Posted by: AAPGAI | May 19, 2010

Rainbow Trout Behaviour

ron_holloway In response to recent correspondence received on the behaviour of todays stocked rainbows I can only call upon my own experiences in the rearing of brown trout in stew ponds for subsequent stocking out into still waters and rivers. Albeit my work was with brown trout for stocking the lower beats of the Itchen and Test yet I believe the principles discussed here would apply equally to rainbows.

It is suggested that over the years since the introduction from the West Coast of the USA of the two recognised separate strains of “rainbows” into this country back in the middle 1800’s the stocks of “generic rainbows” that are now available from commercial hatcheries in the UK consist of a mixture of these two strains. The present genetic make up of what are now called “rainbows” now consists of a mixture of the land locked traits of the Mt Shasta or Kamloops strain and the anadromous Steelhead strain both of which were introduced into this country around the same time. These two varieties of rainbow have now been so intermixed that now we have just the one generic “rainbow” which is now recognised as a species and has been now reclassified as oncorhynchus mykiss from salmo gairdneri

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Hatchery managers have over the years been very wary of introducing fresh blood into their hatcheries for fear of introducing diseases, and rightly so. This has inevitably created closed hatchery strains of rainbows that have been inadvertently selected for good growth over the years by the hatchery manager. These fish have over many generations adapted to their locality with brood stocks selected from within the close confines of these populations which produce trout that survive and grow well in these intensive stew pond conditions. Little thought how ever seems to be given to how these trout behave when released as stock fish!

I believe it may take many generations of artificial trout rearing to alter the actual genetic finger print of a particular strain of fish. So after 100 years or more of intensive rearing the actual genetic make up of a species may not be altered all that much despite intensive inter-breeding. Therefore the natural traits that we observed in the early generations of rainbow trout are still latent within today’s generations. So what has happened to change their observed feeding habits and behaviour patterns once these trout have been released into our rivers and still waters? As with most animals including humans the behaviour patterns and habits of young adults reflects the manner in which he or she was brought up during the formative years of their upbringing. So the same principles apply, in my opinion, to trout reared in stew ponds be they brown or rainbow.

From experience I am now of the opinion that fish and rainbow trout in particular can in hatchery conditions be trained to hunt for food and be manipulated to feed in different ways. It is the changes in the manner that stocked rainbows feed and hunt for food once released into our fisheries and in the manner they act and fight when hooked is what seems to concern many of todays more discerning anglers.

In the early days of the commercial manufacture of high protein trout foods, except for the swim up fry and par stages when fine granulated food was fed, most stew bred trout from these life stages onwards were fed on floating pellets. These floating pellets were very expensive to produce and as the cost of the protein content of these pellets in the form of White Sea fish meal also increased then commercial trout producers demanded a cheaper form of food. So the slow sinking and fast sinking pellet was devised which still contained the high protein levels needed for growth but now were not required to float. It is far cheaper to produce denser sinking pellets than the floating variety which are very expensive to manufacture as it requires extra heating and moulding processes to cook and shape each pellet to make them float.

The denser the pellet the cheaper it is to produce and the quicker it sinks to the bottom of the stew pond. It was soon noticed that trout would readily learn to stand on their heads and peck up these sunken pellets off the floor of the stew pond just like tench or carp sifting muddy gravel at the bottom of a natural pond. Fast sinking extruded pellets were therefore found to be the most economical food form for producing fast growing trout.

trout pellets

I suggest that feeding sinking pellets has been common practice, for many years, in most commercial hatcheries. Therefore the many generations of rainbows that now have been produced have been continually trained during their lives in the hatchery to seek food on the floor or to take fast sinking food on its way to the floor of the rearing pond. These stew bred rainbows have not had the chance in their entire lives to learn to seek and find and take food from or near the surface of the water. They have for many generations adapted to and trained not to look up to seek food but only to look down! It has been shown quite conclusively that trout can be taught to feed in certain ways and once learnt they do not forget even if feeding methods are changed.

However even if certain rearing regimes can be shown to have significant effects on the subsequent behaviour of released rainbows then serious consideration should be given to other hatchery practices. One that could have even more significant effects on the post release behaviour of rainbows and that is the increasing practice of rearing and releasing triploid rainbow trout into fisheries. For the fishery manager the whole principle of rearing and using these trout is quite understandable economically as these fish are virtually 100% sexless and so do not sexually mature. This obviates many problems that fishery managers have had to contend with over the years. When natural trout were used anglers would complain at certain times of the year of catching egg bound females and ugly black males streaming with milt. Despite this plus in their favour I sincerely believe that triploid rainbows are far more benign in their behaviour after release than the old fashioned rainbow trout. So as a species the now “generic rainbow” trout has undergone many changes since it was first introduced as two unblemished natural wild varieties of game fish all those years ago. So it is of little wonder that the behaviour patterns of today’s very “mixed up” rainbow trout has altered somewhat. The EA and the Game Conservancy together are undergoing researches into the pro’s and con’s of triploidy in brown trout so it will be very interesting to see what their findings are and whether their findings can be applied to rainbows. There is little doubt in my mind that hatchery reared rainbow behaviour has changed significantly over the years. Rarely these days do hooked rainbows regularly tail walk or spend more time in the air than in the water or make long runs out to the backing when being played as did the rainbows of twenty to forty or more years ago.

If asked what could be done to improve the angling qualities of stocked rainbows I would suggest that fresh new genuine wild blood could be introduced back into our rainbow hatcheries to perk up the dull resident stocks. Thoughtful game keepers regularly introduce new strains of cock birds or hens to their resident pheasant stock. The principle objective being to maintain and improve the quality and diversity of their resident stocks.

hatchery With hatchery trout, feeding with floating pellets may help to train stock trout to at least look up in the world not down! New or different rearing regimes could be considered that include reducing stocking intensities in rearing ponds. The objectives should be to produce fitter stock fish, and get away from the soft flabby fish that have been ad lib fed in sluggish water in stew ponds that have little through put of water. To produce top quality stock trout that perform well when released, experience teaches that they should be reared in suitable densities, in quite fast moving water and fed on floating food. These conditions have been shown to produce fit, lean, hard fleshed stock trout that show greater sporting qualities when released than trout that are over fed on sinking food, fin damaged by being raised in over crowded ponds in sluggish water. Finally until proven otherwise we have to question the effects or benefits of using rainbow triploids against introducing more natural rainbow diploids from a reputable disease free source.

As for angling methods I believe that anglers have over the years by observation and fishing experience adapted and developed their methods to cope with the changing habits and characteristics of today’s stocked rainbow trout. One thing is for sure the fighting qualities of todays rainbows are not a patch on the trout I used to catch (occasionally!) in my youth some 50 years ago!

Ron Holloway

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