Buried behind the brains of most fish species, the miniscule aragonite-type crystalline calcium carbonate structures known as otoliths are like skeleton keys to marine scientists – tools capable of unpicking numerous locks to reveal data about a given fish’s diet, health, and, most notably in the realm of stock assessments, age.
By counting the calcified layers continually deposited on an otolith over a fish’s lifetime, just as with a tree cross-section, scientists have, since the 19th century, been able to work out both how old a fish is as well as at what rate it has grown. Such findings have been the bedrock of population dynamics, lighting a pathway to management for many valuable fisheries.
This classic approach of characterizing the demography of fish populations through aging its members has been a vital tool for science and ICES historically. Yet, as marine science switches from the traditional track of single stock models and dynamics to the multitrack toolbox of ecosystem-based management, new and innovative ways of reading and processing data from otoliths are coming into play. Using these ‘black boxes’ of information – as otoliths have been described – to further map the ecosystem picture is an increasingly prominent subject in otolith science.
This transition was reflected in the themes of ICES Fifth International Otolith Symposium (IOS2014), held over five days in Majorca in late October. The conference saw over 300 researchers and scientists gather for a programme comprising a spread of speakers and workshops, with the focus being placed on the use of these calcified tissues to build further ecosystem-based understanding through the development of ecological indicators.
IOS2014 also proved the ideal arena in which to address wider questions – such as those of population connectivity, trophic ecology, and migrations – that have augmented the portfolio of otolith research techniques since the last symposium was held.
One standout example of the heightened relevance of otolith studies for ecological and management issues was the presentation given by Woods Hole Oceanographic Institution Senior Scientist Simon Thorrold, who told attendees about his use of otolith geochemistry to ascertain alternative population attributes to the ones typically examined.
“We’re definitely scratching the surface of what we can learn from otoliths,” says Thorrold, whose research team assessed the dispersal of westslope cutthroat and steelhead trout in Montana’s Flathead River. For mature fish, this meant using otoliths to gauge population connectivity through the processes of natal homing (adult trout returning to a native river to spawn) and native straying (those who return to a different river).
Thorrold, who detailed a similar project of larval retention and dispersal around Papa New Guinea’s Kimbe Island in his talk, moved on to contemplate the foodweb question.
“Where we’re at now is taking the next step: if we know where fish are based on this chemistry, perhaps we can also learn something about what they’re doing at that location, where they’re feeding in the foodweb – and how generalist or specialist a fish might be.”
Such studies on movement have centred on the inorganic constituent of otoliths, present in the mineral form of aragonite, and the technique of laser ablation, with results necessitating a rethink of the spatial scales of fish populations. The argument being, as Thorrold puts it, “that the inorganic component is more likely to reflect environment variations – temperature, salinity and water composition – that fish might be experiencing.”
But it’s a new methodology used by Thorrold that leads to intriguing questions on foodweb architecture: the idea of retrospectively defining carbon and nitrogen foodweb sources.
“We analyze stable carbon isotopes and specific amino acids in the organic component of the otolith. And there’s one kind of amino acids that fish as well as other consumers like copepods cannot synthesize themselves,” he explains.
“Basically carbon propagates up the foodweb with an isotopic composition that’s specific to the end member at the base of the foodweb, be it phytoplankton or macroalgae (seaweed). Each one of these end members has a unique isotopic signature which propagates up. So we can analyze a large fish’s otoliths and basically determine where the carbon that fish has assimilated has come from. There’s a similar test you can do with nitrogen to tell us what trophic level the fish was feeding at. ”
“We’re getting to a point now where we’re looking at combining carbon and nitrogen isotope analyses in these amino acids to give a pretty detailed look at the structure of the foodwebs these fish are feeding in.”
Supporting this idea, one of Thorrold’s research efforts revolved around using this organic element to look at movements of fish across a coral reef seascape constituting reefs both on and off the continental shelf.
“For two of the seven species we looked at there was a lot more macroalgal carbon on the shelf reefs than the oceanic ones. So there were differences in primary productivity and where that was coming from. It highlighted the importance of understanding what is fuelling the reef fish populations in those two habitats.”
“In some sense carbon is the currency of ecosystems, so understanding where that carbon is coming from for forage fish species or apex predators or commercial fish species is pretty critical if you want to conduct ecosystem based management.”
For all the advances in otoliths helping to reveal details of foodweb and ecosystems discussed by Thorrold and others at IOS2014, and despite science’s proficiency at being able to ascertain oceanic conditions experienced by fish in the past, the use of otoliths to project forward remains a different matter.
“In some sense these otoliths are measuring what’s important for the fish. So if you’re looking at population projections, you’ll have to have climate data and response of species of interest to change. That’s a very difficult thing to do. But if we want to be looking at the impact of climate change on fish populations and the cost of not acting on it, then that’s the information that you need,” adds Thorrold.
Otolith cross sections dyed and under a microscope