Zoo director Derek Grove said they had built up £1 million of reserves for projects – but had dipped into the cash pot to care for the animals.
It means the plans for the re-introduction of brown bears – and the upgrade of other major exhibits – put on hold with the entire development programme delayed by years.
The second lockdown has led to a drop of around £170,000 in income at the zoo due to it being forced to close, with costs still hitting £250,000.
Mr Grove said: "Over the past five years the zoo has built reserves to enable us to extend, adapt or rebuild several of our major animal habitats, and during this year planning permission was granted to re-instate the brown bears back into the Bear Ravine, to create an additional outside area the Bornean orangutan, along with erecting new housing for the Sumatran tigers and extending their paddock.
"During the previous lockdown a substantial proportion of the £1 million cash reserves set aside for these projects were subsequently used to support the care of the animals and now, although we were able to set some funds while we were open from July to October, these cash reserves are now diminishing again.
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"We have started preliminary work to create space for the new outdoor facility for the Bornean orangutans and we still aim to undertake this project over the coming year if cash reserves allow.
"The sad thing is that projects that should have been completed this year are being postponed and the entire development programme for the Zoo and Castle is being delayed by years."
Mr Grove said it looked "highly unlikely" the zoo, along with the vast majority of animal parks, would be able to access any of the Government's £100 million Zoo Support fund.
Zoos must be 12 weeks from bankruptcy to qualify for the funding, but campaigners have argued any responsible park would already be trying to find new homes for its animals by this time.
Mr Grove, however, has thanked people for their generosity so far – with their fundraising page having hit more than £40,000 from supporters.
He added: "Once again this year Dudley Zoo and Castle is relying on our supporters to help secure our future during the second period of lockdown.
"Through their generosity our JustGiving campaign has hit above £40,000 and this equates to £47,500 once Gift Aid is added from donators able to contribute.
"We are also incredibly grateful for the many donations and well wishes we receive through the post, and Chloe Riley one of our fine young animal care staff raised over £1,000 by undertaking a half marathon around the zoo on her day off, rather her than me I’m afraid, I’d still be running."
To donate to Dudley Zoo and Castle, visit https://www.justgiving.com/campaign/DZCanimalcarefund