Have fish diets changed over the last 150 years? Auckland University of Technology Master’s student Roen McLeod is seeing whether the eye lenses of Te Papa’s historic fish collection could one day reveal where they were feeding and what they were eating.
The natural history collection built and cared for by the team at Te Papa is impressive not only due to its size and diversity, but also the opportunities it offers to access valuable information from the past.
The Te Papa fish collection alone is home to almost 300,000 specimens ranging from tiny, parasitic male anglerfish fused to their girlfriend, all the way to enormous ocean sunfish and great white sharks.
With rare specimens collected from remote locations – and some older than 150 years! – it is exciting to consider how they might help us better understand our world and how animals have responded to environmental change over time.
Comparing the diet and movement of a historic fish specimen to its modern equivalent can give us insights about their lives, enabling us to better protect key prey and habitat during relevant seasons for taonga (treasured) fish species.
Top dog or someone’s lunch? The clues are in the atoms
One tool that can help us trace the diet and movement of a fish across its life is stable isotope analysis (SIA). Stable isotopes are the heavy (have extra neutrons) and light versions of the same element. By interpreting carbon and nitrogen stable isotope ratios (expressed as δ13C and δ15N, respectively) found in the building blocks of body tissues such as proteins, scientists can figure out an animal’s relative place in a food web, or food chain, and the habitat they have been feeding in.
Stable isotope analysis relies on measuring isotopes from the building blocks that make up body tissues, but chemical preservatives can interact with these cellular components. Preservation commonly involves chemicals such as formalin and ethanol, which function by penetrating into the body and hardening cellular structures, removing moisture, and preventing bacterial growth/tissue degradation.
New science on old fish: can we reveal how their diets have changed?
To confidently use SIA on museum samples to reveal diet and habitat of animals over their life, it is important to first test whether chemical preservation changes the δ13C and δ15N values of different body tissues.
With this goal in mind we, Master’s student Roen McLeod and supervisor Dr. Amandine Sabadel from the Science of the Environment and Ecosystem Dynamics (SEED) lab at Auckland University of Technology, have been working in collaboration with the amazing fish team at Te Papa.
Previous work done by Leo Durante and team focused on the δ13C and δ15N values of preserved fishes’ muscle tissue. To build on this, we chose to measure the stable isotope ratios of pilchard, Sardinops sagax, eye lenses across a range of preservation types and durations.
Muscle tissue is biologically active and replaced at a relatively fast rate, so its δ13C and δ15N values only reflect recent (the last ~ 3 months) diet and habitat.
Conversely, inert tissues like eye lenses undergo no further biological activity once they are formed. This means their δ13C and δ15N values represent the diet and habitat from the time period they were created. Like an onion, eye lenses grow in layers.
Measuring the stable isotope values of each layer builds a picture of diet and habitat across the life of the fish, from larval development (the core of the lens) through to recent adult growth (the outermost layer).
The impact of the different preservatives and preservation times on the pilchard eye lenses has been clear to see during the dissections so far, as shown in the following image, but the SIA results to come later in the year will be the key to unlocking a better understanding of our historic taonga species. We look forward to sharing the final learnings, so stay tuned!
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