While the lack of in-depth understanding of the mechanisms driving population dynamics and recruitment is a concern in the shorter term for the management of stocks, it is especially problematic when attempting to predict the future of cephalopod populations with regards to global climate change. There are many uncertainties surrounding the impacts of climate change on marine ecosystems, many of which are linked to our incomplete understanding of marine carbon-cycling and ocean- atmosphere circulation mechanisms (Mann and Lazier 2006). Changes will nevertheless occur, one of the most definite being an increase in temperature. By the end of the century, the global ocean temperature is predicted to rise between 1.8ºC and 6.4ºC depending on future carbon emission scenarios, with an average of 4ºC under the highest emission scenario (namely A1FI, IPCC 2007). The various models agree that the maximum warming will occur in the high latitudes of the northern hemisphere, mainly due to melting ice and changing surface albedo (reflectivity of sun’s radiation), whereas the minimum is expected in the Southern Ocean, due to ocean heat uptake. However, generalisation is difficult as regional differences exist because of localised atmospheric and oceanographic features. For example, while sea surface temperatures (SST)
2070 under the A1FI scenario (CSIRO 2007), coastal water temperatures in Tasmania are expected to rise, in the same time period, between 1.4ºC to 4.1ºC due to Tasmania’s complex current system.
As cephalopods adapt rapidly to varying environments and appear to thrive in warmer conditions due to accelerated growth and their opportunistic nature, it has been suggested that cephalopods will prosper with climate change, providing there is sufficient food availability (Bildstein 2002). However, higher temperatures also lead to smaller hatchlings and potentially smaller adults (Pecl et al. 2004b), implying that hatchling size and post-hatching growth rate will likely be opposing forces acting on the size at age of adult cephalopods (Pecl and Jackson 2008). As temperature also has a direct effect on cephalopod metabolism, as well as that of their prey and predators, climate change is certain to have consequences for species abundance and activity rates (Bailey and Houde 1989).
Rising ocean temperatures are only one of the many consequences of global warming. Levels of atmospheric CO2 are expected to rise, increasing the quantity of CO2 that permeates the surface mixed layer of the oceans and resulting in an estimated drop in pH of 0.14 to 0.35 units by 2100 (IPCC 2007). At present, the natural variation of seawater pH throughout the world’s oceans is 7.5 to 8.3 (Seibel and Fabry 2003) so
ocean acidification could result in a pH range of 7.15 to 7.95 under the highest emission scenario. In cephalopods, oxygen binding and blood transport is extremely sensitive to changes in pH (Miller and Mangum 1988; Pörtner et al. 2004). Therefore the acidification of oceanic waters is likely to limit oxygen uptake in many species, with consequences for activity rates, growth, reproduction and survival (Seibel and Fabry 2003; Rosa and Seibel 2008). Due to their high rates of activity and elevated metabolism, squids are more likely to be affected than octopus and cuttlefishes (Zielinski et al. 2001), whose blood oxygenation only becomes markedly affected at a water pH<7.4 (Sepia officinalis, Zielinski et al. 2001) and pH<7.2 (Octopus dofleini, Miller and Mangum 1988) respectively against pH<7.5 for squids (Illex illecebrosus, Pörtner and Reipschläger 1996).
The predicted increases in precipitation and freshwater runoff are likely to decrease salinity in some areas, which would affect incubation and embryonic development (Paulij et al. 1990; Cinti et al. 2004; Sen 2005) as well as adult survival (Chapela et al. 2006). Increased turbidity could also affect species with ritualised mating behaviour based on sight, reducing breeding success as observed in Loligo vulgaris reynaudii (Roberts and Sauer 1994; Roberts 1998; Rodhouse 2001). The potential increase in the incidence of severe weather and more common and intense El Niño events (Easterling et al. 2000) is also likely to affect the population structure of cephalopod species, as has occurred during past El Niño/La Niña events
(Arntz et al. 1988; Jackson and Domeier 2003; Ish et al. 2004; Zeidberg et al. 2006; Chen et al. 2007).
Other predicted consequences of climate change include modification of the patterns of ocean stratification and/or deep-ocean circulation, changes in the productivity and location of upwelling areas, as well as in the intensity of many currents (Mann and Lazier 2006), which would have consequences for nutrient availability and the distribution of migratory species and those with planktonic stages (paralarvae). Climate change will also bring about modifications in biogeography, as poleward shifts in the range of many species are expected. Such migrations to more suitable thermal environments have already taken place, with the appearance of subtropical and tropical species in temperate areas, such as the observations of the squid Alloteuthis africana and the common paper nautilus Argonauta argo in Spanish waters (Guerra et al. 2002). For completely benthic species, which include some octopus and cuttlefishes, the lack of larval dispersion by means of currents and the limited movement capacities of adults might prove problematic under changing temperature conditions, and animals may be forced to undergo shifts in their depth distribution to match their thermal preferences.
Given the complexity and the potential cascading effects of climate change, predicting the future for particular cephalopod populations is challenging and has not yet been attempted, mostly because the crucial understanding
of population dynamics that is required to make predictions is lacking for most cephalopod species. Importantly, understanding population dynamics requires a sound knowledge of the characteristics of the individuals constituting the population (Vanoverbeke 2008), in particular the mechanisms dictating individual growth and how biotic and abiotic factors influence developmental and reproductive processes in the wild. This knowledge is currently lacking and is the focus of the present research.