Honey, I Shrunk the Sharks!

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, "size-selective harvesting" has caused the average size of the several commercial fish species to decrease over time. This occurs because either a) the fishing gear used is designed in such a way that individuals of a certain size are commonly caught or b) because minimum size limits have been imposed on fisheries as a protective measure. For example, if fisheries managers attempt to boost population growth by setting a minimum size of fish which can be legally landed, over time the average size of the fish in the population will decrease, as the larger individuals are more commonly caught. But does the same process happen to sharks? And what affect might this have?

The sandbar shark (Image source: www.churaumi.okinawa/en/fishbook)

A recent study sought to answer this question; looking at the size of sharks that were landed in fisheries in Onslow Bay, North Carolina since the 1970s (Benavides, 2020). The scientists assessed 12 commonly caught species:

The dusky shark (Image source: www.fishesofaustralia.net.au)

• the sandbar shark (Carcharhinus plumbeus),

• blacknose shark (Carcharhinus acronotus),

• spinner shark (Carcharhinus brevipinna),

• silky shark (Carcharhinus falciformis),

• finetooth shark (Carcharhinus isodon),

The blacktip shark (Image source: www.wikipedia.org)

• bull shark (Carcharhinus leucas),

• blacktip shark (Carcharhinus limbatus),

• dusky shark (Carcharhinus obscurus),

• tiger shark (Galeocerdo cuvier),

• smooth dogfish (Mustelus canis),

• Atlantic sharpnose shark (Rhizoprionodon terraenovae) &

• the scalloped hammerhead (Sphyrna lewini).

Image Credit: Diane Rome Peebles, Source: www.fisheries.noaa.gov)

They analysed how the average "fork length" (FL) had changed over time. The FL is a measure sometimes used to describe the size of a shark and is measured from the tip of the snout to the middle of the caudal fins.

This research reported that, over the time-frame, the average size of sharks caught had declined significantly. These declines were most pronounced in the blacknose shark, dusky shark, smooth dogfish and the Atlantic sharpnose, and the largest declines were found in the sandbar shark; with average FL reducing by 21cm over the period (Benavides, 2020). Their models suggested that, in the worst-case scenario, relative declines ranged between 15 - 63%, with 5 species showing a decline of 45% or higher. This means that in this area, these species have radically shrunk in size, compared to their potential maximum size (Benavides, 2020).

Decreasing Fork length (FL- a measure of shark size) in several shark species over time (Benavides, 2020)

This does not mean that fisheries are magically shrinking sharks, but that the size-selective fishing gear has extracted the larger individuals and reduced the average size of the population. They stated it was likely that the implementation of minimum size requirements in recreational and commercial fisheries in the area has been the main driver for this pattern. They also stated that the same trend probably exists in other species in the Onslow Bay region which were did not assessed in this study (Benavides, 2020).

In other fish species, size-selective harvesting is thought to create a "selection pressure"; this means that over time a species adapts to new pressures in the environment. In this case, it 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. All of these changes could have serious population-level implications and affect how successful protective measures will be for these sharks (Benavides, 2020).

The scalloped hammerhead (Image source: www.oceanographicmagazine.com)

However, the researchers also stated that some previously implemented fisheries- management strategies, may be slowly having a positive effect. They reported that over the past 5 years, the average size of the spinner shark,

dusky shark, tiger shark and scalloped hammerhead were showing very early signs of increasing after their decade-long declines. They suggested that this pattern was likely due to tighter fisheries management measures over the past 20 years (Benavides, 2020).

The nature of conservation, is that we will not see improvement and be able to critically analyse management plans, for many years after protective measures have been implemented. This study shows us that continuous monitoring of shark populations will be necessary in order to ascertain how management strategies are working, but that these protective measures CAN be effective. If we implement management strategies to protect even certain portions of the population of threatened shark species, it is very possible that a few years down the line, we will see benefits... if we are patient.

References

Benavides M (2020). Variability in coastal shark populations across multiple spatiotemporal scales. PhD Thesis, University of North Carolina, USA. Access online.

By Sophie A. Maycock for SharkSpeak.