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Fisheries

Northern Atlantic cod make an epic comeback

Paper:

Rose, G., & Rowe, S., (2015). Northern cod comeback. Canadian Journal of Fisheries and Aquatic Science. 72:1789-1798. Doi: 10.1139/cjfas-2015-0346

gangster cod

Credit: Joachim S. Müller, flickr creative commons, modified by Megan Chen

 

Guess who’s back, back again? Northern Atlantic cod are back, tell a friend!

Figure 1: Collapse of the Northern Atlantic cod stock.  Credit: Lamiot, Wikipedia Commons

Figure 1: Collapse of the Northern Atlantic cod stock. Credit: Lamiot, Wikipedia Commons

Do you remember Eminem’s hit song “Without me”? If you do, you might have appreciated the previous sentence, and you might also remember learning about the great collapse of the Northern Atlantic cod (Gadus Morhua) fishery. The collapse of cod was the textbook example how overfishing can drive fish stocks to collapse. I know what you’re thinking: Fish stock? Sounds delicious! However this biology term does not refer to a broth made by simmering produce and protein, but it refers to a group of fish within a well-defined area that participate in the same reproductive activity. At their heyday, bottom-dwelling Northern Atlantic cod numbered around several million tonnes (t), and supported a major Newfoundland fishery (Newfoundland is a province in Canada for all you Americans). Beyond its economic impact, cod fishing not only fed communities, but also supported livelihoods for hundreds of years, until its spectacular crash—Figure 1. Closely after the collapse, a moratorium or a temporary ban on fishing for cod was announced by the government in 1992. Newfoundlanders were hit hard—many people not only lost their jobs but also their cultural identity.

At the time, it seemed as if the last chapter of a cautionary tale had been penned—however; there is a twist! A new chapter is being written: The great Northern Atlantic cod stock is rebuilding. In their paper, Rose & Rowe (2015) track this burgeoning comeback and examined the conditions for cod’s return after two decades.

 

The Cod Complex

The cod stock is complex. That is to say, individuals of this stock don’t ride together and die together. After spawning offshore, the cod stock splits—hightailing it inshore to feed via one of three major migration highways: Hawke Channel, Notre Dame Channel or the Bonavista Corridor—see Figure 2.

The cod complex - the three migration channels.  Credit: Rose & Rowe (2015).

Figure 2: The cod complex – the three cod migration channels off the coast of Newfoundland. Credit: Rose & Rowe (2015).

 

Tools of the Trade

Figure 3: Cod spawning in the Notre Dame Channel in May 2015.  Seabed is red, and grid measures 1 nautical mile vertically and 50m horizontally.

Figure 3: Echogram of cod spawning in the Notre Dame Channel in May 2015. Seabed is red, and grid measures 1 nautical mile vertically and 50m horizontally.

Echo sounders are a type of sonar equipment that measure distance with sound; these were used to survey densities of cod. Sound was broadcast into one or all migration highways (Hawke Channel, Notre Dame Channel and the Bonavista Corridor) every year during months in 1984-2015 where it bounced off aggregations or groups of cod and formed an image called an ‘echogram’ (much like how a bat or dolphin uses echolocation)—Figure 3. Sampling intensity varied over the years, with more sampling when more fish were located, and most of the surveys were done in the Bonavista Corridor. Less than 1% of cod aggregations consisted of other species, so this technique is pretty darn accurate.  Capelin (Mallotus villosus), a small fish and key food source for cod, was also surveyed using this method in separate studies.

Trawling is a commercial fishing practice that involves dragging a net on the bottom of the seafloor. In the Bonavista Corridor, 59,999 cod were caught, measured by length and tagged from 1984-2015. Less cod (8,216) were trawled, measured and tagged in Hawke Channel from less surveys.

Analysis was done with data collected. For example, survey data of cod was randomly sampled in order to make comparisons over 25 years. As well, condition or ‘healthiness’ of the cod was calculated based on mass and length.

 

The Ebb & Flow of The Cod Stock

Figure 4:

Figure 4: Estimates of (a) biomass and (b) productivity in the Bonavista Corridor from 1995-2014. From Rose & Rowe, 2015.

In the early 1990s, the stock shifted and concentrated south to the Bonavista Corridor, leaving the two northern migration highways sparse. Even so, the stock in the Bonavista Corridor dropped from 450,000 t in 1990 to a vestige population in meager condition in 1994 that was difficult to even locate. In 2003, eleven years following the moratorium, cod in Bonavista Corridor had only increased slightly to 5,000 t and most of them were small and young.  At this point, people were disappointed and confused as to why the cod were not recovering as they had hoped.  However, by 2007, cod in the Bonavista Corridor had increased to 17,000 t and by 2008, it increased exponentially to around 75,000 t! The year 2008 was also when the first large spawning event was seen since the moratorium. The Bonavista Corridor cod is still continuing to grow substantially, and most recent numbers in 2015 put it over 200,000 t—see Figure 4. Furthermore, adult cod spawning aggregations are starting to appear in Hawke Channel and Notre Dame Channel. The size and condition of the cod is also improving while mortality rates are decreasing—a trend to warm any Newfoundlander’s heart. So what allowed and is allowing cod to return?

Conditions for comeback

Figure 5:

Figure 5: Mean bottom sea temperatures from 1985-2014 at a station off of St. John’s, Newfoundland. From Rose & Rowe, 2015.

Two main things:

(1)  Capelin that had declined in the 1990s in parallel with cod–in part due to unselective trawling techniques for cod–increased under warmer ocean temperatures and relief from fishing effort—see Figure 5.  Note: while warming temperatures are helping this cod stock to recover, this may not true for cod stocks in New England.

(2)  Very low fishing effort has granted cod consistent refuge against commercial harvest.

What’s next for cod? Given that their prey source is rising, and adult cod are beginning to form spawning aggregations in each of the three migration highways, increasing recruitment, or number of fish that survive to maturity is next on the checklist. While still a far cry from the millions of tonnes of cod that used to grace Newfoundland’s coasts, it is projected that if capelin continue to increase, and fishing effort stays low, this stock can rebuild to historical, pre-crash levels in less than a decade.  While this is a cause for a cautionary fist pump, perhaps the most compelling part of this story is hope: if cod, the icon of overfishing can come back, there’s no telling what else is waiting to make their great comeback of the century (with our help, of course).

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