Could Eating Smaller Meals Sometimes Benefit Predators More Than Eating Larger Meals?

By Jessica Daly, SRC intern

Sometimes, predators have the ability to choose what size prey to consume when feeding, but little is known about how this decision is made. Several previous research experiments have examined the relationship between prey size and “predator gape size,” or how long it takes to chase, capture, and consume the food. It has been hypothesized that predators may select their prey in part because of possible effects of prey size on digestion and metabolism, but evidence is required to support this.

Metabolism is the rate at which food is broken down into energy. Aerobic scope (AS) is the difference between an animal’s resting and maximum aerobic metabolism, and tells how well the animal can take in oxygen and provide energy to its cells. The specific dynamic action (SDA) measures how much energy is needed to digest and break down food, and is part of the AS. While eating bigger prey will provide more energy to a predator, bigger meals take more energy to metabolize and absorb than smaller meals. If the animal uses a lot of energy breaking down food, that means that it will have less energy to do things like fight or escape from a predator. Because of this, it was hypothesized that it might actually benefit the predator to intentionally choose smaller prey.

A barramundi, the predatory fish which was used as the model organism for this experiment. [Thorne, Nick. 14 July 2006. ]

This idea was tested with the predator fish barramundi, which usually eats large amounts of food at once. Twenty-four juvenile fish were fed different amounts of food, somewhere between 0.6 and 3.4% of their body mass, for two minutes. They were then transferred to individual respirometry chambers for 42 hours. Information gathered from the chambers allowed the scientists to calculate the metabolic rate and SDA for each fish. They were then transferred to a tank for a three-minute “chase” exercise, to simulate a predator attack. After the three minutes, the fish were returned to the respirometers to calculate the aerobic metabolic rate, SDA, and SA after exercise. The growth rates of the fish were also calculated over a period of seven weeks.

Graphs depicting the relationship between growth rate (top) and growth efficiency (bottom) vs. food intake. Each barramundi is a data point. [Norin and Clark.]

The study found that the fish who were fed larger meals had less excess energy, and higher growth rates came at the cost of a decreased AS. This means that after eating a larger meal, the fish spent so much energy metabolizing it that there was less energy left over afterwards. Less energy stores means that a fish can’t swim as fast or as long, and is more likely to be caught and eaten by a predator. The evidence from this experiment suggests that the increased risk of predation outweighs the benefits of the larger size that come with eating large meals. It would likely be beneficial for the barramundi to choose its prey size based on its environment. For example, if the fish were in a relatively safe and isolated area, it might be better to consume more food, whereas in a high-traffic area with high predator abundance, it would be more beneficial to eat less.

Works cited:  

Norin T, Clark TD. 2017. Fish face a trade-off between ‘eating big’ for growth efficiency and ‘eating small’ to retain aerobic capacity. Biol. Lett. 13: 20170298.

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.

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.

Sneaky Predators

By Arina Favilla, SRC intern

“Everything you see exists together in a delicate balance, ” Mufasa wisely tells Simba in The Lion King right before a pouncing lesson. This is true of any ecosystem on the planet—the sun provides energy for plants to grow, plants are grazed on by herbivores, who are eaten by consumers, who are prey to other predators. Any prey-predator imbalance can have cascading effects on the entire ecosystem, particularly when invasive predators are especially sneaky predators, beating Simba in the element of surprise.

The element of surprise is difficult to accomplish in the aquatic environment because there are several cues (smell, sight, vibrations) that warn prey of a nearby predator and illicit a fast-start response, allowing them to get as far away as quickly as possible. It is debated whether this fast-start response is an autonomic response, similar to a knee-jerk reflex, or whether an individual can optimize their escape response in accordance to the threat.

Image of the red lionfish (Pterois volitans) displaying its characteristic fins and venomous spines. (From Wikimedia Commons)

Image of the red lionfish (Pterois volitans) displaying its characteristic fins and venomous spines. (From Wikimedia Commons)

McCormick and Allan (2016) investigated the red lionfish’s (Pterois volitans) success as a predator by determining the response of prey. The red lionfish, native to the Pacific Ocean, is a threatening invasive species in the Caribbean because of their success as predators easily devouring 8-10% of their body weight each day. They quickly decimate reef fish populations and destroy the delicate balance of a reef ecosystem. Moreover, recent research suggests lionfish are successful, sneaky predators by avoiding associative learning, a survival mechanism that allows prey to associate cues with dangerous predators leading to effective fast-start responses and successful escapes.

The study compared the response of whitetail damselfish to two predators, the red lionfish and the common rockcod, as well as a non-predator fish, the three-lined butterflyfish. First, the damselfish were conditioned to associate potential risk with the sight and odor of the two predator species coupled with chemical alarm cues. Previous studies have shown tropical fish species, including damselfish, can quickly learn to associate cues of a predator as a threat. Damselfish were then exposed to olfactory cues (seawater from the predator or non-predator tank) and/or visual cues (predator or non-predator tank placed adjacent to the damselfish tank) before being startled by a stimulus (release of a metal weight at the water’s surface) to provoke the fast-start response.

Comparison of the different aspects of the damselfish’s fast-start response when forewarned through chemical (white), visual (light grey), or a combination of cues (dark grey) of either one of two predators (red lionfish or rockcod), a non-predator (butterflyfish), or controls. The optimal fast-start response would have a short response latency time, high average response speed and maximum speed, and large distance travelled. Damselfish exposed to controls had the lowest response while those exposed to the rockcod had the highest response. Both the butterflyfish and lionfish elicited similar intermediate responses. (McCormick and Allan 2016)

Comparison of the different aspects of the damselfish’s fast-start response when forewarned through chemical (white), visual (light grey), or a combination of cues (dark grey) of either one of two predators (red lionfish or rockcod), a non-predator (butterflyfish), or controls. The optimal fast-start response would have a short response latency time, high average response speed and maximum speed, and large distance travelled. Damselfish exposed to controls had the lowest response while those exposed to the rockcod had the highest response. Both the butterflyfish and lionfish elicited similar intermediate responses. (McCormick and Allan 2016)

McCormick and Allan (2016) found that the damselfish had greater fast-start responses when forewarned about the predatory rockcod through olfactory or visual cues, but showed similar ineffective fast-start responses—slow to react and slower speeds—when exposed to the cues for the lionfish as well as the non-predator butterflyfish and controls (Figure 2). In other words, the damselfish misidentify the lionfish as a non-predator, reducing its chance of escape if attacked. These results suggest that lionfish are capable of circumventing associative learning, leading to higher success rates in attacking prey. The findings of this study begin to explain the success of lionfish as predators, but further studies are required to better understand the mechanisms lionfish use to avoid forewarning of prey.

Works cited

McCormick, M. I. and B. J. M. Allan. 2016. Lionfish misidentification circumvents an optimized escape response by prey. Conservation Physiology 4:1–9.

Early Life History Predator-Prey Interactions and Habitat Use of the American Eel and American Conger

By Alison Enchelmaier, RJD Graduate Student

Declines in the American eel, Anguilla rostrata, have raised interest in studying the species’ early life history. Potential causes could include overfishing, increased predation, and habitat loss; but determining the cause is difficult due to the American eel’s complex life history. One potential factor, predation, is important to consider as the refuge value of estuarine nursery habitats are being reevaluated (Musumeci et al., 2014).Another factor is habitat competition. American eels arrive in North Atlantic estuaries in their early juvenile stage called glass eels, from the winter to spring. This overlaps with another species called Conger oceanicus. Commonly called the American conger, this species arrives in estuaries in the spring to summer. Considering both species have similar life histories, overlapping arrival to estuaries, and the American conger is a piscavore it is possible that both species compete for habitat space and C. oceanicus may prey on A. rostrata (Musumeci et al., 2014).

Figure 1 (1)

Early juvenile stage of eel growth called glass eels. Photo credit:

Musumeci et al. (2014) examined the habitat use of both American congers and American eels by placing them in bowls containing sand and PVC pipe shelter. Each eel was observed to see how often they buried themselves in the sand, rested on top of the sand, or sheltered in the PVC pipe. American eels spent nearly even time sheltered (39%), buried (30%), and on top of the sand (31%). Alternatively, American congers divided their time lying on top of the sand (52%) and sheltering (48%). No congers buried themselves in the sand (Musumeci et al., 2014). From these observations it appears that there is some overlap in both species habitat use, though only American eels bury themselves.

To examine predator-prey relationships, Musumeci et al. (2014) placed two eels of varying sizes together for 24 hours.  Predator-prey pairs were one American conger and one American eel, two American congers, or two American eels. American congers successfully preyed on American eels in 45% of the trials. American congers did not seem to have a size preference, as they preyed on American eels of similar and smaller sizes.  American eels did not prey on American congers in any of the trials. When two congers were paired together cannibalism occurred in 16% of the trials, usually when the eels were different sizes. Only 1% (one instance) of American eel trials resulted in cannibalism.

Figure 2 (1)

Predator-prey interaction between an American conger (larger, predator) and American eel (smaller, prey). Photo credit: Musumeci et al., 2014)

Interactions between American congers and American eels are likely as they have been collected together for several decades. Both species arrival to estuaries and habitat preferences overlap, which means that they may compete for habitat space and demonstrate predator-prey interactions (Musumeci et al., 2014). Based on predation trials, it appears that American eels are potential prey for American congers, which has not yet been observed in nature. American conger cannibalism was frequent enough to indicate that it may occur in nature, though it has not yet been observed in field. The lack of American eel cannibalism contradicts culture system observations of cannibalism between similarly sized juveniles (Musumeci et al., 2014).American conger predation on American eels may affect the American eel’s survival and habitat use. This new information shows that predation and habitat space competition could play a part in the American eel’s decline.


Reference: Musumeci, V.L., Able, K.W., Sullivan, M.C., & Smith, J.M. (2014). Estuarine predator—prey interactions in the early life history of two eels (Anguilla rostrata and Conger oceanicus). Environmental Biology of Fishes. First published online: 1 November 2013.


Predator identity and its indirect effects on fishing

By Laura Louon,
Marine conservation student

Few would be surprised by the fact that fishing causes a reduction in the population of the targeted fish. That is a direct effect of fishing. But nothing in the ocean happens in a vacuum; if you decrease the number of individuals of one species, you are bound to see an effect on at least one other species, if not the entirety of the ecological community. When developing holistic management and conservation plans, it is therefore imperative that managers also consider the indirect effects of decreasing the population of a species in an ecosystem as to make the correct decisions. But how do you measure, and hence predict, these indirect effects?

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