New study suggests the modern ecological structure of fisheries is dangerously unstable

C.O.A.S.T. report on a new study from the University of York: “It is well documented that the increase in demand for fish, combined with advances in fishing technology has led to the decline of many large bodied finfish fisheries around the world. As a result, catches of shellfish such as prawns, scallops and lobsters have rocketed as they begin to thrive in unnaturally predator-low environments, often degraded by the passage of trawls and dredges. Although these shellfish fisheries may initially yield greater economic value than the fisheries they replaced, a recent paper published by the University of York (* details below) highlights that they are likely to be unstable in the long-term, and at great risk of collapse from disease, species invasions and climate change.

Note to Illustration: The ecological effects of intensive fishing. From left to right, fishing effort increases over time. As a result, large predatory fish become depleted and fishers are forced to target new species. Consequently, the marine ecosystem becomes progressively more damaged and biodiversitybiodiversity Biological diversity in an environment as indicated by numbers of different species of plants and animals. is reduced. In this unnaturally predator-free environment, the ecosystem can become dominated by highly valuable shellfish, or by harmful algal blooms, highly invasive gelatinous planktonplankton Plankton is a generic term for a wide variety of the smallest yet most important organisms form that drift in our oceans. They can exist in larger forms of more than 20cm as the larval forms of jellyfish, squid, starfish, sea urchins, etc. and can be algae, bacterial or even viral down to as small as 0.2µm. They are nutrient and light dependent, and form the essential foodchain baseline for larger dependent aquatic lifeforms. Fish species rely on the density and distribution of zooplankton to coincide with first-feeding larvae for good survival of their larvae, which can otherwise starve. Man-made impacts such as dredging, dams on rivers, waste dumping, etc can severely affect zooplankton density and distribution, which can in turn strongly affect larval survival and thus breeding success and stock strength of fish species and the entire ecosystem. They also form the essential basis of CO2 take up in our seas ecosystem, hence Global Warming. and jellyfish. Illustration and text: University of York.

“The paper highlights the issue close to home, with the dramatic shift of species landed in the Clyde changing from one dominated by finfish such as cod and haddock, to nephrops prawns; now contributing up to 84% of the current landed weight. The ability to reverse this shift may prove difficult owing to a range of factors such as declines in important nursery habitat and planktonicplanktonic Free-floating, drifting prey for fish species as well as high levels of juvenile by-catch. Great concern for the sustainability of nephrops stocks is also documented due to the high risk of infection by parasites, which may be enhanced by the lack of predators and greater number of smaller-bodied individuals (found to be more susceptible to disease) in regularly fished areas.

Illustration: A huge catch of young scallops in New Zealand. This fishery was highly productive in the 1980s and 90s, but has now completely collapsed. (Photo credit: Peter Duncan).

“With few species left to target and recent assessments suggesting that the Firth of Clyde nephrops stock may already be exploited above the Maximum Sustainable and Economic Yield, if the nephrops population were to collapse the social and economic consequences for Clyde fishermen (and in turn coastal communities) would be severe.

“The paper therefore outlines the requirement to implement management regimes that will promote the recovery of diverse ocean ecosystems. This includes a combined approach to fisheries management including fishing effort and gear control as well as establishing Marine Protected Areas (MPAs) to ensure fisheries sustainability and resilience in the future.

* Howarth, L.M., Roberts, C. M., Thurstan, R.H., Stewart, B. D. (2013). The unintended consequences of simplifying the sea: making the case for complexity. Fish and Fisheries, 2013.

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