Blue Jellyfish numbers up
November 28, 2009 by BayKeeper
Filed under Education, Wild Things
The thousands of dead Blue Jellyfish (Catostylus mosaicus) on Port Phillip Bay beaches in recent months prompts the obvious questions: why are there suddenly so many; and why did they die?
Is it a natural cycle, or was it something they ate? Blue Jellies are said to feed on micro-organisms, small fishes, and organic particles floating in the water column.
Blue Jellyfish - what's going on there?
Until a couple of years ago, I’d found Blue Jellies in the Bay through spring and summer, but absent in winter. Their occurrence coincided with greater abundance of microscopic algae that they feed on, which bloom due to warmer conditions, increased daily hours of sunlight, and increased nutrient levels flushed into the Bay by spring rains and summer thunderstorms.
Although the year round presence of the Blue Jellies has occurred since the Channel Deepening Project, and dredging does release nutrients into the water column, it is impossible to tell if this is the main cause of the Blue Jelly explosion. Overfishing of their predator species in eastern Australian waters may be a contributing factor.
But it is interesting to note that the biomass of Blue Jellyfish was greater than the combined total of all other species recorded by the Baywide Anchovy Sub-program in both June 2008 and June 2009; and the greatest concentrations of Blue Jellies were in the central area of the Bay, a few kilometres south of the Northern Dredge Material Ground.
Remember all the dead blue jellies on the beach from Altona to Seaford in August last year, I think that was the first time we had seen so many dead’uns on our beaches. Might be interesting to go back and look at what happened on both occasions…. major rain event… sudden drop in temperature… and why were so many in the Bay anyway…. also did this build up and die off of blue jellies happen anywhere else?
Would be great to have a system that tracks where the buggers are so we can avoid them.
One point to consider:
The explosion in jellyfish seems to correspond with the years of drought. With so little water entering the bay from the rivers, the waste water flow from Werribie must be significant. This is rich in nutrients – hence a potential cause of algal/plankton bloom, providing jellyfish food. With the drought, little water is flushed out of the bay, allowing nutrient concentrations to keep increasing.
Does anyone know of a correlation between jellyfish locations and currents from the Werribie outlet?
Also: On ABC radio, a Tasmanian scientist was interviewed, & one of her statements was that trevally eat jellyfish. How are trevally numbers going?
You’ve made some interesting suggestions Michael. But conventional wisdom is that if there’s a drought on land it slows down production in the marine environment. Alongside the Werribee Treatment Plant, the Yarra is the major contributor of nutrient to the Bay; and it’s the tidal movements at the Heads that flush the Bay, not river flows. Unless there’s been a big increase in flows from Werribee, a drought will ordinarily mean less nutrient in the Bay. The ‘whale in the bath-tub’ has got to be the disposal of millions of tonnes of nutrient-rich Yarra sediments in the Bay, as has occurred over the past couple of years.