size of fish

[Hopey]

What makes certain species thrive in either numbers or size in certain waters compares to others

Examples:

• the wye is known as one of the best barbel waters in the country. It is large and the water is very clean and oxygenated. As a result the barbel are numerous. However a fish of 12lb is very big, whereas much smaller rivers, for example the Loddon can hold fish to nearly 20lb?
  
• Or in some waters the number of chub is high, but a fish over 4lb is unlikely whereas in the Lea, it is very difficult to catch, but if you do get one 6lb+ is very realistic, and record fish are certainly a possibility
  
• Equally why can certain ponds contain carp that just keep growing and growing whereas in other waters they seem to reach a natural peak around upper 20lbs?
  
• With predators it kind of makes a bit more sense….i.e perch flourishing where there are no pike or pike flourishing where there is a lot of food…trout reservoirs etc….but even then you might have one stretch of river full of jacks, and another similar stretch which has a high number of bigger fish
  

Not sure if there is an easy answer….maybe there is and I sound stupid (!)….but always baffled me…?

Thoughts?

[Dick Dastardly]

Three simple factors (or kind of four but as you will see the first two are kind of linked)

 

1 AVAILABILITY OF FOOD

2 LACK OF COMPETITION FOR FOOD

3 WATER QUALITY (also kind of linked with the first two as this can dictate them)

4 LACK OF ANGLING PRESSURE

[Anderoo]

I think you've pretty much answered it - for fish to grow to a high average size they need little competition (so usually lower numbers of individuals), lots of available food for their various stages of life, a life as free from stress as possible (e.g. not getting caught all the time, not being constandly predated upon), and not to use their available resources fighting a difficult environment (e.g. living in a river with a strong flow).

 

So a Wye barbel is likely to be smaller than a Lodden barbel because presumably the Loddon has more available food and less flow to fight. A Lea chub has almost no competition and stacks of food (crays), and also little flow to contend with. Little ponds can produce huge carp, if they have plenty of natural food and are left in relative peace.

 

Things do run in cycles though, a water might hold a few big pike but once they die off there's likely to be an explosion in jacks to take their place; over time they reduce in numbers, once again leaving a few big ones.

 

A simplified/simplistic view, but I think probably about right.

[ayjay]

Just another connected point to add, I can remember reading (but not where) that Roach need a certain type of food at one particular stage of their life-cycle otherwise they will never grow large.

 

I'm unconvinced about the negative effects of fish fighting flow: Barbel are easily capable of dealing with anything we get in this country, they are perfectly able to thrive in areas where it would be impossible to fish for them in a practical manner.

[Dick Dastardly]

All fish need "a certain food at certain times" or more accurately a certain SIZE of food. Without this they end up being left in the size range the last size of available food dictates. This can be for just a few seasons (as long as there is an abundance of that particular size) or until a food source of that size becomes available or simply forever! The basic reason behind many "stunted" fish (species) population problems.

[MICK SMITH39]

For hundreds of years things stayed the same the odd desease and angling pressure knocked fish back, but after a few years ,just as someone has said the cycle would begin again.Happy days.BUT then came cormarants,crayfish,otters,mink and worst of all EE'S. Once all the big chub,barbel etc that people are catching are gone,then that's it.IMO

[Dick Dastardly]

Sorry but in my experience heavy predation ALLWAYS HELPS a boom in BIG fish!

[MICK SMITH39]

Yes ,but what happens when the boom ends.Especially this one,i really think there is no coming back from this one.......not in my life time any way.........................Hope im wrong.

[ayjay]

All fish need "a certain food at certain times" or more accurately a certain SIZE of food. Without this they end up being left in the size range the last size of available food dictates. This can be for just a few seasons (as long as there is an abundance of that particular size) or until a food source of that size becomes available or simply forever! The basic reason behind many "stunted" fish (species) population problems.

I'm struggling to remember all the details, but I do remember that this was one specific sort of beastie, which the article definitely said Roach required at a certain time for optimum growth.

 

This critter also had specific requirements with regards to water quality for it's own life-cycle; with the increased acidification of many waters the critter was not as readily available as it had been in the past and often now Roach will never find it at all, let alone at the right stage in their development.

[Steve Walker]

Some of the many factors which can determine availability of food;

 

Growth of plants and algae - is the water fertile, in the sense of containing enough nitrate and phosphate? But not too much, such that everything ends up covered in blanketweed or blotted out by algal blooms? Is the pH acceptable, not too acid? Are there extensive productive areas of shallows? Do they get much light? Is the bottom a good substrate for plants to grow on? Is the water turbid with sediment?

 

Is there habitat for invertebrates to live? Are the flow and the substrate conducive to this?