Increase in ‘up-welling’ may affect marine life
THE beach is changing, every day it is different.
Sometimes the beach seems empty with few creatures lying along the high water mark, but usually there will be pieces of driftwood, soft sponges or hard coral, red-bait polyps, shark egg cases and shell fragments.
But every now and again something unusual happens – either a rare find or sometimes lots of things wash up at the same time.
Mass washout events of invertebrates and fish species occur intermittently across our beaches over time.
Until recently these events may have gone unnoticed. However, today an unusual event is quickly advertised through Facebook and other online media.
The most recent event on the Garden Route included the washout of relatively high numbers of fish.
Two days before washout we experienced strong and sustained easterly winds.
Driving along the coastline, this wind starts to cause the inshore surface waters to move away from the coastline, which starts to pull up colder, deeper water to the surface.
The easterly winds essentially drive a large and effective water pump.
If the water temperature drops quickly, fish are unable to adapt and are stunned by the cold water.
Unable to swim, they then wash ashore.
Although a natural event which provides plenty of easy food for many sea birds and beach scavengers, satellite-derived wind data show that since the early 1990s the intensity and variability of “up-welling” along the south coast has increased, with potential implications for marine life.
Changes in wind patterns and water temperature have the potential to impact on the productivity and species composition of plankton which can influence the abundance of small pelagic fish (such as sardines, anchovies) with consequences for larger predators (both fish and birds).
In addition, an increase in up-welling will increase the offshore movement of surface waters and along with it fish larvae drifting southwards from the more easterly spawning grounds may well be lost.
Ultimately the consequences of increased up-welling are likely to be complex and variable.
During a February up-welling a number of species washed up along the coastline, but surveys of the event indicated that the majority of the fish impacted were juvenile red tjor-tjor, santer and maasbanker.
Of these, santer are an important species within the ski-boat line fishery and we will have to wait and see if this up-welling event will have impacted catch rates in three to four years, when these fish would have been large enough to catch.