Freshwater And Saltwater

Posted: Fri Jan 01, 2010 8:17 pm

hello all,

ive been browsing the forum and noticed that alot of people are getting concerned about freshwater moving the fish out to sea,

the truth is freshwater and seawater do not completly mix,

freshwater is thin and seawater with its salt content tends to be layered, because the freshwater is thinner it tends to sit on top of the seawater,

i personally dont think that frshwater is a problem and is normally gone in a tide,

hope this gives people more confidence to fish those upriver marks,

cheers shane happy

Member Joined: Oct 17, 2009 Posts: 7

not strictly true, marine biology is a little more complex than that. In the Thames we have an area of brackish water - Greenwich through to Dartford
Brackish water is water that has more salinity than fresh water, but not as much as seawater. It may result from mixing of seawater with fresh water, as in estuaries. Technically, brackish water contains between 0.5 and 30 grams of salt per litre, brackish conditions change with rainfall or as more recently a thaw of snow.
The River Thames flowing through London is a classic river estuary. The town of Teddington a few miles west of London marks the boundary between the tidal and non-tidal parts of the Thames, although it is still considered a freshwater river about as far east as Battersea insofar as the average salinity is very low and the fish fauna consists predominantly of freshwater species such as roach, dace, carp, perch, and pike.
The Thames Estuary becomes brackish between Battersea and Gravesend, and the diversity of freshwater fish species present is smaller, primarily roach and dace, euryhaline marine species such as flounder, bass, mullet, and smelt become much more common. Further east, the salinity increases and the freshwater fish species are completely replaced by euryhaline marine ones, until the river reaches Gravesend, at which point conditions become fully marine and the fish fauna resembles that of the adjacent North Sea and includes both euryhaline and stenohaline marine species. A similar pattern of replacement can be observed with the aquatic plants and invertebrates living in the river.
The salinity of an estuary is influenced by the amount of freshwater that flows into it from rivers and streams. The more freshwater flow an estuary receives, the lower its salinity.
So yes the rain can adversely effect the fishing - well thats my excuse anyhow !

Posted: Fri Jan 01, 2010 9:14 pm

As a layman I would have thought that a fast flowing river due to rainfall including water coming off higher ground would have the effect of pushing the saltwater further seawards?.
Cheers
keith big grin

Posted: Fri Jan 01, 2010 11:40 pm

the norweigen fjords have varying layers of salinity depending on wether its summer or winter and they have no problems catching lots of cod,i feel the spratt invasion has more of an impact than the salinity of the water...

Posted: Sat Jan 02, 2010 1:53 pm

very informative post gents.........keep it up!!!

my tuppence for what its worth.
whilst fishing afloat on the thames i always try to watch the fish finder, although i know its only a gauge and weed etc can affect its readings. generally the depth for seing fish on the fishy screen is about 30ft.......on my last trip out, post thaw.....the fish were more commonly found in 40ft depth. could it be the top melt water/temp change driving them down that little bit more?

Posted: Sat Jan 02, 2010 2:03 pm

A very interesting read. Very much appreciated that people have taken the time to make comment on this subject. It all still adds upto "being in the right place, at the right time" and that to me seems the only rule of thumb and this can be as much about LUCK than judgement. Think if I could forcast the fishing, some of the fun would dissappear.
Mark.

Regular Joined: Nov 02, 2008 Posts: 291

no they just avioding you chucking up kol ha ha ha big grin

Posted: Sat Jan 02, 2010 4:41 pm

spew rattled your cage!!!

Guest

as i fish the thames at allhallows most of the time in my case i find that when it has been raining quite hard when the tide has been out the fishing is a lot more slower and less fish than when it hasent been raining having said that i find that if it rains when the tide is in it dont seem to make the the fishing slow. so as it is quite shallow there fishing from the shore line maybe its the fresh water being pused up by the salt water on the flood of the tide that makes it slower if coachga is right. cant coment on the deeper parts as never been out in a boat
cheers steve

Posted: Sat Jan 02, 2010 9:01 pm

The reality is very few fish can live in both saltwater and fresh water. Migratory species such as Sea Trout and Salmon are best known. A larger number are tolerant of degrees of the mix of both, commonly referred to as brakish water. Best known are Flounder, Mullet and Bass. Even these species require a certain amount of salinity to survive. The lesser the fresh water flowing down a river the greater distance up the river system is brakish water. Conversely a huge melt of snow or heavy rain produces a massive river flow of fresh water pushing the distance further offshore that sea fish can tolerate. True saltwater species such as Cod and Whiting are particularly intolerant of fresh water. The shallower the intertidal area the greater the effect. So yes, freshwater flooding does have an effect on inshore angling in regions where river systems empty to sea.
Proves what I've said for years. Freshwater is for drinking. Saltwater is for fishing!

Posted: Sat Jan 02, 2010 10:01 pm

Has made for some very interesting reading all these posts. And am right in saying it applies to river humber as well, which has the ouse and trent flow into it. That would explain why that 10 hrs fishing humber has been a blank.

Posted: Sun Jan 03, 2010 11:14 am

living by the river an having been fishing it for over 20 years i have had days when its rained hard on the day and the days previous to that and still caught whiting as far up as thamesmead 3 hours before high tide... also had the same results at erith and greenhithe to.... the only noticeable thing was the whiting as far thamesmead were generally pin whiting when the river was in a state of flushing out the freshwater...... the freshwater has never affected the sole an bass catches either..... i fished a pier at thamesmead one year and 4 hours before high water had a freshwater common bream on rag and on the same session started catching whiting an hour later 3 hours before high tide. maybe the fresh an salt species adapt better than we expect to changing conditions??????
jay

Posted: Sun Jan 03, 2010 5:53 pm

dick19666 wrote:

the norweigen fjords have varying layers of salinity depending on wether its summer or winter and they have no problems catching lots of cod,i feel the spratt invasion has more of an impact than the salinity of the water...

Seeing the amount of small fish traveling up the thames i agree more with dick's quote .But maybe its a combination of both

Posted: Sun Jan 03, 2010 9:21 pm

Please view the attached link to the scientific publication on the effects of saltwater on freshwater sprcies and the effect of freshwater on saltwater species. The latter section of the report points out that other than a few species, the effects are fatal. Osmosis is the killer if the change is excessive. Deep Norwegian fjords and some deep Scottish lochs can accomodate both fresh and saltwater fish due to the depth of water allowing the two water columns enough space to fully seperate.

http://www.newton.dep.anl.gov/askasci/bio99/bio99295.htm

Posted: Tue Jan 05, 2010 12:00 pm

very imformative reads people,

great to see so many getting involved,

keep it up people

cheers shane happy