Puzzling Over Large Aggregations of Sharks, by Don Orth

Ecological principles should guide the wise use and management of fisheries. However, occasionally it appears at first glance that some principles don't apply. My example today is the extreme inverted pyramid.  Animal ecologist, Charles S. Elton,  introduced the pyramid of numbers in 1927 and coined the term “food chain.”  Later we would adopt the term “food webs” (May 1983).  But Eltonian pyramids would remain as characteristics of ecosystems. Pyramids of numbers and biomass were replaced by energy pyramids (Lindeman 1942) where organism biomass is constrained into rigidly delineated trophic levels. Early work by Elton and Lindeman did not include coral reef ecosystems; however, recently investigators have revealed the shapes of pyramids in coral reef ecosystems where shark are apex predators.

Gray Reef Shark Carcharhinus amblyrynchos was one of several sharks studied by Mourier et al. (2016). Photo by Albert Kok  Source

Basically, energy pyramids graphically depict the declining energy as one moves up the trophic levels in a community.  The producers derive and transfer energy from nonliving sources into the biotic community. 

(i) Bottom-heavy pyramids of numbers (N), (ii) bottom-heavy pyramid of biomass (B), and  (iii)   inverted biomass pyramid.  From Tribelco et al. (2013).

Elton observed a strong relation between the trophic level organisms in food chains and their body sizes.   Consequently, we would expect to see a decline in biomass (abundance) with corresponding increase in body mass.  This is called the biomass size spectrum. It makes intuitive sense and complies with the laws of thermodynamics.  Numerous investigators have explored the application of the biomass spectrum to identify constraints on the structure of aquatic communities and as an indicators of perturbation (Jung and Houde 2005; Sprules and Barth 2016).      

Example biomass size spectrum    Source

A recent investigation of a biosphere reserve located in French Polynesia, largely protected from human influences, revealed a unique energy pyramid and size spectrum.  Investigators used a series of video-assisted underwater visual census surveys across the entire shark school to provide precise estimates of shark numbers.  Here the density of apex predator, the Gray Reef Shark, averaged of 600 reef sharks, two to three times the biomass per hectare documented for any other reef shark aggregations!  Imagine 14 to 40 sharks per hectare.  It is unexpected for the largest apex predators to be so abundant. 

 

Gray Reef Shark aggregations in Fakarava Pass, in the Tuamotu Archipelago of French Polynesia.   Source: Mourier et al. (2016).

So, how is it that we observe cases of extreme inverted pyramids?  The extreme reef shark densities do not make ecological sense.  The observation was made in a biosphere reserve where human impact was negligible.  Is this what we expect in pristine reefs?

This large shark aggregation would need 147-350 kg of fish per day --  that is 91 tonnes per year. Yet, the fish production is only 17 tonnes per year.  The math doesn't work here.  The extreme inverted pyramid is a paradox.  It cannot exist, unless there is a subsidy from outside the area.  Either the sharks move out of the area to feed or else fish enter the area from elsewhere and become shark food.   Investigators tagged the sharks and tracked their movements.  Predators typically make foraging excursions to enable them to feed on multiple pyramids.  However, that was not the whole story.

Examples of shark foraging in the pass at night on the Camouflage Grouper Epinephelus polyphekadion (A, B, and C) and the Whitemargin Unicorn fish Naso annulatus (D).  Source:  Mourier et al. (2016)

What Mourier et al. (2016) discovered were large aggregations of groupers (17,000 were counted in one aggregation) that moved into the area during the grouper spawning season.  This migration brought in 31 tonnes of shark food that was produced elsewhere.  Other fish also migrate in large aggregations, thereby transferring fish production from elsewhere.  These spawning aggregations provide the energy subsidies needed to support large aggregations of sharks.  When the spawning aggregations became scarcer, the sharks shifted to making foraging excursions.  For a quick video review of this work, click here. 

The full potential of the size spectrum theory approach linked to energy pyramids has yet to be realized as more studies must be done from a wide range of ecosystems (Tribelco et al. 2013).  It is a data-hungry approach, but few worthwhile scientific investigations are data free.  Our challenge is finding study regions not heavily influenced by the removal of the top predators. 

This story about super abundant shark aggregations and their reliance on spawning aggregations of groupers for energy subsidies is an important discovery for fisheries management.  It illustrates the futility of single species fisheries management.  Sharks cannot be managed via harvest regulations alone.  Even if no sharks were harvested from this population, the population may decline depending on conditions for other fishes outside the biosphere reserve. Conservation of fish spawning aggregations, which are often targets of exploitation (Sadovy and Domeier 2005), can help conserve shark populations, especially if combined with shark fishing bans.  Simpfendorfer and Heupel (2016) emphasized the critical need for managers to protect the areas over which the sharks disperse to feed, which requires a better understanding of the movement patterns to inform management plans.  Fisheries managers cannot draw boundaries in open ecosystems without knowing the actual movement patterns of all elements of the community.    

References

Elton, C. S. 1927. Animal Ecology. The Macmillan Company, New York. 260 pp.

Jung, S., and E.D. Houde. 2005.    Fish biomass size spectra in Chesapeake Bay. Estuaries 28:226-240.

Lindeman, R. L., 1942. The trophodynamic aspect of ecology. Ecology 23: 399–418.

May, R. M. 1983. The structure of food webs. Nature 301: 566–568.

Mourier, J., J. Maynard, V. Parravicini, L. Ballesta, E. Clua, M.L. Domeier, and S. Planes.  2016.  Extreme inverted trophic pyramid of reef sharks supported by spawning groupers.  Current Biology 26(15):2011-2016.  DOI: http://dx.doi.org/10.1016/j.cub.2016.05.058

Sadovy, Y., and M. Domeier. 2005.  Are aggregation-fisheries sustainable? Reef fish fisheries as a case study. Coral Reefs 24:254. doi:10.1007/s00338-005-0474-6

Simpfendorfer, C.A., and M.R. Heupel.  2016.  Ecology: The upside-down world of coral reef predators.  Current Biology 26:R701–R718,

Sprules, W.G., and L.E. Barth. 2016.  Surfing the biomass size spectrum: some remarks on history, theory, and application.  Canadian Journal of Fisheries and Aquatic Sciences 73(4): 477-495, 10.1139/cjfas-2015-0115

Trebilco, R., J.K. Baum, A.K. Salomon, and N.K. Dulvy.   2013.  Ecosystem ecology: size-based constraints on the pyramids of life.  Trends in Ecology & Evolution 28(7):423-431.