You may recognise the following structure of a news story about fish: A scientist says a fish stock is in a bad state. Then a fisherman says that’s nonsense because he can see himself that there is plenty of fish. The audience is led to believe that, well, it can’t be that bad if the fisher says that nothing has changed.
But, in fact, the sea has changed a lot. Generations of fishermen and managers have only known a state of degradation and have gotten used to scarcity as something normal, according to French researchers who this week published a study in the journal Ocean & Coastal Management in which they have studied the long term trends in the Celtic Sea and the Bay of Biscay.
A sharp decline in fish abundance in this area took place before the 1970s and this depleted state is what we have gotten used to, the study shows. The scientists at Université Européenne de Bretagne have analysed stock status and ecosystem indicators of the Celtic Sea and the Bay of Biscay from 1950 to 2008 based on landings, stocks assessments data and additional auxiliary observations.
Celtic Sea and the Bay of Biscay. Image: Eric Gaba, Wikimedia Commons.
Sharp decline between 1950s and 1970s
“While there has been a tenfold increase in fishing pressure since world war II, total fish abundance has been divided by six. All indices confirm a sharp decline in biomass between the 1950s and 1970s, and we have stayed at a low level since then. Even though the situation has improved slightly for some stocks the last few years, we must not forget where we have come from and the fact that the level of biomass is still very low,” Didier Gascuel, one of the researchers, said in a press release.
According to the study, the apparent stability in landings over the last 50 years masks the fact that the fisheries have been sustained at the cost of a dramatic increase in fishing pressure, and a change in species composition and fishing grounds.
This means that as soon as one resource has been overexploited, fishers have adopted by exploiting new areas and species and by improving their technology – in many cases aided by EU subsidies.
Caught fish is now younger and smaller
Over the studied period, the maximum size of fish has decreased by 32 cm on average for all the species. The large predatory fish are the most affected. For the most exploited species there has also been a change in age structure: the older fish have disappeared and the fishery is dependent on catching younger and younger fish.
“A more moderate fishing pressure would allow for an abundant resource and bigger catches, which would guarantee a better profitability of the industry. Looking back only 20 or 30 years may lead you to think that the ecosystem has not changed. The long term perspective, on the contrary, shows us how much it has deteriorated,” said Didier Gascuel.
Reduce fishing
The concept of “shifting baselines” – i.e. where scientists fail to identify the correct baseline for how abundant a fish species population was before human exploitation and thus work with scarcity as the normal state – has previously been demonstrated in other parts of the world, but this is the first long term study of the Celtic Sea and the Bay of Biscay which shows that the same syndrome applies to this area in Europe.
To reach sustainable levels, fishing pressure on most demersal (living near the bottom of the sea) stocks would need to be cut by at least half, the study concludes. But, Didier Gascuel notes, “achieving such a change seems to be a challenge in the context of the reform of the Common Fisheries Policy.”
Cefas provides an excellent appraisal of the precautioary approach which helps explain some of the terms and concepts used in the paper above:
Precautionary approach
In order to understand how exploitation of a fish stock (that is, fishing mortality) can be managed sustainably, it is useful to explain the ways in which catches and stock abundance respond to different levels of fishing.
The figure below shows how catches from an unfished stock would increase in line with exploitation, up to a point where the total mortality on the stock causes so many fish to be caught at a relatively small size (and discarded or landed) that the potential of the stock to increase through growth is not realised. This "growth overfishing" is common to most marine fish stocks today.
However, providing sufficient fish survive to become adults and spawn, they may still have the reproductive capacity to replace themselves. Stock collapse can only occur when fishing mortality reaches a level (Flim) such that removals from a stock are so high, and its spawning capacity is so diminished, that fewer and fewer juveniles are produced.
The impact of "recruit overfishing" is illustrated by the history of the herring stock in the North Sea, which collapsed in the 1970s. The figure below shows how the abundance of juvenile herring has changed in relation to the spawning stock biomass (weight of mature fish each year), such that at spawning-stock levels below approximately 800 thousand tonnes (Blim), recruitment is reduced. So, not only is the size of the stock being reduced by too high a level of exploitation, but there are fewer juvenile fish to replace those that are caught, and stock levels are likely to fall even lower.
To avoid such stock collapses, fishing mortality needs to be kept at levels which will ensure that stocks are sustained and remain productive (i.e. well below Flim).
The precautionary approach, however, requires fisheries managers to take account of uncertainties in managing stocks. This is done by setting reference points, levels of fishing mortality or spawning stock, at which action should be taken to avert potential stock collapses due to overfishing.
The green zone in the figure above represents the situation in which an exploited stock is within safe biological limits. That is, the spawning biomass is above the biomass reference point (Bpa) which is judged to give a reasonable certainty that, in spite of year-to-year fluctuations, the stock will stay above Blim. The other boundary of the "safe" zone is the level of fishing mortality (Fpa) which is sufficiently below Flim that there is a low probability of stock collapse.
A stock that lies within the amber zone, either because its spawning biomass is approaching Blim too closely or it is being exploited at a level above Fpa, or both, is considered to be outside safe biological limits according to the precautionary approach. In such cases, managers are warned not to allow levels of exploitation that are likely to push the stock into the red zone, but are encouraged to reduce fishing mortality so that more fish survive and the stock returns to the safe (green) zone.
Scientific advice on the status of fish stocks in the northern Atlantic is now being given on the basis of these fishery reference points. They are consistent between stocks, and enable fishermen, managers, consumers and environmentalists to judge whether these renewable resources are being managed sustainably.
Report abstract:
Stocks status and ecosystem indicators of the Celtic Sea and the Bay of Biscay were analysed from 1950 to 2008 based on landings, stocks assessments data and additional auxiliary observations. The apparent stability in landings over the last 50 years masks the fact that the fisheries have been sustained at the cost of a dramatic increase in fishing pressure, and a change in species composition and fishing grounds. Major changes occurred between 1950 and 1970s with a major decrease in the global biomass index. The mean asymptotic length of fish landed has declined by 32 cm (19 cm for demersal fish alone) and the mean trophic level by 0.25. Both the Primary Production Required by the fishery and the Fishing in balance index have declined since the 1980s. All indices lead to conclude to a pervasive overexploitation over the last 30 years. Most exploited species considered are characterized by a severe truncation of their length and age structures, the reliance of the fishery on new recruits, and a large proportion of immature individuals in the landings. For most assessed stocks, fishing mortality is higher than Fmax and close to Fpa. Rebuilding the stocks will require a 2–3-fold decrease in fishing mortality.
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