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Boom and Bust
A shark’s reproductive capacity (slow growing, late to mature and producing few young) means there is little margin for high levels of fisheries-induced mortality.
Historically, many shark fisheries have displayed a distinct ‘boom and bust’ pattern.
The early stage of a target fishery is characterised by the landing of large, ever-growing volumes of mature sharks, which in turn encourages expansion of the fishing fleet. Inevitably, the ‘boom’ is often followed by a rapid decline in landings - the ‘bust’ - as an inflated number of vessels have removed the larger mature sharks. This process depletes populations and, more significantly, greatly reduces their reproductive capacity. At this stage the fishery is no longer financially viable and collapses, often leaving populations under threat - as happened to the Northeast Atlantic Spiny Dogfish (Squalus acanthias), Common Skate (Dipturus spp.), Basking Shark (Cetorhinus maximus) and Porbeagle (Lamna nasus) populations. Once overfished, shark populations can take decades to recover.
For many species this is compounded by their tendency to segregate by size and sex: catches made in a particular geographic area will often be almost exclusively mature females, or immature males, rather than a random ‘selection’ - of males and females, mature and immature - from the wider population. Segregating behaviour is recorded in many species of shark, including Blue (Prionace glauca), Porbeagle and hammerheads (Sphyrna spp.). The removal of large numbers of a single demographic within a population can have a detrimental impact on the wider population.
Encouragingly, there are a small number of fisheries in which sharks are sustainably fished, although these are mostly for smaller, faster growing species. Examples include the Australian Gummyshark (Mustelus antarcticus) fishery, as well as fisheries for Spiny Dogfish in the Northeast Pacific and Northwest Atlantic - both of which were previously subjected to unsustainable levels of fishing mortality but have recovered after fishing was strictly regulated and reduced catch limits put in place (1).
1. Miyake MP, et al. 2010. Recent developments in tuna industry: stocks, fisheries, management, processing, trade and markets. Rome, Italy: FAO.
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