Overfishing and the Replacement of Demersal Finfish by Shellfish: An Example from the English Channel
When overfishing occurs in various parts of the world, the response from industry is usually always the same: rather than reducing fishing efforts (or abandoning the practice), they simply fish further down the food chain (or “trophic levels”) until those species are also depleted, and continue the process downwards. This long-ranging study examining statistics from the past 90 years, finds that fishing in the English Channel has followed this same destructive pattern of fishing down trophic levels, with devastating results. The findings show that serious measures need to be taken to stop fishing in certain areas and governments have to take a more proactive role in marine protection, otherwise overfishing of marine species will have irrecoverable consequences.
Historical marine ecology studies such as this can give us a better perspective on our understanding of marine ecosystems. “Overfishing has had profound effects on coastal ecosystems worldwide for centuries,” state the authors. Unfortunately, in many instances, they find that “the historical response to overfishing is an increase in fishing effort, an expansion to new and deeper grounds and a shift to new target species.” This global phenomenon was first described in 1998. The researchers showed “a decline in the trophic level of global fisheries landings from 3.3 units in the early 1950s to 3.1 in 1994. […] Fisheries typically remove top predators first and as a result their direct competitors and prey are able to prosper, affecting the overall productivity and ecological stability of the ecosystem.” The deliberate fishing of top predators has led to a problematic cascading effect. The authors note that “although new and economically viable fisheries have developed for these new target species, concerns have been raised about their long-term sustainability as well as shifts towards homogenized, simplified ecosystems.” In other words, fishing is not only effectively wiping out species, but it is changing the species balance of entire ecosystems.
To carry out their study, the researchers used a 90-year dataset of international catch statistics from the English Channel marine ecosystem, a region that “has numerous important fishing ports and where finfish landings now make up a far smaller proportion of the catch than they did historically.” The dataset spans decades of intensive fishing activity in a narrow strait between England and France that covers 75,000 km sq. with an average depth of around 100 m in the west gradually decreasing to 40 m depth in the east. Examining statistics over a long period of time, researchers found that, although “fishing has exerted pressure in these waters since the Middle Ages,” the advent of motorized fishing vessels increased pressure exponentially. The results are staggering. “There was a clear increase in landings from the English Channel between 1920 and 2010. These increased gradually from 9,146 tons in 1920 to 50,924 tons in 1970 and escalated rapidly to a maximum weight of 177,793 tons in 1982.” From these numbers it’s clear that fishing pressure has caused significant changes in the English Channel, and the researchers note that “the mean Trophic Level of English Channel landings has fallen by 0.1 unit per decade, one of the fastest rates reported among other heavily fished regions of the world providing yet more evidence that ‘fishing down food webs’ is a worldwide phenomenon.” In the English Channel, this means that the wholesale depletion of large finfish species has been replaced by shellfish species, as the ecosystem has become less diverse and more homogeneous.
Not all of this fishing activity has gone unnoticed. The researchers say that “since the 1950s, the concept of Maximum Sustainable Yield (MSY) and annual catch allowances has been at the foundation of all fisheries management.” However, they note importantly that “disregard of scientific advice over the years has caused the deterioration of many fish stocks and has rendered the single-species approach highly ineffective.” The impact of fishing is serious, and the authors emphasize that these intensive industrial fisheries are often conducted for “perverse economic incentives.” For advocates, information from this study would be very useful to help educate the public on the damage caused by fisheries around the world.
Original Abstract:
The worldwide depletion of major fish stocks through intensive industrial fishing is thought to have profoundly altered the trophic structure of marine ecosystems. Here we assess changes in the trophic structure of the English Channel marine ecosystem using a 90-year time-series (1920–2010) of commercial fishery landings. Our analysis was based on estimates of the mean trophic level (mTL) of annual landings and the Fishing-in-Balance index (FiB). Food webs of the Channel ecosystem have been altered, as shown by a significant decline in the mTL of fishery landings whilst increases in the FiB index suggest increased fishing effort and fishery expansion. Large, high trophic level species (e.g. spurdog, cod, ling) have been increasingly replaced by smaller, low trophic level fish (e.g. small spotted catsharks) and invertebrates (e.g. scallops, crabs and lobster). Declining trophic levels in fisheries catches have occurred worldwide, with fish catches progressively being replaced by invertebrates. We argue that a network of fisheries closures would help rebalance the trophic status of the Channel and allow regeneration of marine ecosystems.