The Beacon

Bycatch Spotlight: One of the Biggest Issues Facing Sharks Today

A dusky shark hooked on a long line. (Photo: NOAA Fisheries)

In honor of Shark Week, Oceana is taking a look at one of the biggest issues facing sharks today: bycatch, or the unintentional catch of non-target fish and other marine life. It occurs in multiple fishing gear types and occurs in fisheries throughout the world. Fortunately, this is a reversible situation that can be overcome with collaboration between fishermen and policy makers. Reducing bycatch and promoting responsible fishing is one of Oceana’s biggest campaigns, and Oceana is working to protect marine life through a “count, cap, and control” approach to minimizing bycatch. Read on to learn more about how bycatch affects sharks and what can be done. 

Dusky sharks, for example, have experienced population declines of 99 percent over the last 40 year due to overfishing and bycatch. In 2010 alone, more than 4,000 dusky sharks were captured as bycatch in three bottom longline fisheries in the southeast region of the U.S. But, duskies aren’t the only species to be negatively impacted by bycatch; great whites, tigers, hammerheads, and many other species are also accidentally caught in fishing gear around the world. Throughout the 1990s, fishermen captured 12 million sharks and rays as bycatch every year, just in international waters alone. 

The good news, however, is that bycatch can be eliminated. With cooperation between policy makers, fishermen, conservationists, and researchers around the world, we have the ability to greatly reduce the threat of bycatch and help sharks gain a chance at recovery.

So how do you reduce the number of sharks that get caught as bycatch? The approach depends on several factors, including life history characteristics (such as how they breathe, what the eat, etc.), fishing gear, regional location, and other environmental factors. Longlines and gillnets, for example, are two kinds of fishing gear that are particularly dangerous to sharks.

Longlines—which can catch sharks instead of the intended target at least 20 percent of the time—are  made up of a mainline that can extend for miles, suspended beneath the surface or close to the seafloor, with thousands of hooks dangling from the mainline. Hooking mortality is the term used to describe the time it takes for a shark to die once hooked on a longline, which varies by species.

For example, blue and tiger sharks have low hooking mortality – more than 70 percent of blue sharks and up to 95 percent of tiger sharks survive after being hooked on a longline and then released, respectively. Alternately, night sharks and scalloped hammerheads have much higher mortality rates, surviving only 20 to 40 percent of the time. Between 1992 and 2000, dusky sharks were the second most commonly caught elasmobranchs in the U.S. pelagic longline fishery.

Hooking mortality rates are also dependent on the time that the longline is kept in the water. Some commercial bottom longline fisheries leave their gear in the water for up to 12 hours. In most cases, sharks are less likely to survive the longer the gear is in the water. This is because many shark species are obligate ram ventilators, meaning they have to swim to breathe, and need to keep moving to pass water over their gills in order to get enough oxygen. When hooked on a longline, these fish suffocate because they can’t move. 

Like longlines, gillnets can stretch for miles, floating at the surface or near the seafloor. Once trapped, mortality rates are very high. Requiem sharks, the family that dusky sharks belong to, have very high mortality rates once caught as bycatch in gillnets. Great white sharks, which are protected from targeted fishing, are often caught in set and drift gillnets in California. Reported bycatch from fishery logbooks has averaged more than 10 sharks per year since the 1980s, and that number has increased in recent years.

Post-release mortality is another factor to consider when accounting for shark bycatch. This number describes the percentage of sharks that are released alive but likely die later due to injury or stress from capture. About 80 percent of dusky sharks and 90 percent of scalloped hammerheads die after being released from bottom longlines. Post-release mortality is a particularly important number because it demonstrates that even if a fisherman does everything right after catching a shark accidentally, the shark can still die. By not accounting for post-release mortality, the National Marine Fisheries Service (NMFS) can easily underestimate the negative impacts of bycatch on populations and fail to implement successful regulations.  

The fishing industry has already had some success when it comes to changing the gear it uses. By switching to more selective, efficient gear – changing the shape and size of hooks, bait, and depth and length of gillnets and longlines – fishermen are able to minimize bycatch rather quickly.

Aside from species and gear type, there are several environmental factors that are also known to play a big role in the potential management of shark bycatch. For example, it is known that some species follow environmental cues, like temperature. A recent study found that as temperature increased, the survival of several shark species, including duskies, significantly decreased. Using this information, fishermen could greatly reduce bycatch by learning where sharks will be based on temperature and then avoiding fishing in those areas. Studies have shown that soak time, fishing depth, and hook size can all be altered without significantly affecting the target catch.

When it comes to successfully managing bycatch, it is also incredibly important to account for mortality across all fisheries. Dusky bycatch, for example, is not even tallied across all regional fisheries, which means that some fishermen’s hard work to reduce bycatch in one area might be completely undermined by the behavior of other fisherman in other areas.

Considering how many factors ought to be included in shark bycatch regulations, it is clear that collaborative effort is required to successfully reduce bycatch and allow populations to recover. The fishermen, who are the experts on the water, must work together with researchers and policy makers in order to reduce shark bycatch. In order to be successful, the government must implement regulations to end overfishing that incentivize fishermen to continue down the path of innovative research.


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