Researchers seeking answers to mercury accumulation in African fish

Kenyan fishers are hoping UWindsor scientists can

improve environmental conditions that threaten the stock of Nile perch.

UWindsor scientists are investigating a mercury contamination mystery that could threaten the future of the Kenyan Nile perch fisheries.

Although Ken Drouillard found that mercury contamination in Winam Gulf, the largest portion of Lake Victoria within Kenya, is on the decline in both the lake as a whole and in the lower components of its food web, he also discovered it is on the rise within Nile perch.

“We saw loadings of mercury increasing from 1960s to the ’80s when it peaked and then started declining and you’d expect the whole lake to recover as a result,” says Dr. Drouillard, School of the Environment professor and Great Lakes Institute for Environmental Research (GLIER) researcher.

“We observed that in the lower trophic levels of the food web the mercury dropped, in mussels and even in smaller fish such as Nile tilapia, but then looking at the top predatory fish Nile perch, we found the opposite pattern and mercury was increasing — this was perplexing.”

Nile perch, which can grow up to 2 m and weigh as much as 200 kg, is a high-quality whitefish that Kenyan fishers rely on for their livelihood. Nile perch were introduced into Lake Victoria in the 1950s to devastating effect on endemic cichlid species causing mass extinctions in this ancient African Great Lake. However, as the fish composition changed, so did its fisheries, and now Nile perch constitutes the country’s most important aquatic export commodity.

Drouillard says they were not expecting the toxin anomaly, which could be an early warning indicator that there is a problem with the sustainability of the Nile perch fisheries.

He says the mercury increase is happening despite the decrease in loadings to the system and after disproving several hypotheses as to why that is, fell upon one scientific explanation.

“It is happening because of bioenergetic stressors happening in the lake, which is reducing the fishes’ growth, meaning these fish are growing less efficiently than they used to. That slower growth rate means they accumulate mercury at a higher rate,” he says.

Among the variety of possible responsible stressors are harmful algal blooms, excess nutrients getting added to the ecosystem via aquaculture pens, and intense agriculture or other impacts of invasive species.

One positive aspect of having an early warning indicator, says Drouillard, is that something may be done about it. This could be a prelude of what may be coming to Lake Victoria as a whole.

“The biggest strategy now is to consider how to manage the water quality,” he says.

“This is a canary in the coal mine observation that the fisheries may be in trouble, and we don’t want to focus on mercury, but address other multiple lake stressors which are somehow interacting with toxic contaminants and exacerbating their effects.”

It is also a potential wake-up call for the Great Lakes, as Winam Gulf has similarities to parts of Lake Erie. Drouillard accompanied a team of UWindsor researchers led by GLIER director Mike McKay to Kenya to compare algal blooms in Africa and North America’s Great Lakes.

“Like Winam Gulf, Lake Erie’s shallower western basin has a lot of activities, from agriculture to industrial, and when you add pollutants to these shallow systems, they become more concentrated, with higher impacts as a result.

“This could make us vulnerable to multiple stressors as well.”

As part of Drouillard’s research he teamed up with senior research fellow from Saint Mary’s University, Linda Campbell, who had collected data in Winam Gulf in 1998. Drouillard and Dr. Campbell replicated her 1998 sampling design to generate a strong 25-year pollutant comparison.

The research results appeared in the journal Science of the Total Environment in the article, “Increasing mercury bioaccumulation and biomagnification rates of Nile perch (Lates niloticus L.) in Winam Gulf, Lake Victoria, Kenya.”

Courtesy: https://www.uwindsor.ca/dailynews/2024-02-16/researchers-seeking-answers-mercury-accumulation-african-fish