We should not assume that fishing jellyfish will solve our jellyfish problem

ICES Journal of Marine Science ICES Journal of Marine Science (2016), 73(4), 1012– 1018. doi:10.1093/icesjms/fsv255 Food for Thought We should not assume that fishing jellyfish will solve our jellyfish problem M. J. Gibbons1 *, F. Boero2,3, and L. Brotz4 Department of Biodiversity and Conservation Biology, University of the Western Cape, Private Bag X17, Bellville 7535, South Africa DiSTeBA, Università del Salento, Lecce 73100, Italy 3 CNR-ISMAR, Italy 4 Sea Around Us, Institute for the Oceans and Fisheries, University of British Columbia, 2202 Main Mall, Vancouver, BC, Canada V6T 1Z4 2 *Corresponding author: tel: + 27 21 959 2475; fax: + 27 21 959 2312; e-mail: Gibbons, M. J., Boero, F., and Brotz, L. We should not assume that fishing jellyfish will solve our jellyfish problem. – ICES Journal of Marine Science, 73: 1012 – 1018. Received 28 September 2015; revised 27 November 2015; accepted 1 December 2015; advance access publication 24 December 2015. Whether jellyfish are increasing or not in the global ocean is a subject of some debate, but the fact remains that when they bloom, jellyfish can negatively affect local economies. Despite this, there has been no robust debate about the idea of deliberately removing jellyfish as a means of population control. Here, we discuss the effects of fishing for jellyfish, either as a sustainable resource and/or as a way to simply reduce their nuisance value, on both individual jellyfish populations and the ecosystem. Given that the drivers influencing each local bloom are different, or that the effects of more widespread drivers may be manifested differently at each locale, our priority at population control/use needs to be more basic research on jellyfish. While we do not advocate a no-fishing approach, we emphasize the need to be cautious in embracing jellyfish fisheries as a panacea and we need to consider the management of each bloom on a case-by-case basis. Keywords: ecosystem effects, fisheries, impacts, jellyfish. Jellyfish (Cnidaria, Medusozoa; Ctenophora) blooms often lead to the formation of swarms that can have important direct impacts on our use of the marine environment. Jellyfish at high densities can clog fishing nets and contaminate fish catches. They can block the pump filters associated with coastal power and desalination plants. They can sting people, sometimes fatally, and impact coastal tourism. They can also have indirect impacts on commercial resources by virtue of their zooplanktivorous diet, and can be predators on, and competitors with, valuable finfish and their larvae. Collectively then, the impacts of jellyfish swarms can negatively affect local economies (e.g. Boero, 2013, and references therein). Jellyfish are a natural component of healthy marine ecosystems. According to species-specific cycles, they produce new individuals seasonally, often deriving from benthic polyps (Boero et al., 2008). If the external conditions are favourable in terms of abiotic factors and/or biotic interactions, some species can develop huge populations that lead to swarms following the concentration of individuals by winds and currents (Graham et al., 2001). Jellyfish outbreaks are mentioned episodically in the old literature (see Russell, 1970, for a review), but evidence is emerging to suggest that jellyfish swarms now tend to be more frequent locally, and the apparent merger of the “local” phenomena gives the impression of a global rise (Brotz et al., 2012). The drivers for these increases, however, are probably manifold and the object of some debate (e.g. Mills, 2001; Condon et al., 2013; Gibbons and Richardson, 2013), but some appear to be linked to various anthropogenic factors. These factors include climate change, eutrophication, overfishing, and the increased availability of hard substrata in coastal systems, among others (see Purcell, 2012, for recent review). Jellyfish are probably able to respond positively to these impacts, individually and/or in synergy, directly and/or indirectly, and as either polyps and/or medusae. Although overfishing and climate change are clearly global issues, their impacts and effects at the local level differ widely across the globe. Consequently, local explanations need to be sought for local population increases, because it is only with knowledge of local drivers (including overfishing and # International Council for the Exploration of the Sea 2015. All rights reserved. For Permissions, please email: 1 1013 Effects of fishing for jellyfish the Mediterranean Sea (e.g. Canepa et al., 2014). This makes control problematic because the species themselves evolved in the ecosystems they occupy. As such, natural booms would have alternated with natural busts following internal controls. Even if the long-term data unambiguously demonstrate that populations have increased in size, indicating perhaps a change to the state of the ecosystem, external control requires a thorough understanding of the drivers of population change. Which brings us back to a basic question: what would be the purpose of fishing? Would it be to exploit the resource sustainably, or would it be to simply reduce the population size perhaps with the hope that by doing so, valuable finfish populations might recover? Or both? This apparently win– win strategy, however, might have some profound ecological implications. Sustainable exploitation of single-species populations Kingsford et al. (2000) have usefully reviewed jellyfish fisheries (with a focus on the paraphyletic Rhizostomeae), and highlighted the problems of ensuring sustainability, given the large number of unknowns. Aside from issues associated with determining growth or natural mortality rates, classical fisheries approaches based on stock –recruit relationships are difficult to implement because there is no clear understanding of what represents the management unit or “stock”: is it the polyp population or that of the medusae; or both? Relationships between the number of polyps and the number of medusae may be intuitive, but they are hard to demonstrate. Having said that, Di Camillo et al. (2010) have suggested that there is a correlation between the extent of polyp beds and the abundance of ephyrae of Aurelia aurita in the Adriatic Sea. Makabe et al. (2014) have recently made similar observations for the same species in the Inland Sea of Japan. While it is clear that medusae come from polyps, if they are present in the life cycle, nobody has yet been able to show a clear relationship between the numbers of medusae and the number of polyps recruiting to the seabed from planulae. Indeed, the sudden appearance of native species of medusae within a system after many years of absence, such as the hardly inconspicuous Drymonema dalmatinum in the Adriatic (Malej et al., 2014b), suggest that polyp populations might remain viable for decades, with no need of new planula settlement. So, if polyp beds are self-maintaining without significant renewal from medusae, due to t (...truncated)