Climate effects on white sharks

A number of factors will effect the trophic interaction between seals and sharks in southern Africa. Most important to note is that while the topic has been researched, there is no definitive proximate cause or trigger for white shark migration. Even within shark populations, there is great discrepancy between length, destination and timing of migrations. While some sharks swim to Australia and back to South Africa in one year, other sharks never venture more than a couple hundred miles up the coast to Namibia. Despite these unknowns, making predictions about how climate change will alter aspects of shark behavior is vital to better understanding the intricacies of ecosystem balance in southern Africa.

The above figure quantitatively shows, in blue, a concerning trend of consistent increase in sea surface temperature anomalies over the range of ocean waters focused on in this blog (Tisdale). In red, under estimates of the A1B climate change scenario in the IPCC report, ocean sea surface temperatures are predicted for the same area. Estimates of temperature increase vary, but could be as severe as 6 degrees Celsius in the next one hundred years (IPCC, 2007). Given the range of temperatures for waters where white sharks are typically spotted and for waters across intercontinental migration routes in the southern Indian ocean, a trend as seen above could have drastic ramifications. These could possibly include alteration of migration and changes in timing due to shifting of proximate cues. Even though cues for shark migration aren't fully understood, temperature change could very likely play a role and if severe enough, sharks may decrease of even stop migrating to warm coastal waters such as southern Africa. Based on trends and statisticts from data of thermal cues to migrations in other species, a shift in migrational timing and range is very likely. Yearly temperatures in False Bay, South Africa support the above trend and show increasing temperature elevations during el Nino years and diminished drops during other years.

As noted earlier, cape fur seals have mating and subsequent breeding cycles that are based on photoperiod, or cues from the length of daylight (Stewardson, Bester and Oosthuizen). Though temperature may have indirect effects on breeding cycles such as resource availability, temperature is not the proximate cause of onset for mating in seal populations from which white sharks in southern Africa rely on. Peak months for shark sightings in the bays where seals reside is from April through September. White sharks time their migrations to align with maximum prey availability and since seal breeding cycles will not shift due to causation in relation to photoperiod as this will not change, if sharks receive proximate cues from sea temperature a trophic mismatch (imbalance between participants of varying trophic levels) between this predator and prey could arise. If sharks can no longer time migrations to arrive in southern Africa in cadence with seal abundance, ramifications throughout the food chain will be observed. Below is a figure which demonstrates how increased sea surface temperatures may effect shark migration dates. Important to notice is the missing data point at a projected temperature increase of five degrees Celsius. This is because such a temperature increase might make waters in False Bay unfavorable for shark habitation. If increases deterring sharks from returning to seal islands along Southern Africa occur, dramatic effects will be experienced throughout the south African marine ecosystem.

1 comment:

  1. This page is very informative and you did a great job writing a very focused discussion of migration patterns and climate change. I do think it's important for you to cite more of your sources. It sounds like you've actually done primary research of your own on this topic. If so, you should cite who sponsored that research/if a faculty member was involved etc. Somehow make it clear where your information is coming from.