I Just Want to Go Home
Updated: May 14
Climate change is a hot-topic in the news media in recent years and rightly so!
But, whilst we often hear about dramatic and disastrous impacts climate change may have on a global scale (such as sea level rise, desertification and extreme weather events), it can be easy to overlook how climate change may affect species on an individual scale...
Port Jackson sharks (Heterodontus portusjacksoni) are so named because their limited range in temperate waters of southern Australia, means they are commonly sighted in waters surrounding the town of Port Jackson. Port Jackson sharks are "poikilothermic", meaning they do not maintain a body temperature higher than that of their surroundings like endothermic animals do (such as humans). If the shark finds itself in suboptimal temperatures (too cold or too hot) or if it needs to increase its metabolic rate, it will seek out and move into warmer waters (this is known as "behavioural thermoregulation"). .
A study analysing the population growth of Port Jackson sharks over a 14 year period between 1996 - 2010 found that environmental temperatures could affect population growth. As sea surface temperatures (SST) became warmer, the population growth of these sharks decreased. This pattern has been recorded in other species of fish and it is thought it happens because with increased SST, growth is reduced. The researchers hypothesised that, as the Port Jackson sharks were drawn to a specific location for breeding, they were no longer actively regulating their external environment and persisted in suboptimal temperatures. What this means is that, as temperatures rise (as climate change models predict) the Port Jackson shark population will struggle to stay at a healthy size and they will be vulnerable to declines as a direct result of climactic change (Izzo & Gillanders, 2020).
Confounding this problem, is the worry that Port Jackson sharks may also be vulnerable to "ocean acidification". It is predicted that, as atmospheric carbon dioxide levels rise, higher concentrations of carbon will dissolve into the ocean, which will cause the marine environment to become increasingly acidic. There is concern that Port Jackson shark metabolism and/orgrowth and/or timing of reproduction and/or juvenile survival rates may be impacted by ocean acidification.
It is thought that species of shark which are migratory and/or "pelagic" (aka live in the open ocean) will be more freely able to find more favourable conditions in response to climate change; they can simply move into an area with more comfortable temperatures. This highlights how the impacts that climate change may have on one species cannot be carried over to predict how shifts in environmental conditions might impact upon another species. As extrapolation is impossible, it is vital that scientists continue to investigate how different species of sharks may be affected by climate change on an individual level.