Oxygen-depleted waters of harbour undermining fish restoration efforts
Researchers have discovered that low oxygen levels in Hamilton Harbour are discouraging sport fish and encouraging carp and goldfish
FEDERAL
GOVERNMENT scientists are focusing on a perplexing problem in Hamilton Harbour that is bogging down efforts to restore populations of desired fish species, a major part of the overall restoration of the bay.
They’ve found that through summer months vast sections of the harbour do not carry nearly as much dissolved oxygen as fish species require.
Researchers from Fisheries and Oceans Canada believe low oxygen is discouraging the healthy growth and reproduction of desired species such as walleye, largemouth bass and northern pike. Whereas undesired species, such as carp and goldfish, are flourishing be-
cause their needs for oxygen are much less.
Dissolved oxygen — which mostly comes from surrounding air and photosynthesis in plants — is necessary for fish to breathe, but the amount required varies depending on the species. Hamilton Harbour has chronic dissolved oxygen issues, mostly traceable to too many nutrients getting into the bay from sewage treatment plants and urban runoff.
The issue has been known about for some time. As well as impairing fish development, low oxygen is a factor with the algae blooms that have been an annual late-summer problem in the harbour. But new research — based on work by Matthew Wells of the University of Toronto — has demonstrated an interesting side to the phenomenon. It seems that large winds are “upwelling” oxygen-depleted water around the bay.
“When the wind blows, it pushes the warm water to one end of the harbour, and it gets replaced by cold water that doesn’t have any oxygen. So, that happens every time the wind blows strongly,” Wells said.
Jon Midwood — a Fisheries and Oceans scientist who has
been tracking fish movements in the compromised waters — said, “You get oxygen-depleted water flushing up and affecting shallow areas of the harbour.
“Along the north shore, we have lovely beds of submerged vegetation, which would be great fish habitat; but when low-oxygen water moves, the fish move out.”
The low-oxygen issue is interesting because it’s another example of the complex road to restoration that harbour scientists face. It’s not easy to fix an ecology that is broken.
Hamilton Harbour was deemed an “area of concern” in 1985 “due to severely degraded environmental conditions, caused primarily by local pollution from industrial and municipal activities.” And since then, scientists and other stakeholders have been trying to edge the harbour out of its environmental malaise into a healthy waterway of flora and fauna reminiscent of a time before industrial activity and urban growth.
Many improvements have been made, to the point that Ministry of Natural Resources officials have taken the optimistic step of stocking the bay with walleye on several occasions, most recently in 2016. Those efforts have produced some impressive-sized fish that anglers have been raving about.
But, so far, there is no evidence the walleye have truly taken hold by reproducing. It’s thought that compromised oxygen in the harbour may be a factor.
“The question now is how this will limit our ability to recover the type of fish community we want to have here in the harbour,” Midwood said.
Wells said a better understanding of dissolved oxygen in the harbour and how it affects fish “should help inform where you put your efforts.
“There is no point in putting a lot of effort into habitat restoration where fish aren’t going to go because the oxygen is continuously fluctuating. This can help identify places where you would not get much bang for your buck and places where you would get a bang for your buck.”
One option might be bubbling oxygen into the harbour waters, but that could be prohibitively expensive or unfeasible, Midwood said.
Scheduled improvements to the Woodward Avenue Wastewater Treatment Plant should help the situation, he said. The city is exploring options to upgrade the Dundas Wastewater Treatment Plant, which would also likely benefit the situation once the work is done.