Wet Season Roebuck Bay

annabelleIn the field, Photography

Juvenile Zebra shark (Stegostoma fasciatum) in the waters off Thangoo.

After a period of being landlocked, we packed our cameras and headed out in the dinghy yesterday for a cruise around Roebuck Bay, enjoying unseasonal easterly winds.  Our prevailing wet season winds are westerlies, which make for choppy conditions in Roebuck Bay.

Heading across the bay to the waters off Thangoo, we spotted six of these gorgeous little newborn Zebra sharks (Stegostroma fasciatum) swimming along on the surface. Only about 30cm long at birth and fairly recently hatched, the pups hatch live from a clutch of up to ten eggs after a long incubation period. The eggs are relatively large at 17cm, with sticky fibres which allow them to stick to the seafloor.

Juvenile Zebra shark (Stegostoma fasciatum) in the waters off Thangoo.

Juvenile Zebra shark (Stegostoma fasciatum) in the waters off Thangoo.

Zebra sharks are carpet sharks and one of the closest relatives of the whale shark. As adults, the sharks display beautiful leopard spots.  Ruthless nocturnal hunters, the adults hoover prey into their mouths, crushing it in their muscular, rotund cheeks.

As they evolve into adulthood, the sharks, like most teenagers, go through a spotty stage. The juvenile stage of this species is so different in appearance to the adult, that for some time they were thought to be separate species.

Juvenile Zebra shark (image courtesy Wikipedia)

Juvenile Zebra shark (image courtesy Wikipedia)

We were able to capture this Barramundi mooching around near the surface (but only on film, alas no fishing lines on the boat).  There were plenty of big Barra hanging around the entrance to Crab Creek where the water was alive with mullet and other bait fish.

Barramundi - Roebuck Bay

Barramundi – Roebuck Bay

 

There were also several pairs of Three Spotted Swimmer Crabs (Portunus sanguinolentus), locked together in a mating embrace.

Three Spotted Swimmer Crabs (Portunus sanguinolentus)

Three Spotted Swimmer Crabs (Portunus sanguinolentus)

One of the most remarkable things about the Kimberley wet season is the change in sea surface temperature in Roebuck Bay, and the influence of freshwater inundation from the enormous shallow wetlands on Roebuck Plains.  As millions of litres of freshwater flow off the plains, the inshore waters heat up as tidal mixing occurs near Crab Creek.  In the middle of the bay the water temperature was a relatively cool 28.3°c, but just off Crab Creek the water was 32°c.  In Dampier Creek, where very little freshwater is flowing into the creek, the temperature was back down to 29°c.

Wet season rain clouds cast dramatic shadows over Roebuck Plains in the Kimberley wet season.  Water from the plains drains into Roebuck Bay, bringing an inundation of fresh water. (Annabelle Sandes/© Annabelle Sandes | Kimberley Media 2010)

Wet season rain clouds cast dramatic shadows over Roebuck Plains in the Kimberley wet season. Water from the plains drains into Roebuck Bay through many channels including Crab Creek.  (© Annabelle Sandes | Kimberley Media 2014)

Tidal mixing off Crab Creek

Tidal mixing off Crab Creek

At this time of year, the shores of Crab Creek are home to flocks of migratory shorebirds including Bar-tailed Godwits, which undertake epic journeys from their breeding grounds in the northern hemisphere to feed on the shores of Roebuck Bay.

A flock of migratory shorebirds at Crab Creek on Roebuck Bay.

A flock of migratory shorebirds at Crab Creek on Roebuck Bay.
A juvenile White-bellied Sea Eagle near Dampier Creek

A juvenile White-bellied Sea Eagle near Dampier Creek