Seeing Over the Horizon with Stable Isotope Analysis

What an animal eat is key in shaping its interactions with its environment and other species. Like many fundamental ecological questions, it can be unexpectedly difficult to answer. Obviously, this question is particularly difficult for cryptic or wide-ranging animals, which are difficult to observe directly.

This is very true for marine predators such as dolphins and whales, which can travel large distances, spend most of their lives underwater and often occur in environments which are difficult and expensive to get to (not to mention the risk of getting your field notebook wet).

Killer whales, the largest of the dolphin species – are apex predators in ocean ecosystems. Their recorded prey includes more than 140 species, ranging in size from fishes weighing a few grams to baleen whales weighing tens of tons.

Marion Island, an isolated South African island in the vast Southern Ocean, is frequented by a small population of killer whales [1]. My colleagues (at the University of Pretoria’s Mammal Research Institute) and I have been studying this population for over a decade now and we know, from observing them directly, that they eat seals and penguins which breed at the island [2]. But we are very interested in what they might eat when we can’t observe them, especially when they travel hundreds of kilometers away from the island [3]. This is important because their diet determines what role they play in this particular ecosystem.

We turned to a technique called stable isotope analysis, which makes use of the fact that the proteins in an animal’s diet are incorporated into the animal’s body tissues in a predictable way. Literally, ‘you are what you eat’ [4]. The ratio of carbon stable isotopes – ẟ13C – can be used to trace the source of carbon in an animal’s tissues. In the Southern Ocean it is often used as an indicator of the latitude at which an animal has been foraging [5]. The ratio of nitrogen stable isotopes – ẟ15N – is used as an indicator of the trophic level of an animal (how high it is in the ‘food chain’). From a small piece of skin which we collected from free-ranging killer whales using a crossbow [6], we could measure both these values, and we report the results in a paper entitled “Variation in the diet of killer whales Orcinus orca at Marion Island, Southern Ocean” in the journal Marine Ecology Progress Series [7].

We found, unsurprisingly, that killer whales at Marion Island are indeed apex predators. But the ẟ15N values weren’t quite as high as we expected, which suggests that the killer whales are also eating prey of a lower trophic level than just seals and penguins, perhaps fishes and squids. Also, variations in ẟ13C values suggest that killer whales forage slightly north of Marion Island when they are not patrolling the island’s waters for seal and penguin prey. When we compared these values among the different social units [8], we didn’t find any substantial differences.

Stable isotope analysis is not a silver bullet –  it has some shortcomings including the fact that an animal’s specific diet usually can’t be identified. But in this case, it has given us some insight into the unseen ecology of killer whales in a remote area, and it has brought up some interesting questions which need answering.

References

1. Reisinger RR, de Bruyn PJN, Bester MN (2011) Abundance estimates of killer whales at subantarctic Marion Island. Aquat Biol 12:177–185 http://dx.doi.org/10.3354/ab00340
2. Reisinger RR, de Bruyn PJN, Tosh CA, Oosthuizen WC, Mufanadzo NT, Bester MN (2011) Prey and seasonal abundance of killer whales at sub-Antarctic Marion Island. African J Mar Sci 33:99–105 http://dx.doi.org/10.2989/1814232X.2011.572356
3. Reisinger RR, Keith M, Andrews RD, de Bruyn PJN (2015) Movement and diving of killer whales (Orcinus orca) at a Southern Ocean archipelago. J Exp Mar Bio Ecol 473:90–102 http://dx.doi.org/10.1016/j.jembe.2015.08.008
4. DeNiro MJ, Epstein S (1976) You are what you eat (plus a few per mil): the carbon isotope cycle in food chains. GSA Abstracts with Programs 8:834−835
5. Jaeger A, Lecomte VJ, Weimerskirch H, Richard P, Cherel Y (2010) Seabird satellite tracking validates the use of latitudinal isoscapes to depict predators’ foraging areas in the Southern Ocean. Rapid Commun Mass Spectrom 24:3456–60 http://dx.doi.org/10.1002/rcm.4792
6. Reisinger RR, Oosthuizen WC, Péron G, Cory Toussaint D, Andrews RD, Bruyn PJN de (2014) Satellite Tagging and Biopsy Sampling of Killer Whales at Subantarctic Marion Island: Effectiveness, Immediate Reactions and Long-Term Responses. PLoS One 9:e111835 http://dx.doi.org/10.1371/journal.pone.0111835
7. Reisinger RR, Gröcke D, Lübcker N, McClymont E, Hoelzel AR, de Bruyn PJN (2016) Variation in the diet of killer whales Orcinus orca at Marion Island, Southern Ocean. Mar Ecol Prog Ser 549:263–274 http://dx.doi.org/10.3354/meps11676
8. Reisinger RR, Janse van Rensburg C, Hoelzel AR, de Bruyn PJN (in press) Kinship and association in a highly social apex predator population, killer whales at Marion Island. Behavioral Ecology http://dx.doi.org/10.1093/beheco/arx034

Talk on Killer Whales at Bayworld

A killer whale breaks the surface in Rockhopper Bay at Marion Island.

I will be giving a public talk entitled “Cold killers, and other finned and flying beasts of the far south” at Bayworld / Port Elizabeth Museum on 29 March 2016.

The talk is part of the Friends of Bayworld AGM, but business will be brief and everyone is welcome.

When:
Tuesday, 29 March 2016 at 17:15 for 17:30.

Where:
Exhibitions & Events Centre, Bayworld / Port Elizabeth Museum, 7 Castle Hill

For more information contact:
Rainique Kaiser on 041 584 0650 or rainiquek [at] bayworld.co.za

Talk summary:
Killer whales (Orcinus orca) are the ocean’s apex predators and are among the most widely distributed animals. They have fascinating foraging and social behaviour which varies from place to place, and there may even be different species of killer whales. I will talk about these aspects, mainly using examples from my research on sub-Antarctic Marion Island. I will also talk about a new project with which I and my colleagues are trying to map important habitat for killer whales, seabirds and seals around Marion Island using tracking data.

Marion Island Research Documentary

The long-running South African environmental TV show, 50|50, has compiled a special episode on the Mammal Research Institute’s seal and killer whale research at Marion Island. And yes, I do make an appearance!

The Effect of Satellite Tagging and Biopsy Sampling on Killer Whales

Still frame from a point of view video showing satellite tagging of an adult male killer whale (M007) at Marion Island. The tag can be seen in the dorsal fin. © RYAN REISINGER

Satellite Tagging and Biopsy Sampling of Killer Whales at Subantarctic Marion Island: Effectiveness, Immediate Reactions and Long-Term Responses
PLOS ONE 9(11): e111835

Cetaceans spend the vast majority of their lives under water and are highly mobile and often wide-ranging, which makes them a challenging taxon to study. Two field methods – tissue biopsy sampling and satellite-linked telemetry (or satellite tagging) – are becoming widely used in cetacean studies because they allow the collection of data which are difficult or impossible to obtain by other means. Tissues obtained by biopsy sampling can be used for a range of analyses including genetics, stable isotopes, fatty acids, contaminants, hormones and trace elements and can so address aspects such as population structure, diet and animal health. Satellite tagging can elucidate the movement, distribution, behaviour and habitat use of cetaceans in relation to their physical environment. Such data are critical to understanding the ecology of a species and its environmental role and, consequently, are vital to conservation or management efforts. The need for such information is particularly acute given the anthropogenic pressures many such populations and species face.

However, researchers must carefully consider their methods not only from an animal welfare perspective, but also to ensure the scientific rigour and validity of their results. The latter point is critical where methods may affect the subsequent behaviour or performance of individuals, thereby biasing the results obtained. From an ethical perspective researchers have an onus to assess the tradeoffs between the ‘importance’ of research, its likely benefit and its effect on animals before conducting work; from a scientific perspective the responsibility is to design robust and valid studies. Researchers should further evaluate animal effects and research methods post-hoc, refine these where needed and, importantly, publish such results.

We have been satellite tagging and biopsy sampling killer whales at Marion Island since 2011 and in our new paper in PLOS ONE we looked at the immediate reactions of the animals to being tagged or sampled and whether killer whales showed any longer term responses.

We never observed severe reactions to tagging or biopsy sampling, there was typically no observable reaction or a flinch, shake of the body, some acceleration and/or an immediate dive. We analysed individual sighting histories over several years, and we could detect no significant mid- (1 month) or long-term (<24 months) changes in killer whale occurrence where we tagged and biopsy sampled.

However, we will continue long-term monitoring of individuals after biopsy sampling and tagging to provide continuous assessment of potential impacts on the study animals. We reccomend that such monitoring should be implemented in other studies where animals are biopsied or tagged, especially considering the increased use of these methods.

Read the full paper in PLOS ONE:
http://dx.doi.org/10.1371/journal.pone.0111835

This is an emotive issue which is likely to elicit some strong responses.

Marion Island Killer Whales 2006-2013

Our photographic identification catalogue of killer whales (Orcinus orca) at sub-Antarctic Marion Island (2006-2013) is now available on figshare:
http://dx.doi.org/10.6084/m9.figshare.971317.v1

SA Science Lens

I was awarded runner up in the Science in Action category of the 2013-2014 Southern African Science Lens competition, run by the South African Association of Science and Technology Advancement (SAASTA).

The winning images are at http://saasta.ac.za/index.php?option=com_content&view=article&id=339

Congratulations to my friends Otto Whitehead and Morgan Trimble on their winning entries.

Killer Impact

Killer whales (Orcinus orca) are the oceans’ apex predators; over 140 species of marine vertebrates have been reported as killer whale prey.  Their top predator status and eclectic diet, along with their large energy requirements, means that killer whales can have big effects on marine ecosystems.  In the Southern Ocean elephant seal (Mirounga leonina) populations crashed between the 1950s and 1990s.  Since many killer whales prey on elephant seals, some researchers suggested that killer whales may have caused the declines (a top down explanation) while others suggested that environmental changes were the primary drivers of the crash (a bottom up explanation).  At Marion Island in the Southern Ocean, nearly 15,000 elephant seals have been marked and resighted through their lives to try and understand how the environment is changing. Recently researchers have also started studying the killer whales there (which can be individually identified by their unique dorsal fins) to fathom how they can influence other species.  In this photograph, an adult female killer whale – M002 (right) – and her three year old calf – M027 (left) –  try to drag a one year old elephant seal – GR346 – off a ledge.  The coloured ‘tags’ used to uniquely identify the seal can be seen in its hindflippers.  The photograph illustrates predator-prey relationships, and how datasets on two species can intersect.

Killer Impact.

Gauging the Threat – White Shark Numbers Published in PLOS One

Dorsal fin identification images of one of the sharks (named “Darwin”) in the study. DOI: 10.1371/journal.pone.0066035.g003

South Africa was the first country to afford white sharks (Carcharodon carcharias) protection and is reputed to host the world’s largest remaining population of these apex predators. Yet no studies in the country have accurately determined a population size of living individuals.  I assisted Alison Towner, Michelle Wcisel, David Edwards and Oliver Jewell of the Dyer Island Conservation Trust (DICT) to produce a population size estimate using data they have been collecting at Gansbaai, South Africa. The study has just been published in the journal PLoS One:

Gauging the Threat: The First Population Estimate for White Sharks in South Africa Using Photo Identification and Automated Software Alison V. Towner, Michelle A. Wcisel, Ryan R. Reisinger, David Edwards, Oliver J. D. Jewell Research Article | published 12 Jun 2013 | PLOS ONE
http://dx.plos.org/10.1371/journal.pone.0066035

DICT have been taking dorsal fin photographs of sharks during cage diving trips since January 2007. They identified 532 individual sharks from their dorsal fin characteristics: notches, scarring, pigmentation patterns and the overall shape.  From these ‘photo-ID’ data, they could build sighting histories for each individual, or records of the dates each animal was seen over several years.  I used these sighting histories for a ‘capture-mark-recapture’ analysis which let me calculate the approximate size of the population.  Our best population size was 908 sharks with a 95% confidence interval of 808 – 1,008 sharks.  This estimate is a lot larger than those for other white shark hotspots around the world (e.g., 219 sharks in California) but is about the same as a 1996 estimate for sharks caught in shark nets off KwaZulu Natal (KZN), South Africa.  As we concluded in the paper:

“This suggests white shark numbers have not shown marked recovery from; 1) the deployment of shark nets and drum lines along the KZN coastline in 1952, which are still in place to date; 2) the heavy fishing pressures white sharks experienced in the 1970’s and 80’s; and 3) a lack of protection in neighbouring Mozambique. Despite the species being protected since 1991, such a low estimate and lack of recovery rate suggests the Southern African white shark is not receiving adequate protection for population growth.  These results highlight the need for effective protective measures within the entire home range of the Southern African white shark.”

Junior Captain Scott Medal

Flanked by my supervisors Dr. Nico de Bruyn (left) and Prof. Marthán Bester.

I’m very pleased to announce that I’ve been awarded the 2012 Suid-Afrikaanse Akademie vir Wetenskap en Kuns Junior Kaptein Scottgedenkmedalje (South African Academy of Science and Art Junior Captain Scott Memorial Medal).  The medal is awarded to the best zoology or botany MSc thesis at a South African university.

Salt Water 03

It’s been a (long) while since the last Salt Water. I intended to write about the end of winter, how the elephant seal breeding season was looming, but in the meantime spring has come and gone, as have the elephant seal females. Winter let go of the island reluctantly and we had heavy snowfalls well into October. But the island was humming with animals – feathered, finned and flippered, adult and hours old – by then.

Download from Issuu.

Salt Water 02

Around 08:15. Sunrise. But only barely. The sun will describe a low arc through the sky before disappearing behind the western horizon eight and a half hours later. The days are short and the light low, but Marion doesn’t share the extremes of the high latitudes which have near or complete darkness for most of the winter.

Download from Issuu.