Arxiu d'etiquetes: fisheries

Eat less sardines to eat more sardines

A recent study published in November 2014 in Proceedings of the National Academy of Sciences of the United States of America (PNAS) demonstrates the effect of fishing in forage fish population collapses. In this article, we are going to do a review of it.


Forage fish is important in two ways: in the first one, they support the largest fisheries in the world; and in the second one, they transfer energy from plankton to predators, like large fish, seabirds and marine mammals. So, if fishing depletes foraging fish, the consequences can be catastrophic in both the ecological and the economical way. When a fish population collapses, there are three additive reasons:

  1. High fishing pressure for several years before collapse.
  2. A sharp drop in natural population productivity.
  3. A delayed response to reduce fishing pressure.
Forage fish: The vital link of the ocean food webs (Picture from Grist)
Forage fish: The vital link of the ocean food webs (Picture from Grist)


Forage fish are small fishes that eat plankton (phytoplankton or zooplankton) and are eaten by larger predators that are higher on the food web. They play a key role in marine ecosystems by transfering energy through the food chain. So, the collapse of theses species can produce widespread ecological effects. They include sardines, capelin, herrings, anchovies, mackerels, sand eels and menhaden. Further information here.

Some examples of forage fish (Picture from The Pew Charitable Trust).
Some examples of forage fish (Picture from The Pew Charitable Trust).

They also provide benefits to people since these species support the 30% of the worldwide fisheries by weight, as well benefiting aquaculture and livestock industries.

It is important to know that these species, naturally, suffer large cyclical fluctuations in abundance. For this reason, the fishing pressure on these species can aggravate the situation.


Almost one-half of the forage fish studied populations collapsed at a limit of 25% of the average population biomass and almost one-third do it at a limit of 15%. Severe depletions of these populations have commonly occurred and the reasons of this population collapses are a combination of three elements: rapid decline in natural productivity (average population productivity dropped 2-3 years before collapse), continuous high fishing rate (50-200% higher that average annual fishing rates) and retarded response of the fishery to low productivity.

Fishing rate is an important factor that contribute to collapse of populations because it was higher in collapsed populations (1/0.44 vs. 1/0.26 years), while the mean natural productivity was similar in both cases. Models predict that only 4 of the 15 studied populations would have collapsed due to a decline in natural productivity. Moreover, the average minimum biomass levels would have been six times greater if fishing had not occurred during the years immediately leading up to collapse. So, it can be concluded that high fishing rates promotes population collapse.


There are two managements tools to protect predators that feed on forage fish. The first case consist on developing early warning indicators of changing stock productivity, but it has been successful in some particular cases. The second one overcome the limitations of the first and consist on a risk-based approach, when fishing is restricted during the high-risk periods.

It is important to emphasise that reducing fishing when forage fish are moderately limited would have small effect on fishery catches while providing substantial ecological benefits. 



Fishing and general state of fisheries

This is the first post in the topic Impacts of the fishing, whose goal is to expose the impact of fisheries in the sea populations and them ecosystems.


Fishing consists on the exploitation of the aquatic populations that exsists naturaly. Since 1950, it has been observed an increase of the natural captures until 1990 due to the incorporation of motors and sonars in the ships. As of 1990, the capture are around 90 million tonnes. Nowadays, the fishing pressure is so high that the fisheries tend to collapse because the adults cannot produce enough juvenile fishes. The following picture shows the evolution of the natural captures (blue) and the evolution of aquaculture production (green) (FAO).

evolució pesca

The captures of the fisheries have different components. On one side, the capture of the species of interest (goal species) and, on the other side, the other species with no interest (bycatch). Bycatch can be divided into species with low value or discards. Discards consists on species captured unintentionally because they economic value is non-existent, species legally protect or with a bad appearance. Fishing by trawling discards 40% of the captures.

A fishing stock consists on a population or a piece of it that is exploited as a whole. Depending on the exploitation of the stocks, these can be non-fully exploited (blue), when the fishing pressure don’t affect the population; overexploited (red), when the juvenile fishes aren’t enough to recover the adult population; or fully exploited (green), when the stock tends to collapse.


A 10-times reduction on the fishing resources of the continental shelf in just a century has been reported (Christensen et al. 2004), and a reduction too in the resources of the abyssal plain (Devine et al. 2006) and the open sea.

The climate change and the fishing are the main factors that impact the sea. The effects of the fishing take place in the organisms and ecosystems.


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