Jellyfish in ecosystems, online databases, and ecosystem models

Abstract

There are indications that pelagic cnidarians and ctenophores (‘jellyfish’) have increased in abundance throughout the world, or that outbreaks are more frequent, although much uncertainty surrounds the issue, due to the scarcity of reliable baseline data. Numerous hypotheses have been proposed for the individual increases or outbreaks that are better documented, but direct experimental or manipulative studies at the ecosystem scale cannot be used for testing them. Thus, ecological modeling provides the best alternative to understand the role of jellyfish in large fisheries-based ecosystems; indeed, it is an approach consistent with new ecosystem-based fisheries management practices. Here, we provide an overview of online databases available to ecosystem modelers and discuss general aspects and shortcomings of the coverage of jellyfish in these databases. We then provide a summary of how jellyfish have been treated and parameterized by existing ecosystem models (specifically focusing on ‘Ecopath with Ecosim’ as a standard modeling toolset). Despite overall weaknesses in the parameterization of jellyfish in these models, interesting patterns emerge that suggest some systems, especially smaller and more structured ones, may be particularly vulnerable to long-term jellyfish biomass increase. Since jellyfish also feed on the eggs and larvae of commercially important food fish, outbreaks of jellyfish may ultimately imply a reduction in the fish biomass available to fisheries. On the other hand, jellyfish, which have been traditionally fished for human consumption in East and Southeast Asia, are now seen as a potential resource in other parts of the world, where pilot fisheries have emerged. It is also argued here that reduced predation on the benthic and pelagic stages of jellyfish, both a result of fishing, may be a strong contributing factor as well. For marine biologists specializing on jellyfish, this means that their research might become more applied. This implies that they would benefit from adopting some concepts and methods from fisheries biology and ecosystem modeling, and thus from using (and contributing to) online databases, such as SeaLifeBase and FishBase, developed to support such research. This would remedy the situation, documented here, wherein jellyfish are either infrequently included in food web models, typically constructed using the Ecopath with Ecosim software, or included as a single functional group with the characteristic of an ‘average’ jellyfish. Thus, jellyfish specialists could readily improve the jellyfish-related components of such models, and we show how they could do this. Also, it is suggested that when such improvement is performed, the resulting models can lead to non-intuitive inferences and hence interesting hypotheses on the roles of jellyfish in ecosystems. This is illustrated here through (a) an investigation of whether jellyfish are keystone species and (b) the identification of conditions under which (simulated) jellyfish outbreaks may occur.