Wet Season Ecosystem Revitalisation

Posted by Jane McNeil on

Late January saw the real start of the wet season in Far North Queensland, with large rainfall totals falling across much of the state. The wet season is a key annual event for most ecosystems, marine and freshwater included. It’s not hard to see evidence of the huge effect this has on many of our local species. A great example can be seen with the choruses of native frogs that can be heard along local marshes and ephemeral water courses as they utilise the wet conditions to lay their eggs. Underwater, other wet season associated events are occurring, though they aren’t always as evident. For example, young barramundi use these flood events to access freshwater wetlands and similar habitats, as they provide ideal environments for growth in their early life stages. An interesting point of focus throughout the wet season are the estuaries, as they probably undergo the most relative change of all aquatic ecosystems. With the huge influx of fresh water, estuarine conditions change significantly. Both the temperature and salinity in estuaries drop sharply, which affects fish species to a varying degree, as each has a specific tolerance to these variables. For example, species such as barramundi that can live in freshwater are going to be more tolerant to this rapid salinity drop than estuarine or marine specialists. However, for most species, including more tolerant species such as barra, the rapid salinity and temperature drop is still unfavourable in the short term, and fish may respond as such. This tends to mean the rivers themselves are rather inactive during these periods of significant freshwater flow, and fish may move to other areas with more suitable conditions. Keeping this in mind, it is certainly possible to target fish while the rivers are discharging large amounts of fresh water. Look for more stable coastal waters, away from cold and fresh flood discharge for your best shot.

Once the main floods are past, rivers will normally see a burst of life. Prawns tend to appear along the coast in decent numbers, feeding on detritus released from estuaries in flood. These prawns and their offspring are a major food source for just about every predatory coastal species at this time of year. Prawn aggregations draw predators from all around, including species such as manta rays which seldom venture to our beaches excluding during juvenile prawn blooms. For species that naturally inhabit these areas, the prawn influx provides easy access to large amounts of high-quality prey. Opportunistic predators such as barramundi eat a lot and grow fast during these periods of high productivity. Barra, mangrove jack, salmon, grunter and many other species can be taken along local beaches with relative consistency once these aggregations of prawns appear. By the time this abundance of prawns along the coast has dropped, the local systems will have just about returned to their pre-flood state, at least to an extent. Because estuary are highly dynamic systems, they tend to change structure relatively rapidly, with new sandbars, deep holes and snags forming regularly. A large amount of these changes occur throughout the flood season for obvious reasons. Floodwaters are especially effective at moving sand, mud and other structure down a river, and estuarine systems are often completely different after large flooding events. As such, fish won’t always be in the same spots as they have been in previous years, if conditions have changed enough to make a particular spot unsuitable. For this reason, it is always a good idea to revisit and assess favourite spots after the wet season, as they have likely changed. Revisiting and relearning your local systems after the wet is a great way to set yourself up for the year ahead!

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