November 3, 2016 — “Fishing pressures…or environmental pressures…are different from place to place even within what is considered to be a single management area, and that effect is multiplied when you consider going from one management area to another” says Coby Needle. This implies that there is no singular reason for the observed differences in stock status. However, there do seem to be general trends based on the latitudinal position of stocks.
In general, the northern stocks are doing better than the southern populations. “In the NE Atlantic, the more northerly stocks like Barents Sea and Icelandic cod are generally in much better shape than the ones further south…There is some long-term environmental trend affecting their recovery” says Robin Cook. However, “it’s not as simple as it was 2 or 3 years ago when we probably thought it was all related to global change; the southern stocks were suffering while the northern stocks were benefitting from a warming Arctic” says Chris Zimmermann. One example is the disappearance of North Sea cod from the southern spawning grounds, where there has been no spawning activity for the last 10-15 years. “Newspapers say ‘there’s no spawning of cod in the North Sea at all.’ That’s not the case – it’s just the southern spawning grounds. That’s certainly related to global change” says Chris Zimmermann. “Distributions are further north because the more southern populations are less successful” Robin Cook agrees.
Why do we see these differences among northern and southern populations on both sides of the basin? More specifically, why are northern stocks faring better than their southern counterparts? Factors contributing to these observed differences include variations in (1) environmental conditions and (2) management structures.
Environmental conditions
Northern stocks
On both sides of the basin, first order oceanographic factors tend to influence changes in productivity more strongly than the southern stocks. “Norwegians and Icelanders both agree with first order oceanographic factors playing a big role in changes in productivity they see in their stocks” says Jake Rice. In other words, northern stocks are more affected by fairly dramatic environmental conditions that we don’t see in the more southern areas; the most notable being shifting location and intensity of Arctic cold water that is absent in temperate southern stocks.
Stocks in the higher latitudes are very susceptible to the position of the polar front, which is the difference between colder Arctic water and warmer temperate water. “Cod really don’t do well when the water gets down below 0°C” says Jake Rice, implying that colder Arctic water leads to slower growth rates. Northern stocks “are therefore more vulnerable to environmental changes than in more temperature regions like the North Sea, where water is generally warmer and less variable” says Robin Cook.
If northern stocks are more vulnerable to environmental conditions, why are they doing better than southern populations? In short, global warming. Over the last 10-plus years, the northern stocks have been benefitting from a warming Arctic, which not only includes favorable environmental conditions (warmer water), but has also led to second order trophodynamic benefits. More specifically, the re-emergence of cod’s key food source: capelin. “Capelin is a key species in the Arctic and sub-Arctic ecosystem in the NE Atlantic, and there’s a clear connection between the availability of capelin and stock development of cod” says Chris Zimmermann. This has led to higher recruitment and thus the improvement of northern Atlantic stocks.
Southern stocks
The southern populations, conversely, are more heavily influenced by trophodynamics. Trophodynamics in the North Sea, Celtic Sea, and Irish Sea likely play a much larger role, particularly North Sea stock because it is a species-rich ecosystem. Unlike the northern populations, which are subject to cold Arctic water inflow, southern stocks are located in temperate waters. “The concern is that it might be getting too warm, and conditions are more favorable for some other species than they are for cod” says Jake Rice. In the western Atlantic, Georges Bank and Gulf of Maine stocks have declined in recent years through “a combination of historic overfishing and poor environmental conditions. Generally those stocks do well in cold winters and there hasn’t been a cold winter for a number of years…that has resulted in decline in recruitment success, etc.” says Steve Murawski. Increasing SST may be putting more stress on southern populations on both sides of the basin and may be altering their distribution. For instance, the center of spawning populations for European stocks are now further north than in past years because southern populations have not been as successful. “There’s a number of different possible drivers, temperature would be one…uneven fishing pressure…would be another one…Another potential driver keeping southern stock depressed could be multispecies effects” says Coby Needle.
Management
“Cod is extremely well-monitored…There are more than two dozen management stocks of Atlantic cod around the basin” says Steve Murawski. While these many programs each keep an eye on decreases in productivity, changes in recruitment, etc., “there’s a vast difference in the management approaches, certainly historically and even now” says Steve Murawski. These differences have lent a hand to the observed differences in stock status.
In general, poor environmental conditions combined with high fishing mortality rates contributed to the decline of many cod stocks in past years. One example is the Northern cod stock, where “a combination of overfishing and poor recruitment due to extremely cold weather conditions at Newfoundland conspired to result in a collapse in that stock from which it still is recovering” says Steve Murawski.
“I believe humans have the most important impact on the development stocks” says Chris Zimmermann. The number of participants in a fishery is an important factor, a stark example of which exists between the Barents Sea and North Sea stocks. The Barents Sea stock, which is in good shape, is harvested by 2 nations only (Norway and Russia) and includes a simple management system. In contrast, in the North Sea, which is low but rebuilding, there are several nations fishing on at least 2-3 different life stages. Five nations constitute the majority of the fishery targeting cod (Scotland, England, France, Denmark, and Norway) while minor impacts may come from bycatch of cod in fisheries targeting other species. “There are just too many participants, so it’s very difficult to reach an agreement, and it’s not only participants targeting the adult cod, but it’s also different fisheries catching juvenile cod as bycatch in the brown shrimp fishery, for example” says Chris Zimmermann. “What has happened is distribution of fishing effort hasn’t changed, but the distribution of the fish has changed dramatically, so they are much more distributed to the north” says Coby Needle. This means that nations fishing in northern areas are now catching a higher proportion of cod than their southern counterparts, which has interesting management implications. Despite these differences, the North Sea is now rebuilding, which, according to Steve Murawski, is because “managers have adopted a more conservation management approach. That, in combination with more effective management (actually meeting their targets), has resulted in improved prospects for that stock.”