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Say a Barbel weighted 16.10 in the first two weeks of January, what is the maximum weight it could make come the last day of the season. This is a Trent fish and the river was in flood almost all this time. I know this isn't an exact science, but interesting nonetheless.
I’m not sure how anyone could make an meaningful estimate on that. At that stage I can’t see that the fish is growing in size, simply fat deposits that will increase and decrease. We’re no longer talking about growth rates of growing year classes. There are so many factors that will impact on this, such as food availability and quality, climate, flow, genetics etc etc.
In a closed environment like a lake, where things are less subject to environmental factors things are probably a bit more predictable. So the carp boys can make better assumptions based on averages. For a barbel in a river, I don’t think this is possible.
Fish growth is governed by many factors such as geographical location, water temperature, food availability, age of fish, level of competition for resources etc etc. In general terms fish will feed most willingly in the warmer months in order to attain weight gain so as to sustain them through leaner times (winter), due the many variables involved findings regarding fish growth rates will differ from venue to venue.
There's an interesting paper where barbel populations were studied on the River Lee. The findings suggest that there was a significant increase on first year growth when compared to other rivers. However, growth reduced after this stage and the maximum size of fish in relation to age group was smaller than that from other rivers. I know that this isn't specific to the Trent but it may give you an insight to fish growth.
If you're interested in reading the paper it can be found online under the banner: Age and growth of European barbel Barbus barbus (Cyprinidae) in the small, mesotrophic River Lee and relative to other populations in England.
whats proved right for me is a 14lb fish in June, will make 15 to 16lb in March. One 16lb 5oz fish I did catch in March, made 15lb 6oz in October. Then 14lb 5oz in June, then 16lb 5oz again the following March.
There was a Kennet fish I caught that went from 13lb 12oz in June to 19lb 2oz in October. I have heard of Barbel putting on stupid amounts of weight very quickly too. A lean 9lb 12oz Bristol Avon fish put on almost 3lb in three weeks after a heavy baiting campaign(not mine). With your Trent fish its impossible to guess at without photos. It could have been full of Boilies at 16lb 10 oz and at its maximum weight or been a massive lean empty fish and able to put on 3lb or more in that time. In fishing there are lots of times/ways that fish are weighed wrong. Its nothing to worry about as long as you are doing it correctly.