November 2014 saw the release of Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report Synthesis Report.  “Science has spoken”, UN Secretary General Ban Ki Moon iterated on the climate crisis challenge that we now face.  “There is no ambiguity in their message. Leaders must act; time is not on our side".  The Earth’s surface is warming, and it is with 95% certainty, the IPCC concludes, that the dominant cause is human activity.  The climate crisis impacts every part of our planet, including the ocean, where rising sea levels and global ocean temperature increases are a significant cause of concern.  On a global scale, measurements show that between 1971 and 2010 the upper 75 meters of the ocean has warmed by 0.11⁰C.  Between 1901 and 2010, the global mean sea level has risen by 0.19 meters.  Ocean acidification, regarded as climate change’s “evil twin” is also a growing cause for concern, with greenhouse gas emissions - the primary contributing factor to the climate crisis - also altering the carbonate chemistry of the ocean.  These figures may seem small, but they are not insignificant to the fish that occupy the ocean.

The majority of marine fishes are ectothermic – they do not generate their own internal heat, instead relying external heat to give them the warmth necessary for physiological processes. Although the thermal tolerance of each species varies, their distribution is inextricably tied to ocean temperature.  As Dr William Cheung of the University of British Columbia’s Fisheries Centre and colleagues highlighted in a 2009 study, this generally means species shifting polewards towards the cooler (albeit warming) waters to stay within their thermal niche.  Adding invertebrates into the calculations, a 60% species turnover of biodiversity in the Arctic and Southern Oceans is predicted by 2050.  For fish living in the poles, whose temperature limits are on average 2 - 4 times narrower than lower latitude species, warming waters may be the end of the line - there may simply be nowhere left for these fish to go.

No species is an Island - what changes to one species invariably impacts another.  A number of distributional and abundance changes of phytoplankton correlated with ocean warming have been detected across the globe, including the timing of phytoplankton blooms. Similarly, zooplankton has also demonstrated changes in their distribution and abundance, correlated not just to ocean warming but to the timing and location of phytoplankton. 

The knock on effects of such lower trophic level distributional changes continues throughout the food web. Reviewing observed changes of a number of commercially caught species in the Northeast Atlantic, Franz Mueter implicated poor feeding conditions during warm periods arising from a reduction in large zooplankton species in reduced survival of juvenile walleyed Pollock.  The reason for the reduction in zooplankton cannot be exactly determined, but is likely the result of either a “mismatch between the timing of the spring [phytoplankton] bloom and the prey needs of [the zooplankton] …or to a reduction in post-bloom production resulting from intense stratification and reduced nutrient supply into the surface layer”.

Warming waters does more than alter distributions.  If temperature exceeds a fish’s thermal tolerance, it may experience increases in metabolic demands.  Couple that with predicted decreases in oxygen associated with warmer waters, growth and even survival becomes an issue.  Studying the growth rates of the long-lived banded moowong in the Tasman Sea using otoliths – a bony structure in the inner ear which have ‘growth rings’ similar to those found in trees, marine biologist Dr Anna Neuheimer, now at the University of Hawai’I, and co-authors were able to determine that over the past 90 years, individuals living at the northern end of their distributional range, which has experienced the most warming, have suffered greater declines in growth rates compared to those in areas that have not warmed as quickly.  The researchers also found evidence of metabolic stress, with some fish increasing their oxygen consumption by 44% when approaching spawning swim speeds, eventually going into anaerobic stress. 

In the North Sea, Dr Alan Baudron from the University of Aberdeen demonstrated that a number of commercially caught species including haddock, herring, and Norway pout, have reduced their maximum body length by up to 29% since 1970.   It appears that the fish may grow more rapidly during their juvenile stages, resulting in fish maturing at a smaller size.  Numerous studies have demonstrated that smaller female fish don’t just produce fewer eggs than their larger counterparts, but eggs of reduced quality.  In short, what offspring are produced have a reduced chance of survival.   

Interestingly changes in productivity – the number of recruits per spawner - though can be highly variable not just between species but, as demonstrated by fisheries scientist Dr Franz Mueter of the University of Alaska Fairbanks, between populations of the same species.  Focusing on sockeye Salmon in British Columbia and Alaska, Mueter revealed that whilst historical increases in regional sea surface temperature resulted in a decrease in the salmon’s productivity in central and southern British Columbia, northern British Columbian and Alaskan populations saw increases in productivity.  Exactly why this would be remains unclear.

Sea level rise may not seem to be an immediate problem for fish, but for those who utilize shallow water habitats, sea level rise could be problematic.  Exactly how shallow habitats will respond to changing sea level varies depending on local conditions in which they exist.  For example, in some regions corals may respond with vertical growth, ensuring the continuation of habitat.  Conversely tidal creeks and low marshlands used both by residents and transient marine fish may become inundated and eventually lost because human development prevents their inland migration.  Alterations in oceanic currents which are utilized by marine fishes are also a cause of concern.  Predicting alterations to ocean currents with any degree of certainty is extremely difficult, though there is some evidence of climate-induced changes such as for the Kuroshio Current, which is predicted to increase its speed as a result of increased wind stress.  What this means for fish like grey mullet that make use of the Kuroshio for larval dispersal and adult movement is uncertain. 

The impacts of climate change on marine fishes go far beyond those outlined here, and are likely to be highly variable depending on the species and regions they are in.  In some respects we may be powerless to halt some of the climate-induced changes impacting marine fishes.  Even if we were to halt all carbon dioxide emissions with immediate effect, work by scientists such as NOAA Atmospheric Scientist Dr Susan Solomon suggests that the time lag between alteration of carbon dioxide concentrations and climactic response will not be seen for some 1,000 years after emissions stop. 

These impacts go beyond just the fish themselves.  As highlighted in a paper lead by Dr Alistair Hobday, senior research scientist at CSIRO Marine and Atmospheric Research, many of ocean warming ‘hotspots’ – where over the last 50 years ocean surface temperature has changed the most rapidly, and is projected to continue to do so - often occur where humans heavily rely on marine resources for food security and livelihood.  Mitigating climate change is no longer sufficient in itself.  We also need to adapt to ensure that we are able to thrive in this changing new ocean climate. 

This story was written for The Marine Professional, a publication of the Institute of Marine Engineering, Science & Technology (IMarEST).