An Examination of Intraguild Predation Events Between Sharks and Pinnipeds or Cetaceans, and Their Importance

By Brenna Bales, SRC intern

Popular opinion conjectures that sharks are always the dominant predator in their specific environments. The famous, terrorizing shot of the great white shark leaping out of the water with the unsuspecting seal in its jaws is iconic to Discovery Channel’s “Shark Week” highlight reel every year, boosting this notion of shark dominance. But what many don’t realize is that the seal is more capable than it may seem in those dramatized, slow-motion clips; In some cases, the role may be reversed.

Intraguild predation (IGP) is the killing or consuming of species that are also potential predators, a combination of both competitive and parasitic/predatory interactions (Polis et. al., 1989). What is so special about this type of relationship between two species? Fundamentally, the act of killing a predator and using it as a source of energy impacts more than just the two species directly involved. Instead, it affects other populations indirectly, and can stabilize or destabilize an environment in different ways. For example, population declines based on age differences can occur, as a species A adult will predate over a species B adult; However, a species A juvenile is preyed upon by a species B adult (Polis et. al., 1989; Figure 1). If there are less species A juveniles due to predation, then there will subsequently be less species A adults, thus reducing threats on species B at all ages. These changes trickle down the trophic levels, affecting species A and B prey items and their respective food sources, as changes in apex predator populations have cascading trophic effects (Myers et. al., 2007).

Figure 1: Three types of intraguild predation: (a) simple, (b) reciprocal, (c) reciprocal age-dependent.
https://openi.nlm.nih.gov/imgs/512/178/2853694/PMC2853694_442_2010_1575_Fig1_HTML.png

Overall, IGP can be age-, density-, and resource-dependent, disturbing established trophic and population dynamics such as above. There are two IGP descriptors, each with two categories: symmetry (asymmetrical/symmetrical) and age structure (important/relatively unimportant) (Polis et. al., 1989). We will first examine an asymmetrical IGP event. Observations by Chris Fallows in waters off Cape Point, South Africa led to the discovery of the predation of Cape fur seals (Arctocephalus pusillus pusillus) on blue sharks (Prionace glauca). This is the first evidence of asymmetric IGP on a mid-sized predatory shark by a pinniped (Fallows et. al., 2015). Therefore, these observations were exciting. The seal only consumed the viscera (main internal organs such as intestines, stomach, liver, etc.) after chasing and tossing the shark for several minutes (Figure 2). It proceeded to perform similar actions and kill 5 out of 10 sharks in the vicinity. This behavior is significant because in the past, the opposite interactions have been observed, in which adult blue sharks chased juvenile and adult Cape fur seals (Stewardson 1999). This behavior is similar to the situation between species A and B explained above; Although, in the end, not enough evidence is presented to strictly attribute age-dependency to the relationship between blue sharks and Cape fur seals.

Figure 2: A Cape fur seal feeds on a blue shark. Taken from: Fallows, C., Benoît, H.P. and Hammerschlag, N., 2015. Intraguild predation and partial consumption of blue sharks Prionace glauca by Cape fur seals Arctocephalus pusillus pusillus. African Journal of Marine Science, 37(1), pp.125-128.

Another first for shark-pinniped IGP observations was an incident off the coast of New South Wales, Australia. An Australian fur seal (Arctocephalus pusillus doriferus) was observed feeding on an approximately 1.4-meter wobbegong shark (Orectolobus ornatus) (Allen and Huveneers, 2005). It was inferred that the incident was predatory due to the fact that the shark’s bodily condition would have appeared different had it been opportunistically ripped off a hook from a long-line, and rejection by a fisherman was unlikely. In addition, wobbegong sharks have never been found in the diet of any other predatory animal, although elasmobranchs such as the puffadder shyshark (Haploblepharus edwardsii) and the spiny dogfish (Squalus acanthias) have been (Allen and Huveneers, 2005). This supports the need to examine changing predatory roles in marine environments, as more than one threatened species at the top of the food chain can have serious conservation implications, and the removal of one species may have consequences not previously considered.
Switching from pinnipeds to cetaceans, recently popularized killer whale (Orcinus orca) and great white shark (Carcharadon carcharias) interactions have flipped the public’s vision of the great white as the ocean’s most fearsome predator. However, these interactions have been observed and officially recorded since the 1990’s around the Farallon Islands by researchers such as Peter Pyle and Scott Anderson. Both species consume pinniped prey, and the killer whales will occasionally consume the white sharks, clearly an example of IGP (Pyle et. al., 1999), and one that contradicts long-held beliefs.

There are two cases in which the same type of IGP interactions have different outcomes: when prey is abundant, and when it is not. If resources are abundant, competition will be low, and species A and B will, for the most part, coexist. However, when the situation is reversed and prey resources are limited, species A may begin to prey upon species B, thus becoming an IGP situation. The major sharks that are thought to prey on cetaceans are the white shark (Carcharadon carcharias), tiger shark (Galeocerdo cuvier), bull shark (Carcharhinus leucas), sixgill shark (Hexanchus griseus), sevengill shark (Notorynchus cepedianus), dusky shark (Carcharhinus obscurus), oceanic whitetip shark (Carcharhinus longimanus), shortfin mako shark (Isurus oxyrinchus), pacific sleeper shark (Somniosus microcephalus), greenland shark (Somniosus microcephalus), cookie-cutter shark (Isistius brasiliensis), and portugese dogfish (Centroscymnus coelolphis) (Heithaus 2001). However, reciprocal predation/consumption by cetaceans on these species is rare to non-existent, although aggression is often observed from both cetaceans and pinnipeds (Heithaus 2001; Stewardson and Brett, 2000; Kirkwood and Dickie, 2005).

In conclusion, unexpected intraguild predation between different species is dependent on a variety of environmental, opportunistic, and resource related variables, and expands beyond the marine environment (Polis et. al., 1989). As declines of elasmobranch, pinniped, and cetacean populations continue, lower trophic levels may or may not even be affected when other species subsequently become dominant predators, with only the upper trophic levels being affected. Nonetheless, these are important considerations to factor into an ecological assessment or examination, as climate and anthropogenic stressors mount on wild populations.

Literature cited:

Allen, S. and Huveneers, C. (2005). First record of an Australian fur seal (Arctocephalus pusillus doriferus) feeding on a wobbegong shark (Orectolobus ornatus). Proceedings of the Linnean Society of New South Wales 126, 95-97.

Bowen WD, Iverson SJ. 2013x. Methods of estimating marine mammal diets: a review of validation experiments and sources of bias and uncertainty. Marine Mammal Science 29: 719–754.

Fallows, C., Benoît, H.P. and Hammerschlag, N., 2015. Intraguild predation and partial consumption of blue sharks Prionace glauca by Cape fur seals Arctocephalus pusillus pusillus. African Journal of Marine Science, 37(1), pp.125-128.

Heithaus, M.R., 2001. Predator–prey and competitive interactions between sharks (order Selachii) and dolphins (suborder Odontoceti): a review. Journal of Zoology, 253(1), pp.53-68.

Kirkwood, R. and Dickie, J., 2005. Mobbing of a great white shark (Carcharodon carcharias) by adult male Australian fur seals (Arctocephalus pusillus doriferus). Marine mammal science, 21(2), pp.336-339.

Myers, R.A., Baum, J.K., Shepherd, T.D., Powers, S.P. and Peterson, C.H., 2007. Cascading effects of the loss of apex predatory sharks from a coastal ocean. Science, 315(5820), pp.1846-1850.
Polis, G.A., Myers, C.A. and Holt, R.D., 1989. The ecology and evolution of intraguild predation: potential competitors that eat each other. Annual review of ecology and systematics, 20(1), pp.297-330.

Pyle, P., Schramm, M. J., Keiper, C. & Anderson, S. D. (1999). Predation on a white shark (Carcharodon carcharias) by a killer whale (Orcinus orca) and a possible case of competitive displacement. Mar. Mamm. Sci. 15: 563±568.

Stewardson CL. 1999. Preliminary investigations of shark predation on Cape fur seals Arctocephalus pusillus pusillus from the Eastern Cape coast of South Africa. Transactions of the Royal Society of South Africa 54: 191–203.

Stewardson CL, Brett M. 2000. Aggressive behaviour of an adult male Cape fur seal (Arctocephalus pusillus pusillus) towards a great white shark (Carcharodon carcharias). African Zoology 35: 147–150.

Why have global shark and ray landings declined: improved management or overfishing?

By Patrick Goebel, SRC Intern

A decline in shark and ray landings could be thought of as a success for in improved management strategies. However, in the case of Davidson et al (2015), that is too good to be true. Sadly, the decline in global shark and ray landings has been attributed to overfishing and other ecosystem influencers.

Sharks and rays are commercially valuable for their fins, meat, liver, oil and skin with their fins and meat in the highest demand. The demand for shark products is relatively new, as their commercial value has only increased with the decline of other valuable fisheries. The increase in fishing pressure combined with the lack of laws regulating the shark and ray fishery has resulted in population declines.

The rapid decline in shark and ray populations resulted in new management strategies. Davidson et al (2015), investigated these new management strategies to determine if declines in shark and ray catches were a result of the fisheries management performance or over.

fishing. Figure 1. Global distribution of (a) country-specific shark and ray landings averaged between 2003 and 2011 and mapped as a percent of the total. (b) the difference between the averages of landings reported in 2001-2003 and 2009-2011

fishing.
Figure 1. Global distribution of (a) country-specific shark and ray landings averaged between 2003 and 2011 and mapped as a percent of the total. (b) the difference between the averages of landings reported in 2001-2003 and 2009-2011

Shark, ray, skate, and chimaera landings from 1950 (earliest years of reporting) to 2013 were investigated. In total, 126 countries shark and ray landings were modeled against indirect and direct fishing measures and fisheries management performance.

The peak of shark and ray landings was 2003 and has declined by about 20% in the past decade. As stated in Davidson et al (2015), the reduction in shark and ray landings are related to indirect and direct measures of fishing pressure rather than management implementation. This shows that sharks and rays are being harvested at an unsustainable rate. Furthermore, Davidson et al (2015), highlighted several countries that deserve prioritization for conservation and management action. The greatest declines were reported in Pakistan and Sri Lanka, both of which have little to no management. If new management strategies are not implemented into these countries, sharks and rays will continue to be harvest at damaging rate.

Davidson, Lindsay NK, Meg A. Krawchuk, and Nicholas K. Dulvy. “Why have global shark and ray landings declined: improved management or overfishing?.” Fish and Fisheries (2015).

Shark Conservation in the Galapagos Islands

By Daniela Escontrela, RJD Intern

The Galapagos Islands are a popular tourist destination for many people around the world. The pristine environment combined with the vast amounts of life and different species make many people come to the Galapagos every year. However, one of the most important species people come to watch are the scalloped hammerheads and the whale sharks, along with other sharks.

Living on the islands for over two months I can say I have seen most of the iconic animals that the Galapagos are known for. However, one of the best experiences I’ve had is snorkeling and diving with the sharks here. On a snorkel trip to Punta Morena, we were riding back to Puerto Villamil after a long day of snorkeling. All of a sudden, our boat slowed down and veered off to the right. It took me a second to realize why we were going in that direction, and then I saw it. A huge dorsal and caudal fin were sticking out of the water. Being marine biologists, everyone’s first instinct was to jump in the water with this massive creature. However, our boat captain and first mate were wary about this. They kept saying it was a “Tiburon gato” which we gathered was a tiger shark. However, we were positive it was a whale shark as the dorsal fin had the characteristic white spots of a whale shark. After much debate about what it was, the boat captain rode over closer to the shark and in that moment the shark swam right under our boat, and that’s when we were one hundred percent sure that it was a whale shark. The boat captain and first mate were still very skeptical, but regardless we all quickly suited up and jumped in the water to swim with this huge, majestic creature. We swam with it for at least ten minutes as it circled the boat several times. My favorite part was its gorgeous white spots that were running down its body. But as quickly as this adventure began, it then ended as the whale shark dove deep into the abyss. It was interesting, the men working on this boat run snorkel trips almost on a daily basis and have definitely seen a whale shark before, but had never jumped into the water with one.

Figure 1 Shark Paper

An image of me swimming along the whale shark spotted on our way back from a snorkeling trip to Punta Morena. Photo credits to Emily Rose Nelson.

Another incredible experience with sharks was when we were diving in San Cristobal at Kicker Rock (Leon Dormido). The dive began with more sharks than I had ever seen in one dive, small blacktips and Galapagos sharks casually swam by, eyeing us with caution. Towards the end of the dive I was filming two Galapagos sharks that were directly below me when I heard a clinking noise. I looked up, expecting to see another small Galapagos or blactip shark, but what I saw was something I was not expecting. A school of at least ten scalloped hammerheads swimming towards me, one of them coming within five feet of me. They were magnificent, something I had wanted to see my whole life. There slender bodies, big dorsal fins and beautiful cephalofoil were enthralling. They surrounded us for a couple of minutes, some swimming under me, some above me. I didn’t know where to look, they were too beautiful. But they soon swam away and I was out of air from all the excitement.

Figure 2 Shark Paper

An image taken of one of the scalloped hammerheads spotted on a dive at San Cristobal as it swam away from us. Photo taken by me.

I have always been a strong advocate of shark conservation. Back in the states, I am part of the RJ Dunlap program which focuses on shark research and conservation. However, this trip gave me an insight into the shark finning problem that I had never had before. Besides being able to see these majestic creatures underwater, I have also lived with a host family the past two months where my host dad used to be a shark fisherman here in the Galapagos. It was interesting to hear his story, he had been a pepino diver, and when the pepino fishery was banned, he switched to shark finning. He says he didn’t have an alternative, he had to feed his family and the shark fishing brought in good money for him and his family to survive. Eventually, shark fishing was also banned in the Galapagos and he had to find another job. Luckily, he was able to find a job with the government. However, he tells me he would like shark fishing to continue. This time not because of the money, but because a lot of the locals are terrified of the sharks. He himself is scared of sharks, every time I tell him about my day’s stories involving sharks, he cringes and tells me to be careful.

It’s interesting, on one hand I love sharks and would love for the massive killing of sharks to end completely. However, then there’s the people that rely on these jobs to support their family; this is something I had never fully considered before. You can’t ask someone to stop their job when it’s the only source of income they have and the only way they can get through life. It made me realize that banning shark finning all together is unrealistic. However, other things can be done to reduce the number of sharks that are killed every year. We need to set up a comprehensive education program to teach locals of these small fishing communities about sharks. From talking to many locals and hearing their stories, I have come to realize that a lot of them don’t have much education when it comes to things about the natural world, especially sharks. People need to be educated about sharks, the threats they face and how catastrophic it could be to lose sharks. They need to learn that taking out sharks from the environment could cause environmental impacts, such as throwing the food web off balance. In addition, removing sharks from the environment could cause ecotourism to cease, causing them to lose a crucial part of their income.

In addition, we can’t just ask shark fishermen to stop fishing without giving them alternatives, especially when this is their only source of income. When the park banned shark finning and pepino fishing, they started to set up a lot of road blocks so the fishermen couldn’t use their boats for tourism and ferrying people between islands. The park needs to help those fishermen displaced by the ban to find new jobs, this way it will be less likely that the fishermen will go back to illegally fishing for sharks.

There also needs to be more enforcement against illegal fishing. The park currently only has one patrol boat for the island of Isabela. It’s not only enforcement to prevent local fishermen from fishing sharks. They also need to have more stringent penalties for foreign vessels that come into the Galapagos Marine Reserve to fish for sharks.

A combination of education, alternative jobs and enforcement could help the Galapagos in their conservation effort for sharks. It is imperative that these issues be faced because if the Galapagos Islands were to lose sharks, the rest of the marine reserve could be severely affected. One of the major things that attracted me to the Galapagos was the idea that I would be able to see iconic species like the whale shark and the scalloped hammerhead. Luckily, I had that experience along with the chance to meet people that once depended on these species; future generations should get the chance I had of seeing all these incredible animals and sights.

Comprehensive Review of IUCN Shark and Ray Extinction Risk: Factors increasing risk, under-management of fisheries, and shortcomings in current conservation activities

By Kyra Hartog, RJD Intern

The natural world is changing rapidly in the face of land and coastal development, climate change, fisheries, and other human impacts. With these changes come conservation concerns for the various species that inhabit these areas impacted by human activities. The International Union for Conservation of Nature and Natural Resources (IUCN) Red List is a valuable conservation tool that allows scientists to determine the conservation status of various plant and animal species around the world. In their recent paper, Dulvy et. al provide a comprehensive assessment of all species of sharks and rays (chondrichthyans) under the IUCN Red List criteria, which provides insight into the rapidly changing biodiversity of the world’s oceans.

The IUCN Red List classifies species in categories ranging from “Least Concern” to “Extinct” based on certain criteria such as reduction in population size, change in geographic range, and number of reproductive individuals in a population, among other measures. Dulvy and his colleagues applied these criteria to 1,041 species of sharks and rays and evaluated each species’ status based on these criteria. They found that over a quarter of the species of sharks and rays could be classified as threated under IUCN criteria, mostly due to overfishing and habitat degradation. The found that large-bodied, shallow-water species had the highest extinction risk of the sharks and rays and that the overall risk for chondrichthyans was higher than that of other vertebrates. Though shark and ray populations have changed significantly due to overfishing and habitat destruction, it is unclear whether these changes are reversible or if they signal a larger problem regarding overall marine species extinction risk.

hartog 1

Figure from Dulvy et. al representing A) the increased reported catch of shark and ray species over time, B) the increased contribution of rays to the global reported chondrichthyan catch, and C) Shark and ray fishing nations based on % of contribution to global reported catch, number of threatened species in each area, and % of contribution to the global fin trade based in Hong Kong

Sharks and their relatives exhibit some of latest maturing and slowest reproducing species of any taxonomic group. Populations of chondricthyan fishes also exhibit extreme life history characteristics such as low population growth rates, weak density-dependent juvenile survival, and increased sensitivity to fishing mortality. Though sharks and rays are often caught as bycatch of fisheries targeting some other species, they are often kept due to the increasing value of their fins and demand for meat, liver oil, and gill rakers (from Manta and other devil rays). These fishing pressures, combined with effects of habitat degradation, make for a potentially disastrous future for chondrichthyan species. Commercial and residential coastal development, mangrove destruction, river engineering, and pollution are the main processes causing freshwater, estuarine, and marine habitat degradation. These human activities alone threaten one third of already threatened shark and ray species.

The most acute effects of these activities are seen in those species that require freshwater and those that can live comfortably in both fresh and salt water: one third of the 90 species in this category are affected severely by habitat degradation. Their risk is exacerbated by the specific nature of their habitats and their small geographic ranges. Mangrove destruction, in particular, has increasingly become an issue in Southeast Asia where mangrove forests are being clear-cut for shrimp farming operations. Human perception of sharks as dangerous has led to increased use of shark control nets at beaches and direct persecution due to shark attacks and supposed damage to aquaculture and other fishery operations. Only one species, the New Caledonia catshark, has been directly threatened by climate change but many others have been recognized as climate sensitive. Climate change hotspots like the Mediterranean Sea should also be monitored for changes in species extinction risk.

Shark and ray species are most threatened when they are large-bodied, coastal-dwelling, exposed to fisheries, and within a narrow depth range. This combination of factors is exemplified in the Sawfish family (Pristidae) and has led to their status as the most threatened chondrichthyan family, and possibly the most threatened family of all marine fishes. Other highly threatened groups include shelf-dwelling rays, angel sharks, and thresher sharks. The least threatened groups are those that are small bodied and somewhat out of reach of fishery operations such as catsharks, chimaeras, and soft-nose skates. Conservation of specific areas is prioritized based on the number of threatened species in that area, the level of expected threat for those species, and the number of threatened endemic species (found only in that area and therefore irreplaceable). Hotspots that fall under these criteria include the Indo-Pacific Biodiversity Triangle and the Red Sea. These areas, among some 15 other conservation hotspots, represent a combination of high threat, low safety, and high uncertainty in extinction risk among the chondrichthyan species that live there.

hartog 2

Level of “irreplaceability” among chondrichthyans in global conservation hotspots. Score is based on the number of small-range (endemic) species found within each area.

While there have not been any known global extinctions, 28 populations of sawfishes, skates, and angel sharks have been driven to regional or local extinction. Sharks and rays have the highest number of species classified as “Data Deficient” by the IUCN among all evaluated taxa. Fourteen percent of these species are likely to be threatened based on their life histories and distribution. Dulvy et. al made a novel observation that minimum depth limit and narrowness of depth range may be more important in determining extinction risk than geographic range, possibly due to the wide-reaching nature of fisheries today. No species can be out of reach of the current global fishing fleets but some may be able to escape capture by inhabiting deeper oceanic zones. These global fisheries represent an issue for international management agencies as it is difficult to monitor so many operations. Listing under conventions such as CITES (Convention of International Trade of Endangered Species) and effective implementation of these listings is key to reducing extinction risk of sharks and rays, globally. Repeated Red List status assessments, proper catch reporting, and sufficient management plans must also be employed on regional level in order to see significant changes in chondrichthyan conservation status. This study provides a comprehensive exposure of the under-management of sharks and rays as well as the shortcomings of various management groups in protecting these species from further exploitation. These findings will be invaluable to the future of effective and meaningful shark and ray conservation around the world.

 

References:
Dulvy, N. K., Fowler, S. L., Musick, J. a., Cavanagh, R. D., Kyne, P. M., Harrison, L. R., … White, W. T. (2014). Extinction risk and conservation of the world’s sharks and rays. eLife, 3(e00590). doi:10.7554/eLife.00590

IUCN (2013). The IUCN Red List of Threatened Species. Version 2013.2. <http://www.iucnredlist.org>. Downloaded on 2 March 2014.

Shark Tagging with the University Of Miami President’s Council – February 22, 2014

By Jacob Jerome, RJD Graduate Student and Intern

It’s never hard to get up early knowing that I’m going out on the water with the amazing RJD crew, and Saturday morning was no exception.  Driving over to Key Biscayne I was excited to board the Diver’s Paradise and get the gear loaded for the trip so we could get out on the water. After loading the gear and checking out the new floating platform, we headed over to the Seaquarium to meet Dr. Neil and pick up our guests for the day, the UM President’s Council.

After meeting the diverse and fun group of members of the President’s Council and their guest, Dr. Neil gave the group a rundown of some of the projects that the RJD team is working on along with an overview of the gear that we would be using for the day. Everyone seemed very excited to have the opportunity to participate in our research! Shortly after the briefing, we left the dock and enjoyed the beautiful ride over to our sampling site for the day in Biscayne Bay.

01_Pose with bait (3)

Members of the President’s Council take a minute to pose with the bait.

Once at the site, we set out our first ten drumlines with the help of our very enthusiastic group. During our hour soak time, Dr. Neil showed the group, with the help of our mascot Sharkie, what data we would be collecting if we were lucky enough to catch any sharks. You could see that everyone was getting more excited about the possibilities of the day. Captain Eric got us in position to check our first drumline and to our surprise we had a shark!

02_neil brief (1)

Dr. Neil briefs the group about the workup that will take place for each shark.

As the drumline was pulled in, Captain Eric informed us that we had a hammerhead on the line. This news changed everything. We worked to get the shark to the platform as quickly as possible because we have learned from our research that hammerheads are very sensitive to capture. Unfortunately, the shark got off the line just as we were getting it to the platform. Though we weren’t able to collect any data from this amazing animal, everyone on the boat got to see this rare species up close and watch it swim gracefully away.

As we moved on to our next drumline, we were again pleasantly surprised to find a shark on the line! This time we were able to successfully get a big nurse shark secured on the platform for a quick workup and then release it back into the water.

03_guest line (1)

One of the guests helps bring in the line.

With spirits high, we pulled in the remainder of the lines from the first set. Another shark managed to escape us as we were pulling it in, this time a large nurse shark. After a delicious lunch provided by SALT Restaurant, we began pulling in the second round of drumlines with the help of our guests. We discovered two more feisty nurse sharks during this set and were able to successfully collect measurements, fin clips, and blood samples for our ongoing research. On our third and final set of lines, we were getting skunked by the sharks and began thinking that we were done seeing them for the day. But on our very last line, we were able to pull in another big nurse shark!

04_guest tag (1)

A guest prepares to tag a nurse shark.

Our adventure was a great success — we were able to land 4 nurse sharks all over 7 feet long! To make the day even more successful, one of the nurse sharks was a recapture, meaning it already had an RJD tag in it. Because of this, the data we were able to collect was even more meaningful due to the additional information a recapture provides for our research. Research data obtained, a great  group of enthusiastic guests,  a gorgeous day, priceless! As always, I can’t wait for my next shark tagging trip with RJD.