Rising temperatures

Continuation from Ocean acidification and warming

The increase in greenhouse gases has made a profound impact on ocean water temperatures globally.  In particular, the east coast of Tasmania has been noted as a climate change hotspot due to the area's rapid rate of temperature change.  The rate of ocean warming off southeastern Australia has been about 3-4 times greater than the global average.  Johnson et al. gives insight into the East Australia Current (EAC) and how it has contributed to the warming of waters off the east coast of Tasmania.  Intensification of the EAC has brought stronger, more long-lasting, and more frequent incursions of warm, nutrient-poor water.  This movement has resulted in declining populations of smaller abalone on the coast and declining density of kelp beds.  The EAC is relatively nutrient-poor compared to the sub-Arctic water masses that formerly dominated the east coast of Tasmania [3].

 Johnson et al. studied the blacklip abalone, scientifically known as Haliotis rubra [3].  The blacklip abalone inhabit areas at depths of up to 40 m, although most are found in the first 20 m.  This species is predominantly a cold-water species [2].  Maximum shell length, growth rate, and size at maturity were parameters that were measured in determining how temperature has affected abalone.  Temperatures were obtained from sea surface temperature satellite readings [3].

In the graphs above, the solid and dotted lines represent east and west coast populations of abalone measured in this study.  Size at maturity and therefore maximum shell length in wild, east and west coast populations decline with increasing mean annual temperature (graphs b and c).  The growth rate was measured during the juvenile stage when maximum lifetime growth rates occur.  Growth rate in response to temperature did not show significant effects (graph a) [3].

Smaller abalone bodies and shells could potentially escalate overfishing if consumer demand (food & decorative items) for it were to remain unchanged.  If ocean temperature trends and/or consumer demands do not change,  reduced size abalone will have to be compensated by increased number of individuals captured and consumed.

Vilchis et al. also discussed the impacts of warming temperatures on kelp forest biomass.  Since 1957, the carrying capacity of this abalone food source has declined by two-thirds in southern California.  Similar effects can be observed in areas with elevated water temperatures.  Temperature plays an important role because it is inversely related to nutrient concentration.  Warmer waters are poor in nutrients.  The health of kelp is dependent on nitrate availability, which is dependent on nutrient concentrations in the water.  So of course, reduction in food sources leads to reduction in those that feed on it [5].

Not only will warm waters reduce the growth and viability of Australian abalone species, it will also negatively affect the fisheries production.  Economically speaking, understanding the implications of anthropogenic effects on ocean warming is important to maintain the fisheries industry in this area.