Identifying a potential target for conservation, scientists have created the first risk assessment index of sharks and their relatives. Ranging from great whites to goblin sharks, these species are threatened by our fishing practices as well as human-induced changes in ocean ecosystems.
A new study from the University of Melbourne, published in the journal Fish and Fisheries, lays out a new method to calculate how vulnerable each species is to fishing and other human activities, including climate change, under various scenarios.
Of a number of factors, they three key ones to estimate how vulnerable each species is to a type of fishing.
Sharks and related species have a number of different ways to nurture their young. Blacktip and bull sharks, for example, give birth normally with placentas to nourish the developing baby shark inside them until it’s ready. Some other types produce milk which they feed before giving birth while great white sharks lay unfertilized eggs that become food after being laid by the mother.
In a process known as oophagy, gray nurse sharks give birth to live young who eat their siblings inside of them. These embryos are born big enough to survive outside the mother’s body and begin eating other fish in order for survival after being released into Westernport Bay near Melbourne off Australia. In an interesting phenomenon called chimerism that occurs with elephantfish spawning large eggs in spear-shaped horny cases which accumulate fine silt from becoming buried deep underwater on seabeds where they lay spawnings, each embryo eats all its unborn brothers or sisters so it can grow up bigger than its parents were when they hatched out themselves before going through metamorphosis while maturing over time.
These adaptations make species vulnerable to fishing because the females produce fewer, large and capable young rather than producing a larger number of small eggs that many scale fish produce. This means the mothers often cannot replace those we humans kill fast enough to keep their numbers from dwindling away.
Did you know that Australians eat 10 times more sharks than the global average? The latest research shows only 14% of shark species are in good health. That means 86% could be extinct within our lifetime!
Did you know that Melbourne is one of the top cities for eating these animals at a rate ten times higher than most other parts of society? It turns out flake (a type of fried fish) is regularly consumed by many families – and this contributes to an increasing number being threatened with extinction due to overfishing globally.
In order to protect sharks, fishery managers need accurate data on how each species is affected. Unfortunately, this task seems impossible as it would be difficult and dangerous to collect such information from deep sea creatures like the great white shark (Carcharodon carcharias).
The University of Melbourne study looked at three factors for this study. Prior to getting into the details, they first categorized species based on their ecological groups. the method was applied to all 132 species of local sharks and their relatives in southern Australia.
The result of the research found that sharks have a much lower chance of being caught and/or killed by fishing gear than previously thought. Finally, it takes about twenty years for an individual shark’s population to replace itself in one area where humans are hunting them down for food or sport like they do with tuna fish (whose populations can be replaced within nine months).
The expected rates of temperature change in our waters are much faster than the extinction events in Earth’s history, especially for south-eastern Australian waters. As a result, species that live on sand and mud will move to cooler water or further South where it is deeper off the coast of Australia.
Species that live on or around reefs will likely be more affected, as most reefs are in shallow water, and species currently there may end up being displaced by incoming subtropical species.
Researchers evaluated the risks to sharks under low, medium and high-emission scenarios of greenhouse gases in 2100, and under two fishing scenarios, before and after significant fisheries management reform beginning in 2006 greatly reduced fishing pressure. It was at this time for example, that Australia, banned most fishing below a depth of 700 meters.
The study went on to identify how many species are vulnerable to four types of fishing under these five scenarios.
Gillnets are nets that go along the bottom of the sea and only catch mid-sized sharks, including the commercially important gummy sharks. This allows for large mothers to be alive, which can replace their young ones lost from fishing. These changes in management have helped maintain balance within the species making them all less vulnerable than before 2006 when trawl was introduced into Australia’s waters.
Gillnets are a type of fishing gear that not only catch gummy sharks, but also smaller and some larger shark species. A few kinds of these fish may be rendered moderately vulnerable from this method.
When the risks from the current fishing stress and the high emissions scenario were combined—reserachers found that 25 species of shark were at high risk, 70 species at medium risk, and 37 species at low risk.
Our results provide advance warning about which species Australian fisheries managers need to protect.
Read the full study here: https://onlinelibrary.wiley.com/doi/10.1111/faf.12571