to predation. For example, some hatchery fish take up positions at midstream, high in the water column, thereby exposing themselves to the effects of avian predators, excluding themselves from access to benthic prey items, and exposing themselves to the effects of strong currents such as increased energy expenditure. Also, hatchery fish tend not to distribute themselves throughout a river, lake, or stream completely, resulting in areas of extreme population density and increased competition among individuals. Training hatchery fish to select more appropriate microhabitats presents some interesting challenges to hatchery managers. One solution would be to raise fish in more natural tanks with rock substrate and overhead cover rather than the typical concrete raceways found in most hatcheries. One example where such methods were used involved raising sea run cutthroat trout in the presence of over head cover and with an automated feeding system. Another experiment involving sea run cutthroats involved rearing the fish in natural ponds for a period of time before releasing them in to the wild. Both methods resulted in better survival rates and increased return of fish.

Poor or altered migratory behavior is also a common characteristic of fish reared in “traditional” hatchery environments. Research has shown that, in the wild, certain species fish often interact with one another and learn migratory behavior to foraging and/or spawning areas by observing their conspecifics. Theoretically, hatchery fish could be trained to migrate by placing them with more experienced individuals and allowing them to interact. However, since migratory starting and ending points are specific to natural environments, and could not be recreated in hatchery environments, training of this type would be very difficult. However, the concepts that fish can learn from conspecifics is very important to the training of hatchery fish to forage and avoid predators more effectively once released.

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