The Barents Sea capelin is a small pelagic forage fish but something of a foodweb hero. Born to the south of its range closer to the Norwegian-Russian coastline, capelin head north to the colder waters east of Svalbard, where they spend three to four years, mature, return south to spawn, and then almost always die. Despite this fleeting lifespan, the species has existed and exists long enough to represent a critical ecological link between the zooplankton and krill it feeds on and the larger fish such as cod and marine mammals like minke whales that prey on it.
Knowledge of this relationship between predator and prey, the actual physical interactions that take place in the marine environment, is what is at the heart of what’s known as multispecies advice. ICES delivers this advice based on stock assessments that have factored in foodweb interactions.
Predator-prey interactions are part of an overall description of the ecosystem that comes as part of the advice; without this, managers cannot properly evaluate different scenarios, such as conservation of a protected habitat or limitations on fishing, for their fisheries. Mapping the route towards a desired outcome necessitates the identification of particular interactions that can affect it, with trade-offs sometimes having to be taken into account along the way.
In order to detail an ecosystem’s intricacies, several questions must be asked. Who are the main actors in the ecosystem and how do they interact? This is vital as it’s not simply fish but all species that either directly or indirectly interrelate and affect our likely management decisions. Also, what are the environmental drivers and other human pressures affecting the ecosystem and the fisheries that depend on them? And how have interactions changed over time?
One of the main interactions in developing this approach is predation and charting the variable numbers of fish of a particular species that die through being eaten (predation mortality) by others in the respective ecosystem. The picture is a complex one. Many commercial fish prey on others, and for young fish and other small species mortality rates as such are significantly higher than those caused by fishing. Fish that feed on plankton also eat the planktonic young of the predators (capelin feed on fish eggs and larvae). Predation isn’t always the case of two disparate species however; the cannibalistic nature of cod also has to be considered.
When looking for data signals of multispecies interactions, one issue is the difficulty in distinguishing between observation ‘noise’ – less precise readings – and what is in reality taking place in the populations. Noise can occur due to the observation method or the variability present in the data, as we can only sample a small part of the sea while fish occur in schools and layers and move around. This may impact the time-series as much as the trend in abundance over time.
For Barents Sea capelin, however, there are very strong predator-prey relationships, which means that we in spite of observation noise get a clear signal about these relationships. Taking this into account could be the difference between a catch of several hundred thousand tonnes or a recommendation for a year-long fishery closure.
In the Barents Sea, early recognition of the fact that fluctuating cod predation proved pivotal in evaluating the capelin stock led to calculations of the numbers of cod eating capelin being worked into the capelin model. Multispecies assessments for capelin were included in ICES advice for the first time in 1990. Predation mortality continues to be incorporated in the model.
Multispecies assessments have since been rolled out to cover other Barents Sea stocks such as cod, as well as the North and Baltic seas. In the latter two instances, assessments are reached by running a multispecies model, extracting the number of predation mortalities, and then feeding results in single species models.
Alongside the actual stock assessment, the model also forms the basis for broader information about how the catches of different species trade off against each other. For example, catching more predatory fish would allow for higher catches of their prey – at the expense of lowering the stock of the predator. Such ‘culling’ effects are typical predictions from multispecies and ecosystem models. These are not currently used directly in fisheries management, but help point the way towards a more ecosystem based approach.
Models fall into two categories: multispecies ones, which concentrate on predation between commercially important species and key predators, and ecosystem ones, which include multispecies interactions as part of a wider ecosystem representation (both fishery and foodweb) and are sometimes connected to hydrodynamic and biogeochemical processes. Examples of this include Ecopath with Ecosim and the Atlantis ecosystem model.
It is the job of the ICES Working Group on Multispecies Assessment Methods (WGSAM) to develop these models and integrate their results and use into practical management advice, a story that starts out with research on predator-prey interactions.
WGSAM collaborates with a number of other expert groups including WGMIXFISH, the Working Group on Mixed Fisheries Advice. This is of particular note as both multispecies and mixed fisheries approaches (characterized by various species sharing space and also a fisher’s net) should work together under one management framework due to the intimate interconnectivity of the two aspects.
At present multispecies analyses can be the foundation for advice on what combinations of catches give good overall yields while avoiding negative consequences for the different stocks, whilst mixed fisheries can show which of these options are achievable with the fleets available, and what would be the consequences for the different fleet sectors.
And, though this is an ideal rather than the working reality, a joint 2014 WGSAM and WGMIXFISH session looked into new ways of making the two groups’ outputs fit together, as the move towards ecosystem-based advice would benefit from closer integration of multispecies and mixed fisheries models.
Photo: Frits Ahlefeldt