Zone bound­aries

Anglers Mail - - Anglers -

present at the very top of the ti­dal range, where there is min­i­mal salt in­cur­sion.

Fur­ther down­stream, to­wards the up­per end of the es­tu­ary, where there is a fresh­wa­ter in­flu­ence but with higher salin­ity, you can find pop­u­la­tions of coarse species such as carp, bream and roach. In the same area, it is also pos­si­ble to find sea species with a high fresh­wa­ter tol­er­ance, such as mul­let, bass, floun­der and smelt.

The text­book de­scrip­tions of river zones are fine in the­ory, but the re­al­ity of the sit­u­a­tion is that there is usu­ally not a wellde­fined band, more a grad­ual con­ver­gence of one zone into an­other. It is not un­com­mon to find, for ex­am­ple, trout in the bar­bel zone, carp in the grayling zone or bar­bel in the bream zone. It is only where there are well-de­fined phys­i­o­log­i­cal re­quire­ments, such as the oxy­gen and spawn­ing re­quire­ments of the salmonids, that species zona­tion is more rigidly de­fined.

The in­flu­ence of river en­gi­neer­ing mud­dies zona­tion, too, with dams, weirs, chan­nel straight­en­ing and var­i­ous flood de­fence or agri­cul­tural schemes all mod­i­fy­ing flow and in­ter­fer­ing with the nat­u­ral bal­ance of the flood plain. It is cer­tainly not un­usual to find carp and bream pop­u­la­tions in slow, deep and tur­bid wa­ter above weirs, with bar­bel and oc­ca­sional trout in the fast, well-oxy­genated wa­ter be­low. There are also dis­creet pock­ets of mi­cro­hab­i­tat found along the length of all flow­ing wa­ters.

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