A question

Today I'm celebrating that someone has asked a question to the blog, and a good question at that. How is it possible that catches change so much from year to year? And no, that's not in the survey report. The report presents the results. The hypotheses are for cientific publications.

To answer well we would have to fill several volumes, so there goes a rather sketchy response. The answer lies with the combination of hydrography, climate and anthropic pressure (fishing is often the main type of pressure exerted by man in the case of fishery resources).

All animals have an optimum temperature range. At sea, with some exception, as it happens here in the shallows of the Grand Banks, the temperature decreases with increasing depth. Different species of fish prefer different temperatures, that is why we do not get yellowtail flounder at 1000 m or chimaeras at 50 m. Fish seek their optimal temperature by moving along the depth range or latitudinally. For example, due to global warming, species that were traditionally found in the temperate zone are being recorded in high latitudes. The most controversial example that comes to mind is mackerel, a pelagic species that has entered Icelandic waters for some years (to the delight of the islanders) and has created a first class international conflict, but that is another matter.

Vanessa, Antonio, Jorge and Cristina working fast: a deep water haul

It has also been observed in both the Atlantic and the Pacific that there are patterns of fluctuation in populations of very diverse fish species and in different fishing grounds, which led to think of large-scale processes operating in the background. One of these is the North Atlantic Oscillation (NOA), which is a pattern of variability in atmospheric circulation. It affects weather in the North Atlantic, North America and Europe. It also affects planktonic production, and therefore fish stocks and their distributions. It is defined by an index that measures the pressure difference between the Azores anticyclone and the low in Iceland. In the Great Bank, if the NOA index is positive, the influence of the Gulf Stream (which flows northwards and carries warm water) is greater than the influence of the Labrador Current flowing southwards and carries cold water).

And we must also take into account the damage that humans inflict, which we call anthropic pressure. In coastal areas we create many different problems, on the high seas we focus on a few of them. Like fishing. It is believed that fishing increases the annual variability of exploited populations because it increases the mortality of certain species, and it does so in a differential way: it does not affect the entire population pyramid equally. There are indications that this also affects the biology of species, which can start to reproduce at a younger age and smaller size to compensate for the shortage of older and larger females, which are also the ones that produce more and better eggs.

Jorge sampling the tiniest fish we got today.

The populations - in this case of fish - can recover in the face of difficulties, but this depends on the state of the population and the difficulties. If a well-structured population suffers temporarily from high mortality, either due to adverse environmental conditions or the development of a fishery, it can increase its biomass again (although perhaps not at the original level). But if adverse conditions are added to the anthropic pressure we have all the ingredients for a disaster. Ask the Great Bank cod, of which 14 MILLION TONS (4-5 BILLION fish) were fished between 1960 and 1972 according to Rose (2007) and in this survey we are fishing it by units. In recent years, it has been hypothesized that the very cold period which began in the early 1990s and which decimated the population of capelin, the preferred cod diet, contributed as well to the disappearance of cod.

Speaking of dissapearing: the Grand Banks... of Fog.

The big problem that we have to solve in this area of research is that we cannot find a way to separate the effect of the climate from the effect of fishing. Models are being developed but further research is needed.

In the case of our survey, fishing pressure is very low here compared to what it used to be, our gear is working well (a haul can go wrong, but not all hauls in two surveys), and the Canadian surveys last year also caught less than usual, but right now Diana and Fernando are in the annual NAFO meeting where all survey and commercial catch data are analyzed together and they may have an explanation in a few weeks.

Anyway, Alicia, well done. If you have reached this line you have won a beautiful 10 kg cod that you can collect here in the next 7 days from 9 to 14 h.

The cod haul. Lucía, Vanessa to the left, Jose Luis and a passer-by in the background. In the foreground Antonio and Jorge.