After years of drought, Aaron Graham was ecstatic to see the Macquarie River coming to life again. The water is higher, the river is clearer and the Murray cod that live there are fatter than ever. However, the source of this abundant fish food supply comes from a rather unsavoury place. Dubbo and the surrounding regions are some of the worst-hit areas of the mouse plague, and the fish have been gorging themselves on the thousands of mice that attempt to swim across the river. “We’ve seen the mice get taken off the top of the water,” Graham said. “Having so much food around they’re just getting so healthy and fat. It’s crazy… It has to be about 90% of the fish I’m pulling up have mice in them. “I’ve seen up to 10 mice coming out of a 70 centimetre and 80 centimetre… I think it’s just like the cod’s way of recovering from all that bad stuff that’s happened in the last 12 months.” Photos have spread across the internet of cod across the Murray Darling Basin river system with plump bellies and dozens of mice bodies sitting in their throat. Graham said that is not necessarily a bad thing, except the fish have a tendency to regurgitate everything in their stomachs when they are caught. “It’s when they do it into the boat that gets me,” he said. “I’ve had a few dry heaves, it’s not the prettiest on the nose, that’s for sure. “They swallow them whole so it’s like they’re slowly dissolving… it’s like a musty smell, they just start falling apart, but you can see the hair and the tails on the feet and it’s not the prettiest.” Besides the disgusting new obstacle for fishermen, the cod’s new diet has raised further concerns about the NSW government’s proposed fix to the mice plague. In May, the NSW agriculture minister, Adam Marshall, announced a $50m support package for affected regions, promising to “knock [the mice] into oblivion”. On Thursday the minister announced $1.8m of this funding would go towards developing genetic biocontrol measures. This could include releasing modified mice into the wild that would spread their altered genes throughout the population and cause either more male than female mice to be born, or render all new female mice infertile. While the government says the research is promising, it is being developed to help control future plagues. The centrepiece of its plan to combat the current crisis is the poison, bromadiolone. The state has applied for emergency approval to use the poison along the perimeter of crops. The wide-scale agricultural use of the anti-coagulant is currently banned, partially due to the vastly increased risk of secondary poisoning compared to the industry standard bait, zinc phosphide. “Zinc phosphide… any toxicity in the carcasses is gone in 24 hours, in one day. It just converts into phosphine gas in the gut,” said Xavier Martin, a grain grower from Mullaley, north east of Dubbo, and the vice president of the NSW Farmers Association. “Whereas, bromadiolone… takes 100 to 200 days to fight down in the gut. So it’s a very different product.” This chance of secondary or even tertiary poisonings poses a significant threat to other animals in the Australian ecosystem. “Whether it’s farm dogs, pigs, poultry, they all scavenge. If they see a sick mouse or a carcass they can’t help themselves. It’s the same with [native] barn owls and eagles and hawks, wedge-tail eagles. There is just an endless list of native animals that would have a go at them too.” The APVMA is still considering the NSW government’s permit application after seeking additional information this week. Guardian Australia understands more than 550 farmers have already registered their interest in using the poison if approved. Dr Peter Brown, leader of the rodent management research team at the CSIRO, said these poisons have the potential to “accumulate up through the food chain”, and affect a wide range of animals. This includes the Murray cod living across the state’s river system. Cod in the rivers throughout the Murray Darling Basin have faced serious threats in recent years after three major kills, where potentially millions of fish died suddenly due to a combination of drought, algal blooms and sudden temperature drops. Now multiple farmers and agronomists have written to the NSW government to lodge their concerns that widespread bromadiolone use could once again threaten the species. Despite running a bait shop in town, Graham doesn’t eat fish and releases all the mice-filled cod he has caught, but he says many people do fish for food on the Macquarie River. But he says it’s unlikely people will ingest contaminated fish if the use of bromadiolone is approved – the sight of decomposing mice bodies should act as sufficient deterrent. “I don’t think I’ve heard too many reports of blokes game enough to eat them. I have heard a few that get to the point where they gut them and see the moss and can’t do it.” Even if the bait is approved, and the government’s plan to set up free grain poison treatments hubs goes ahead, the NSW Farmers Association is skeptical about whether it will work. “They’ve put $50m on the table, but certainly for farmers, I don’t think they’ve killed one mouse yet,” Martin said. “Our members have got considerable reservations about what restrictions on use may be and whether that be any way useful.” Although Martin said many elements of the rescue package were useful, such as bait rebates for households and small business, the NSW Farmers Association wants less focus on bromadiolone and more help for farmers using zinc phosphide baits. Guardian Australia has contacted the NSW agriculture minister’s office for comment.