Assessing Exposure to Hypoxia in Cod from the Northern Gulf of St. Lawrence and St. Lawrence Estuary Using Otolith Microchemistry

Description

Elevated manganese levels at the edge of the otolith of a five-year-old cod caught at a depth of 219 metres, in hypoxic water. Few samples have been analysed yet, but so far manganese levels are lower in the otoliths of cod caught in coastal waters.

Low levels of dissolved oxygen, known as hypoxia, which is suspected to be caused in part by climate-related changes in ocean circulation and temperature, are known to decrease the growth rate of Atlantic cod. Otoliths—a structure of the inner ear of fish—accumulate mineral deposits over time, which creates characteristic rings representative of the fish's age, much like the rings of a tree trunk. The ratio of specific elements in the deposits can be studied to determine some of the living conditions of the fish at specific times during its life.

Results: Researchers studied otoliths from modern fish (1980 onwards) and historical samples (fish living from 1650–1850) to study the exposure of cod to water low in dissolved oxygen. In this project, researchers analyzed the ratios of elements that relate to oxygen concentration in the water, as well as ratios that indicate the salinity of the water.

Although this analysis and the interpretation of the raw data is on-going, the results so far indicate low levels of the elements that are associated with hypoxia in the otoliths of cod from the St. Lawrence. This could be because cod spend relatively little time in the deep, oxygen-poor waters each year, or because the hypoxia levels of the St. Lawrence are not severe enough to leave a trace in the otoliths.

Further questions on the relationship between chemical elements and fish habitat conditions still need to be answered and will likely require a specialist in water chemistry to assist the team of researchers. Results from this project are expected to be published in a scientific paper or DFO report in 2014.

Program Name

Aquatic Climate Change Adaptation Services Program (ACCASP)

Ecoregion(s)

Atlantic: Gulf of St. Lawrence, St. Lawrence Estuary

Principal Investigator(s)

Denis Chabot
Fisheries and Oceans Canada

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