Understanding climate, plankton and commercial fishing

If you were to ask people what influences the abundance of fish in the sea, most would probably answer ‘commercial fishing’. While it is true that commercial fishing can deplete fish stocks, another important factor, the ‘environment’, or more accurately, environmental variability, is often overlooked as a determinant of fish abundance.

Atlantic cod  (Gadus morhua)

Certainly, when the environment is benign, commercial fishing, referred to as a ‘top-down’ control of fish abundance (think of fishing as a form of predation) can be the most important influence. However, when the environment changes and becomes unfavourable, the environment will become a key driver of fish abundance, and this is referred to as ‘bottom-up’ control. Understanding the interplay of bottom-up and top-down controls is vital for sustainable fisheries, and especially at a time of climate change and warming seas.

It is the phytoplankton and the plankton food web that determines the abundance of fish and all other creatures in the sea (http://blueplanetsociety.blogspot.co.uk/2015/03/the-importance-of-plankton.html).

Stylised diagram of the plankton food web with respect to the herring, a fish that feeds upon plankton throughout its life. The arrows show the interconnections in the food web, ‘who eats who’, revealing the complexity and how it might easily uncouple.

The plankton live at the sea surface and their habitat is likely to warm due to anthropogenic climate change. More and more studies are now providing evidence that the plankton’s distribution, abundance and seasonality is altering as their habitat warms, uncoupling the marine food chain. In the North Atlantic ocean many of these studies have focused upon understanding the population dynamics of the cod Gadus morhua due to its commercial importance and history of population declines.

In the Northeast Atlantic, warm-temperate, pseudo-oceanic species of copepod have moved northwards by about 10° of latitudes over 48 years between 1958 and 2005 (52–62°N;10°W) as the sea surface has warmed, which is a poleward movement of 23.16 km per year (http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2486.2009.01848.x/full). Cold water species of copepod have retracted towards the poles and warm-water species have moved northwards. This movement of copepods has resulted in a 60% reduction in the preferred food of larval cod in the North Sea, the cold water copepod Calanus finmarchicus, affecting cod recruitment (the number of juvenile cod that survive to become adults).

Cod is also a cold water species and the North Sea lies at the southern edge of this fish’s distribution (http://www.int-res.com/articles/cr2010/41/c041p015.pdf).

The thermal niche of the Atlantic cod (blue area) based on mean annual sea surface temperature (SST) during the period 1960 to 2005 and the probability of cod occurrence. Both observed (1960 to 2005, shaded bars, white text and arrows) and projected (based upon climate change scenario A2, 1990 to 2100, black text and arrows) ranges in SST are shown for Iceland (solid vertical lines) and the North Sea (dashed vertical lines), indicating that under climate change scenario A2, the North Sea becomes too warm for high numbers of cod such that it may be an unviable fishery.
So, not only are cod in the North Sea experiencing fishing pressure, but the warming environment is now exerting bottom-up control too, both through the food web and upon cod directly (http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2486.2009.02063.x/full). You might well argue that cod will just move northwards and so ‘all will be OK’. Unfortunately, it is not so simple as both the habitat (bathymetry) and temperature must be suitable. The North Sea is a shallow, nutrient rich sea and so supports a productive plankton food web. (Shallow seas, like nutrient rich upwelling regions, support the world’s most productive fisheries.) Northwards of the North Sea the ocean deepens and is less favourable for plankton and cod. Here, the shallow seas are restricted to continental shelves along the coast of Norway and surrounding Iceland. The next, large, favourable habitat for cod is the Barents Sea.

Cod feed upon crabs, lobsters and shrimps and in regions where cod have declined through overfishing, such as in the Northwest Atlantic, there has been a large increase in these decapods, which may be due to decreased predation pressure upon them (relaxation of a top-down control). In the North Sea, where cod have declined due to the combined effects of fishing and environmental change, decapods have also increased in abundance. Currently, the abundance of decapods in the North Sea is also influenced positively by warming; they produce more offspring when the sea is warmer and warm-water species have also invaded. And so, in the North Sea, the decline of cod and the warming environment may both be favouring decapods. In turn, this has ramifications for other species and the ecology of the North Sea; there are ‘winners and losers’ in this ecosystem (http://blueplanetsociety.blogspot.co.uk/2015/11/plankton-as-indicators-of-ecosystem.html).

Two recent studies of cod have again shed light upon how important the environment is as a driver of an animal’s abundance. These two studies are focused at the northern and southern limits of the distribution of cod in the Northwest Atlantic. Here, at the species’ southern limit in the Gulf of Maine, warming seas are reducing the abundance of cod (http://www.sciencemag.org/content/early/2015/10/28/science.aac9819.full). In contrast, at the northern, cold boundary of the species’ distribution in Newfoundland, warming seas are having a positive impact upon their numbers through the food web (http://www.nrcresearchpress.com/doi/10.1139/cjfas-2015-0346#.VjjRWqS3KPQ). Interestingly, these two studies also reveal that environment is a more important determinant of abundance than controls upon overfishing.

If the global climate and the sea surface continues to warm it does not mean that we will not experience some seasons and years that are colder than others. (Of course, due to the warmer baseline temperature, these colder years will not be as cold as they might have been in the past.) Again, using cod as an example, in these cooler years we may see an increase in cod abundance among populations that reside at the warmer edge of the species’ niche due to more favourable conditions (such as cooler conditions would create in the Gulf of Maine or the North Sea). In these circumstances, if we only consider commercial fishing activity to influence abundance, we may be lulled into a false belief that a fish stock is recovering due to effective fishery management strategies, only to find that we were wrong when the sea temperature increases again.

Ecosystems by their nature are complex with many linkages among the species they contain. Understanding the interactions among species, and how and why they change, which must include an understanding of the environment, is key to their sustainable exploitation. Consideration of environmental changes is absolutely necessary with regard to anthropogenic climate change. While there is no guarantee that setting quotas will enable a stock to resist adverse climatic conditions, an absence of regulation might well precipitate a stock’s collapse, or might cancel any short-term benefit of improved environmental conditions.

Dr Richard Kirby and Dr Grégory Beaugrand are plankton scientists interested in marine ecosystem dynamics and fisheries.