Ribnjačarstvo »Jelas« s 2305 ha vodenih površina,
od čega je 12% obraslo trskom i rogozom (Phragmites
sp. i Typha sp.) mnogim vrstama ptica močvarica
u vrijeme seobe pruža vrlo povoljne uvjete za gnježđenje,
prehranu i odmor. Neke su od tih vrsta gotovo
potpuno ili u većoj mjeri ihtiofagi, t j . hrane se
ponajviše ribom i tako štete ribljoj proizvodnji u
ovom ribnjačarstvu. U pokušaju da se utvrdi stupanj
utjecaja spomenutih vrsta riba na riblju proizvodnju
odabrali smo za istraživanje deset vrsta ihtiofagnih
ptica, i to:
Ćubasti gnjurac Podiceps cristatus (L.)
Veliki vranac Phalacrocorax carbo (L.)
Gak Nycticorax nycticorax (L.)
Žuta čaplja Ardeola ralloides Scopoli
Velika bijela čaplja Egretta alba (L.)
Mala bijela čaplja Egretta garzetta (L.)
Siva čaplja Ardea cinerea (L.)
Čaplja danguba Ardea purpurea (L.)
Riječni galeb Larus ridibundus (L.)
Obična čigra Sterna hirudno (L.)
te samo jednu vrstu iz grupe omnivora i to lisku
(Fulica atra /L./). Kriterij za izbor tih deset vrsta ihtiofaga
osnivao se na podatku o udjelu ribe u dnevnom
obroku i o brojnosti populacije. Na osnovi želučanih
sadržaja odraslih i mladih ptica utvrđene su
približne količine pojedine ribe za svaku od deset istraživanih vrsta, a na osnovi utvrđene brojnosti svake
populacije i dužine zadržavanja na ribnjačarstvu
u toku godine utvrđene su ukupne količine pojedine
ribe. Istraživanja su provedena od god. 1976. do
1985. Dobiveni rezultati upućuju na podnošljive štete
u razdoblju od 1976. do 1980. jer je u to vrijeme
broj vrsta koje se gnijezde znatno manji nego u
razdoblju od 1980. do 1985. godine, kada se na gniježđenju
javljaju nove ihtiofagne vrste, i to s vrlo
brojnim populacijama tako da enormno štete rastu.
Posebna šteta nije utvrđivana.
Mogućnosti zaštite cjelokupnog ribnjačarstva ili
pojedinih ribnjaka vrlo su ograničene, a djelomično
ili potpuno uništenje pojedinih vrsta ptica ne može
biti prikladno rješenje. Mjere moraju biti kompleksne
i treba da teže smanjenju broja pojedinih vrsta
(posebno onih kojih se populacija povećava), i to
upotrebom aktivnih i pasivnih plašila pa sve do naknade
za učinjenu štetu.The fish farm »Jelas« with a 2305 ha water expanse,
12% of which is overgrown with reeds and
bull-rushes (Phragmites sp. and Typha sp.), offer favourable
conditions for nesting, nutrition and rest to many different species of wading birds. Some of
these species are either completely or only partially
ichthyophagous birds, i.e. they feed either exclusively
or only partially on the fish thus, damaging the
fish production in the fish ponds. To determine the
degree of their influence on the fish production 10
species of ichthyophagous birds were investigated:
Great Crested Grebe Podiceps cristatus (L.)
Cormorant Phalacrocorax carbo (L.)
Night Heron Nycticorax nycticorax (L.)
Squacco Heron Egretta alba (L.)
Great White Egret Ardeol aralloides (Scopoli)
Little Egret Egretta garzetta (L.)
Grey Heron Ardea cinerea (L.)
Purple Heron Ardea purpurea (L.)
Black-headed Gull Larus ridibundus (L.)
Common Tern Sterna hirundo (L.)
Only one species of herbivorous birds, the Coot (Fulica
atra), was investigated of the 13 species which
feed on fish from fish ponds. The decisive criterion
for chosing these 10 species was their participation
in their daily portion of food and the size of the population.
Based on the gizzard contents of both the
grown-up and the young birds the approximate quantity
of the consumed fish was determined for each
of the investigated species, while the approximate
total quantity of the consumed fish was determined
according to the recorder number of each population
and the duration of its sojourn at the fish ponds.
The investigations were Carried from 1976-1985.
The obtained results show that damage was tolerable
in the interval from 1976-1980 because the
number of nesting species was then considerably
smaller than in 1985 when some new ichthyophagous
species were observed to nest at the fish ponds in
very numerous populations, causing a great increase
of damage. Indirect damage has not been investigated.
Possibilities are very limited for the protection of
either the complete area of the fish ponds or separate
ponds. On the other hand, partial or total extinction
of a single species will not give adequate
solution. The meausres have to be complex and should
include a decrease in the number of single species
(particularly those in expansion like the cormorant),
use of both active and passive scarecrows and
damage compensation for the overpopulated but endangered
species
