In the past few decades marine biologists have become aware of many dramatic changes in the world's oceans, including dramatic falls in many commercially exploited Fish species, rising temperatures and widespread pollution and marine litter. During this time several studies have shown that as Fish populations have fallen in many areas, they have been replaced by rising Cephalopod numbers, leading some experts to wonder if this might be a global trend, though to date the data has not been examined to test this hypothesis.
In a paper published in the journal Current Biology on 23 May 2016, Zoë Doubleday and Thomas Prowse of the School of Biological Sciences and The Environment Institute at the University of Adelaide, Alexander Arkhipkin of the Fisheries Department of the Falkland Islands, Graham Pierce of the Oceanlab at the University of Aberdeen, Jayson Semmens of the Institute for Marine and Antarctic Studies at the University of Tasmania, Michael Steer of the South Australian Research and Development Institute, Stephen Leporati of the Department of Fisheries Western Australia, Sílvia Lourenço of the Instituto Português do Mar e Atmosfera, Antoni Quetglas of the Centre Oceanogràfic de les Balears of the Instituto Español de Oceanografía, Warwick Sauer of the Department of Ichthyology and Fisheries Science at Rhodes University and Bronwyn Gillanders, also of the School of Biological Sciences and The Environment Institute at the University of Adelaide, present a detailed analysis of Cephalopod population statistics from around the globe since 1953.
A Caribbean Reef Squid, Sepioteuthis sepioidea, on Bari Reef in the Caribbean Netherlands.
Doubleday & Prowse et al. examined records from all major oceanographic regions in both hemispheres from 1953 to 2013, examining all key taxonomic groups (Squid, Octopus and Cuttlefish) and life history groups: demersal (living close to the bottom), benthopelagic (living on the bottom, but also swimming higher into the water column at times), and pelagic (living in the water column away from the bottom).
A Two-spot Octopus, Octopus bimaculoides. Monterey Bay Aquarium.
In all cases Cephalopod numbers were found to have grown over the period examined. Doubleday & Prowse et al. suggest several possible explanations for this. Cephalopods are highly adaptable and intelligent, enabling them to colonise new areas and exploit novel resources, and have shorter life-cycles than comparably sized fish. In addition almost all Cephalopods, regardless of adult lifestyle, have a planktonic larval phase enabling them to reach suitable new habitats quickly. In addition recent studies have shown that rising temperatures tend to reduce generation times in Cephalopods (i.e. they reach maturity and reproduce more quickly). Furthermore many such larval Cephalopods are subject to predation by Fish species that have undergone dramatic population declines in recent years, potentially enabling more larval Cephalopods to survive to adulthood and reproduce.
Broadclub Cuttlefish, Sepia latimanus.
Doubleday & Prowse et al. note that such a rise in Caphalopod numbers is likely to have knock-on effects, most notably a rise in predation on species targeted by Cephalopods and an increase in food supply to species targeting Cephalopods (including many Humans). However they also observe that the current rise in Caphalopod numbers does not necessarily imply that in future Cephalopod populations will continue to rise, noting that the group are potentially vulnerable to future threats, such as rising ocean acidification and increased targeting by Human predation as Fish populations decline.
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