The Future of Sharks?: A PVA Experiment

Population Viability Analysis: Scalloped Hammerhead Shark Species

 

 Introduction

Many of the recent bans on shark fins were implemented not too long ago beginning last year in 2011 with major advocacy for the conservation of shark species spreading in parts of Asia that have the highest demand for the delicacy. Because shark conservation and awareness has just started gaining momentum, it is hard to speculate what effect, if any, these bans on shark fins would have on their future survival. Using PVA, I wanted to hypothesize future population trends of the scalloped hammerhead. With their large size and high number of fin ray counts, they fetch a hefty price. Current forensic testing of shark fin species identification have pinpointed that three hammerhead species (scalloped, smooth, and great) constitute a significant 5.9% of the total trade with about 2.7 million hammerheads harvested annually for the international market. Scalloped hammerheads were included in the ICUN's endangered species list in 2008 due to overfishing. According to the WWF, this species is "a relatively slow-growing, late-maturing, long-lived shark with a low population growth rate" making them highly vulnerable to over-exploitation and extinction if current fishing trends continue.

Methods

Microsoft Excel was used to perform these population estimates over a 100 year period. We will start with a population of 18,000 scalloped hammerheads. The growth factor for a given year will either be 0.50 or 1.10 depending on whether a major catastrophe has happened. The frequency of these major collapses in population (due to natural disaster, habitat loss, human influences, disease, etc.) will occur approximately once in 50 years. This will often be low in occurrence as expected in reality but if it happens, the population decreases by half. More commonly, the population will grow at a rate of 1.10. I have set a low growth due to the hammerhead having low fecundity and slow maturation. This is multiplied by the predation rate which is set at a low baseline of 0.85 on the assumption that current fishing trends continue and sharks are continued to be mass harvested without any global intervention. Assuming that advocacy for shark protection influences harvesting, the predation rate will be adjusted accordingly.The population size (y-axis) will be graphed against years (x-axis) and depending on whether the population ever hit zero, the population status will be labeled as extinct or extant.

Equations used:

Growth factor for a given year: =IF(RAND() < 1/$A$1,0.50,1.10)

Population growth: =IF(C5*B5*$A$3>105000,105000,IF(C5*B5*$A$3>$A$2,C5*B5*$A$3,0))

Population status: =IF(MIN(C5:C104)=0, "Extinct","Extant")

Results 

1. Shark population in 100 years if the bans have no effect on fishing rates; Current trends continue at a predation rate of 15% meaning survival = (1-predation) 85%: 

Due to the low survival influenced by heavy predation and nonexistent shark management, the population is unable to be sustained with a low population growth rate. Population size steadily declined and either went extinct in about 60-90 years or remained extant with an extremely low number.  

2. Shark population in 100 years if the bans have a 5% decrease in worldwide fishing; Predation rate decreases to 10% meaning survival = 90%: 

By decreasing fishing by even 5%, the population still declined but not as rapidly as the baseline predation rate. The graph above shows how the population would fare if no natural disasters happen and only population growth rate and predation rate were factors. Though the population didn't become extinct, the low growth rate still does not make the population sustainable against the predation rate. After 100 years, the final population would have decreased to about 7,000 which is less than half of the starting population. 



3. Shark population in 100 years if the bans have a 10% decrease in worldwide fishing; Predation rate decreases to 5% meaning survival = 95%:

Even with the occasional population collapses, the population is able to become sustainable and steadily increase over the 100 year period. Over 20 trial runs, the population can either hit maximum capacity (no collapses, multiply by 0.50), increase above the starting 18,000 individuals, or maintain a steady equilibrium with frequent collapses.

4. Shark Population in 100 years if the bans increase illegal poaching by 5%; Predation rate increases to 20% meaning survival = 80%

With only a 80% survival rate, the population quickly becomes extinct in about 30 years, roughly half the time it took when it was at a baseline 85%.

Discussion

From the PVA experiments, we see that even small increments of 5% in survival rate can have a substantial impact on the fate of a population in a fairly short time span. If we have current shark harvesting trends continue without any globally united effort to limit the illegal poaching, we could see extinction of a population of sharks in the next century. If current or future bans are able to decrease shark harvesting, we can either see a shark populations decline on a smaller scale or have populations increase and maintain a steady equilibrium. Conscious effort and advocacy will need to be taken in order to make sure illegal poaching is controlled and not increase any further else the world may lose certain shark species as early as 30-40 years from now. There is still hope left for sharks populations to become sustainable and survive for many generations if we become more proactive about controlling unregulated shark harvesting as soon as possible.


Sources: 

http://www.redorbit.com/news/science/1259813/hammerhead_shark_makes_endangered_species_list/ 

http://www.worldwildlife.org/what/globalmarkets/wildlifetrade/WWFBinaryitem15546.pdf

http://www.tandfonline.com/doi/abs/10.1577/M08-026.1