At last count, almost 5,000 dead trout had been found floating in the Enler River in Comber. Added to that were 326 sea trout, 28 salmon parr and a significant number of flounders.
Anything over 100 adult fish killed makes it a major pollution incident, so joint investigations have been launched by the departments concerned and there is little doubt that a court case is on the way.
Rivers are complex ecosystems that sustain rich arrays of plants, animals and invertebrates. It would be nice to think that every time a river is polluted, someone will face a day in court.
But it's simply not the case. Earlier this month, the Belfast Telegraph revealed that just one in 20 people who pollutes a waterway is prosecuted in the courts.
Most offenders escape punishment, with just a handful of the more than 1,000 incidents a year resulting in a conviction.
The Northern Ireland Environment Agency says incidents of water pollution have fallen sharply over the past 15 years. Last year, 1,149 confirmed cases were reported to the agency, much lower than the 2,087 incidents in 1996.
The agency says 1,200 is still quite high but that covers a host of low-level incidents, including minor pollution such as leaky oil tanks or problem septic tanks. The serious incidents have become less and less common. Of those 1,149 incidents last year, just 57 resulted in a conviction and in 2011, 59 convictions were secured from 1,303 incidents. In many cases, there aren't sufficient grounds to bring an action.
Under the EU Water Framework Directive, Northern Ireland is expected to have 59% of waters in good condition by 2015 – an apparently impossible target, given that we're still hovering somewhere around the 30% mark.
Our two biggest waterways, Lough Neagh and Lough Erne, are in particularly poor condition. Both are high in nutrients from farm run-off and susceptible to algal blooms that suck the oxygen out of the water and damage the natural array of plants and animals. It was a process similar to this that resulted in this week's fish kill, although the high nutrient levels were caused by silty wash water rather than fertiliser and sewage effluent.
It didn't help that when the then Environment Minister Alex Attwood put in a bid of £9m to the Executive to take forward Water Framework Directive work that was needed following the drawing up of river basin management plans, it was flatly turned down. Instead he had to scrape together the money to do some of the work.
And if Northern Ireland doesn't manage to meet those exacting standards by 2015, then we face the prospect of paying millions in infraction fines.
Green groups say we need better regulation in concert with moving away from an over-reliance on chemicals in farming.
One good place to start would be to tackle the large numbers of legal discharge consents issued by the DoE which the Green Party brands "legalised pollution".
The Ulster Farmers Union has campaigned against red tape, saying it is becoming impossible for farmers to do their job because of the sheer weight of bureaucracy.
It seems clear that this week's fish kill was probably a disastrous accident and there was no intention of treating the Enler River as a convenient dumping ground.
But if our beleaguered aquatic habitats are to get the protection they need, then more regulation is needed to make sure these kind of mistakes can't happen, not less.
If we want to be confident that our regulatory system is capable of protecting our environment against the dangers posed by fracking and mining, then at the very least we need a system that can protect a river from dirty water.
There are already fears about the impact of fracking on water quality and people are concerned about the threat of contamination by leakage and spillages of chemicals used in shale gas extraction.
With the lack of an independent Environmental Protection Agency, incidents like this one do not foster confidence that the current regulatory system is fit for purpose.