Canada’s Oceans Now: Arctic Ecosystems 2023 - Ups and downs of cod and copepods

Ups and downs of cod and copepods
The survival of young copepods and Arctic cod depends on multiple conditions in their habitat. Survival to adulthood is highly variable between years.

On the top left, a blue text box explains that Arctic cod and copepods are called forage species. Explanatory text within the box states:

• They are important food for many fish, marine mammals, and birds.
• Larger, more nutritious adults – not younger life stages – are preferred by large predators.

Within this box, there are white silhouettes of various marine mammals, birds, and fish. Below these, there are black silhouettes of a cod and a copepod at both young (blue circle) and adult life stages (yellow circle). The white dashed lines show which animals use adult copepods and Arctic cod as food.

On the bottom left, a purple text box explains that it takes more than one year for an adult Arctic cod to develop from an egg. At the top of the box, many young cod shapes are shown swimming near the water’s surface. Text explains that young cod can be found in the upper water layers. A large arrow points downwards to larger adult cod shapes, with text explaining that adults are found closer to the sea floor. Some of the adult code shapes are empty, showing that some young cod do not survive into adulthood. The number of young cod who survive depends on variability in climate, predation, water temperature, and ice break up.

On the right side three illustrations depict changes in copepods from spring to fall under three different situations.  Text at the top states:

• Copepods grow from eggs and have many young life stages that look different than adults. Some adult copepods migrate to deep water for winter.

Each illustration shows a vertical cross section of Arctic waters along a temporal gradient from spring (left) to fall (right), with ice cover and ice algae on the left, and open water with phytoplankton towards the right. Adult copepods begin on the left by feeding on ice algae, then lay eggs as the ice begins to break up. The eggs develop into larvae which feed on the phytoplankton before growing into adults ready for the winter.

The top illustration shows what happens when the copepod larvae can feed on the best part of the phytoplankton bloom. Copepods want their larvae to have the best access to food (the first part of a phytoplankton bloom); when they do, more larvae will grow into adults. A check mark indicates that there is a good match between the larvae and their food.

Text beside the illustration says:

• Copepods want their larvae to have the best access to food: the first part of a phytoplankton bloom.
• Larvae are developed enough to feed on the best part of the phytoplankton bloom. More larvae grew into adults.

The middle illustration shows what happens when early ice break-up leads to an earlier phytoplankton bloom. A warning triangle indicates that there is not a good match (a mismatch) between the larvae and their food.

Text beside the illustration says:

• Pressure: Timing
• Early ice break-up, earlier phytoplankton bloom.
• In the Beaufort Sea, the larvae were not developed enough to benefit from the best part of the phytoplankton bloom. Fewer larvae grew into adults.

The bottom illustration shows what happens when hatching occurs in time for larvae to be developed enough to feed, however, the phytoplankton species present are not beneficial. A warning triangle indicates that potentially harmful food was present for the larvae.

Text beside the illustration says:

• Pressure: Unfavourable food
• Larvae are developed to use the bloom – the timing is right. However, the phytoplankton species are not beneficial.
• In Baffin Bay, a bloom of diatom phytoplankton potentially released toxins and were not eaten by the copepods. Fewer larvae grew into adults.