Expert topic: Predator series: Lemon shark
by Rebecca Sherratt, Features Editor, International Aquafeed
The Lemon shark (Negaprion brevirostris) is a species belonging to the Carcharhinidae family which can be found in the Western Atlantic Ocean and South-eastern Atlantic. Individuals familiar with farming fish in subtropical waters may be aware of the Lemon shark and its status as a particularly dangerous predator. Lemon sharks have an especially varied diet but a particular fondness for bony fish, crustaceans and shrimp.
In the US especially, Lemon sharks are often caught as bycatch in both gillnet and pelagic fisheries. Their name derives from their yellowish hue, which enables the Lemon shark to blend in with the sandy seafloor and remain undetected by the fish above. The species can also be recognised by their blunt snout, which is shorter than the width of their mouth and notably more rounded than those of other shark species. Lemon sharks lack a mid-dorsal ridge and their two dorsal fins are remarkably similar in size.
Lemon sharks can grow up to 11.3ft long and 183kg and tend to inhabit the shallow waters of mangroves, coral reefs and enclosed bays, which also serve as nursery areas for the smaller fish they prey upon. They rarely enter open waters any deeper than 300 feet, aside from during migrations, preferring continental and insular shelves for their hunting.
This species of shark prefers to live in groups. Known to be social creatures, they regularly communicate for courtship and protection purposes. Feeding is rarely a solitary activity and, once prey is secured, Lemon sharks will feed en masse. Lemon sharks reach sexual maturity at approximately 12-16 years of age and give birth to pups in shallow nursery waters which they often return to throughout their lives.
The belief that sharks are opportunistic hunters is especially true for Lemon sharks. This species will typically relocate to wherever there is prey in abundance and will feed on whatever is available at the time such as catfish, jacks, croakers, crayfish, stingrays and mullet. Despite this, many studies have shown evidence to suggest that Lemon sharks also display preference towards certain species and will actively avoid preying upon certain species in favour of others.
The Lemon shark will adopt a stalking technique to hunt slower species that are easy to capture. In the Bahamas, the Lemon shark's primary diet will consist of mojarras, a species which notably camouflages in an attempt to escape a threatening situation, rather than to simply swim away.
Lemon sharks feed at night through the use of electroreceptors to detect the movement of their prey in twilight waters. Unlike other shark species, which rip their prey apart upon initial contact, Lemon sharks swim up to their prey at impressive speeds, only to brake abruptly and jab the prey with their pectoral fins. Once the Lemon shark has disorientated their prey, they clamp down with their jaw and shake their heads from side to side until chunks of flesh are torn off.
Establishing anti-shark precautions
Sharks can be daunting predators for fish farms, but there are precautionary measures one can take to minimise the threat they pose. One of these measures is shark-resistant netting.
In 2013, Dutch life-sciences company Royal DSM produced an innovative new shark-resistant net for use in warm water aquaculture. Constructed from polyethylene fibres and stainless-steel wire, the net proves to be resistant to bites and can withstand attacks from a variety of aquatic carnivores such as Lemon sharks, hammerheads, tiger sharks and reef sharks. The small holes in the netting also mean that sharks cannot get trapped in the net.
A variety of electrical and chemical repellents are also available on the market for fish farmers who want to minimise the risks of sharks. Alloys and magnets generate small voltages in seawater, which affect a shark's electroreceptors, whilst leaving the farmed fish unaffected.
Many companies have undertaken research into bycatch reduction strategies. Sharks account for over 66 percent of global bycatch, which takes a significant toll on population numbers. Regulations surrounding fishing times, which correspond with when sharks are known to be more active, have been put into place around the globe to try and minimise the damage of such activities.