Honey, I Shrunk the Sharks!
Updated: Sep 13
We are all aware that over-fishing has the potential to seriously deplete fish stocks, but there are also other effects from fishing, which might be more subtle and insidious. For example, some fisheries have selectively over-harvested a one sex over the other and some have overexploited certain size (age) classes. In some places this "size-selective harvesting" has caused the average size of commercial fish species to actually decrease over time - the fish are shrinking. But does the same process happen to sharks? And does it matter? What affect might it have?
A Fish Out of Water
By far the greatest threat to sharks comes from fisheries. When you look at the factors that caused them to decline, pretty much any and all endangered species of sharks are affected by overfishing (IUCN SSG, 2023).
Sharks are caught by fisheries for many reasons. In some places, they are directly targeted for their meat, fins, liver oils and skin. In many other fisheries they are caught as "bycatch" - incidental catches when a different commercially valuable species is the target. In this case they may be landed anyway as a bonus catch or they might be thrown back (IUCN SSG, 2023). Sharks are also commonly accidentally killed as bycatch, as they are very sensitive to stress. To learn more, check out If You Love Me, Let Me Go.
Under the Radar
Other effects from fishing might be much more subtle. To understand how sharks are being affected, a research team assessed the catches of various species of sharks in Onslow Bay, North Carolina since the 1970s (Benavides, 2020).
They found that, over the time-frame, the sharks being caught in these fisheries were getting much smaller. These declines were pronounced in the blacknose shark (Carcharhinus acronotus), dusky shark (Carcharhinus obscurus), smooth dogfish (Mustelus canis) and the Atlantic sharpnose (Rhizoprionodon terraenovae). The largest drop was in the sandbar shark (Carcharhinus plumbeus); with average "fork length" (aka FL - a scientific measure of the length of a shark's body) reducing by 21cm during the period (Benavides, 2020).
Their models suggested that, in the worst-case scenario, relative declines ranged between 15 - 63%, with several species showing a size reduction of 45% or more. This means that in this area, these species have radically shrunk in size - to almost half their original length - compared to their potential maximum sizes (Benavides, 2020).
Size Matters
This does not mean that fisheries are magically shrinking sharks, but that the size-selective harvesting has reduced the average size of the population because larger individuals are disproportionately extracted (Benavides, 2020).
Size selective harvesting can occur because the fishing gear being used is designed in such a way that individuals of a certain size are commonly caught (Benavides, 2020).
However, in this case, the scientists suspected that the implementation of "minimum size requirements" in recreational and commercial fisheries was the main driver for this pattern. This means that fisheries managers have set a minimum size of fish which can be legally landed in an attempt to boost population growth. Sadly, because this has meant that the larger individuals have more commonly been caught, so over time, the average size of the sharks in the population has decreased (Benavides, 2020).
Under Pressure
In other fish species, size-selective harvesting is thought to create a "selection pressure". This natural evolutionary process is what drives animals to adapt and change thanks to pressures in their environment. For example, if brightly coloured individuals are more likely to get picked off by predators, there is a selection pressure to be dull and so over time, that species will become less luridly coloured.
In this incidence, the anthropogenic pressure to stay small (because larger animals are more likely to die in fisheries) might mean that the species evolves to have a reduced growth rate or to have an earlier size-at-maturity, decreased litter sizes or reduced investment in offspring growth (Benavides, 2020).
I Meant Well
Sandbar sharks are flagged as Endangered by the IUCN and their populations are still declining in the wild. Minimum size requirements were brought in to protect threatened sharks in this area, as it was hoped that it would reduce infant mortality in fisheries and increase the mount of sharks that could survive to adulthood and then breed.
However, the problem now is that larger, fertile animals might be being taken out of the population, and the selection pressure to remain small could have serious population-level implications and affect how successful protective measures will be for these sharks (Benavides, 2020).
The populations structures of wild animals, and how they breed and grow, are not simple systems. This can mean that even well-thought out management plans can have unforeseen side-effects.
As scientists have been monitoring shark populations in North Carolina and have learned about this issue, they will now have the opportunity to re-evaluate the protective measures and work to improve them. So hopefully, if a study like this one is repeated in a few years, we can hope to see sharks are bigger and better than ever. We will just have to wait and see.
References
Benavides M (2020). Variability in coastal shark populations across multiple spatiotemporal scales. PhD Thesis, University of North Carolina, USA. Access online.
IUCN SSG (2023). International Union for the Conservation of Nature Shark Specialist Group. Access online.
By Sophie A. Maycock for SharkSpeak.