As sea temperatures rose, the herring population fell in the Gulf of St. Lawrence

Federal scientists are now using environmental conditions to predict what will happen to fish populations in Atlantic Canada.

The new projection method was applied this year for the first time to the population of spring spawning herring in the southern Gulf of St. Lawrence. The forage fish has been in serious trouble for two decades.

An analysis of the impact of ecosystem changes in the Gulf — and a new model showing recovery is unlikely — was one factor in the Department of Fisheries and Oceans' decision to impose a moratorium on the spring herring fishery.

A distinct fall spawning herring population in the area is in better shape. Its quota was reduced by 16 per cent late last week.

Using an ecosystem approach to set quotas

"It improves our understanding of what is happening to the stocks and it helps us manage our expectations about the possibility of rebuilding," said Fisheries and Oceans aquatic science biologist Francois Turcotte, based in Moncton.

In a recently published study, Turcotte links key environmental changes in the southern Gulf of St. Lawrence to the collapse of spring spawning herring.

Change in ocean conditions coincides with fish population decline

He said waters started warming in 1992, and fewer herring survived beyond age two — a phase known as recruitment.

The rise in sea temperatures and decrease in the survival of young herring coincided with changes in the presence of microscopic animals in the water called zooplankton.

Zooplankton are food for baby herring and critical to their survival.

The numbers of energy-rich zooplankton that favour colder water decreased, while less nutritious warmer water zooplankton increased.

"The average level of recruitment of spring spawning herring declined to half of what it was," said Turcotte.

"In recent years the level is 10 times lower than the highest recruitment observed before 1992."

A species with boom-and-bust cycles has not seen any booms lately.

Predicting how many fish will live to age two

Turcotte built a model that uses the presence of zooplankton to predict how many herring can be expected to survive to age two.

The model was tested retroactively and predicted previous year events with enough accuracy that it can be used in future stock assessments, he said.

"Herring is a key species in the Gulf ecosystem. Its role in the ecosystem is to transfer the energy from the lower food chain to the bigger animals … almost everything in the gulf eats herring," said Turcotte.

"So it's very important to have a good understanding of what is driving this population and what is keeping it from recovering. So this way we can protect it and protect the critical stages of its life."

'A consequence of climate change'

Warming sea surface temperatures and less ice cover have been well documented in the Gulf of St. Lawrence.

Turcotte's said making a link to changes in herring recruitment "is a long and complicated road."

"But it is a fair assumption to say that it is a consequence of climate change on the increases in sea temperatures and the resulting impacts on the productivity of the Gulf as a whole," he added.

Fisheries scientist Sebastian Pardo with the Halifax-based conservation group Ecology Action Centre says including environmental conditions makes sense.

"It's looking beyond our traditional way of managing fisheries and incorporating variables that we know are important but haven't been taken into account until recently," he said.

Turcotte said the ecosystem approach used in his study is part of a national initiative at DFO and is a way to ensure climate change is factored into assessments of fish stocks.

"Including environmental data and species interaction is something that we will be doing more and more in DFO," he said.

"This is aimed at moving towards assessing our stocks in a way that considers the stock with its links to other components of the ecosystem and vice versa."