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SEASONAL MOVEMENTS OF YELLOW FIN WHITING IN S.A.


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With the seasons changing and the YFW on the move, I thought I would start with this one.

SEASONAL MOVEMENTS OF YELLOW FIN WHITING IN S.A.

I have been fishing, mainly as a wading fisherman targeting YFW, for the past 18 years.

Over that time I have logged details of most of my fishing sessions on excel spreadsheets.

Recorded my various observations and analysed the data.

This is a summary of my thoughts on the seasonal movements of Yellow Fin Whiting in S.A.

 

The arrival of YFW on suburban beaches in the warmer months and their disappearance over cooler months is a regular event.

So where do the YFW come from?

And where do they go to?

 

It is a total fallacy that YFW are a Summer only species and that it is only in warmer temperatures that it is taken by the land based angler. Yellowfin whiting inhabit relatively shallow waters for their entire life. I catch them all year round, in all temperature ranges, wading in shallow water. Although their location and their behaviour does change with the seasons. So too must the fishing methods used and the areas fished.

 

INDEPENDENT GULF POPULATIONS

The YFW populations in each gulf are likely to be independent of each other. They most likely do not move out of the two SA Gulfs in which they reside. This has been suggested by some limited tagging studies to date. They are possibly similar to the Blue Swimmer Crabs where DNA testing has shown that there are independent populations in each Gulf that do not intermingle. Currently there is a study of YFW being undertaken in this area. To date the last published report from the Fisheries Research and Development Corporation in 2020  https://fish.gov.au/report/341-Yellowfin-Whiting-2020  suggests 2 separate gulf populations. So it is most likely the YFW population’s movements are only, within the gulf in which it resides. I have noticed morphological differences between the 2 populations in the 2 Gulfs. Spencers Gulf fish tend to be slightly shorter but heavier built than the often longer fish in the St Vincent Gulf. And recently as a lure fisherman, I notice Spencers Gulf YFW are harder to catch on lures.

 

A study of data from the commercial fishermen’s catches of YFW, provides the clue to their movements within each Gulf.

Source:  The South Australian Marine Scalefish Fishery Stock Status Report – 2011/12

WHERE do they catch most of their fish? ( Fig.1)

806699492_zFig1catchAreas.png.31dbb2d9d902fcbf4f5c4b293d37d43c.png

See the map of the areas where the Commercial Tonnage is caught. The majority of the commercial catch is taken in the northern ends of the 2 gulfs.

 

WHEN do they catch most of their fish? (Fig.2)

2033819444_zFig2MonthlyCatch2Gulfs.png.e1b55eaf99075d0def877e57cb6bb54f.png

We see in the graphs for the Monthly catches of YFW, that the majority of the commercial catch is taken in the colder Winter months of May, June, July, August.

 

What are the Water Temperatures for those areas at those times?  (Fig.3)

Source:  http://oceancurrent.imos.org.au/SAgulfs/latest.html

zFig3WaterTempSeasons.jpg.2ee4b0a0f1ead6ec9a32b8cbcf5b24e0.jpg

We can correlate the professional catch months and their catch areas with maps for the gulf water temperatures.

Most of the YFW catch by professional fishermen, are caught in the Northern coldest parts of the Gulfs in Winter.

And the commercial catches drop when the YFW population disperse into the cooler Southern Gulf areas during the summer months.

 

The movement of YFW in the gulfs is driven by the availability of food.

It is also driven by a Poikilothermic animal using colder water temperatures to reduce their metabolism and their food needs.

 

As waters cool in the lower gulf in Autumn, their food sources in that area becomes scarce. Ever notice how bungum worms are rarely seen in winter. They along with other benthic dwelling organisms, worms, clickers, and small cockles, tend to move deeper in the sand. Are less accessible and remain dormant in the cooler months. The food that was available when the water was warmer, has disappeared from these southern areas of the Gulfs.

 

The most consistent and reliable food source over winter is in the northern gulfs. The food source here is primarily bloodworms. They are plentiful in the organic rich flats adjacent the mangrove areas of the northern gulf. Also abundant are the seaweed worms residing below the large dead seaweed deposits, that occur in the northern gulf over winter. The seaweed worms become more accessible at this time of the year, as big night tides and winter storm surges, dislodge seaweed banks and their seaweed worm populations. Bloodworms are readily available right throughout the winter period. Bloodworms are actually most active in winter. Constantly moving and relocating all through winter using the larger winter night tides to do this. Not just during their major spawning runs, when they are very visible on the surface in large numbers. Hence they become a consistent food source for the YFW in winter. Taste a winter caught YFW and you can often taste the distinctive iodine flavour that comes from a diet primarily made up of bloodworms.

 

The other main reason for the YFW to withdraw to the colder waters of the Northern Gulfs, is to intentionally enter a state of Torpor. Being Poikilothermic animals they get their body temperature from the surrounding water. Torpor is a state of decreased physiological activity in an animal, usually by a reduced body temperature and metabolic rate. Torpor enables animals to survive periods of reduced food availability. The Northern Gulfs are the winter “Bear Caves” for the YFW.

 

However, they still need to feed. Just not as much. And certainly not as vigorously as they do in summer. This makes winter fishing for them more location and conditions specific for a land based fisher. In particular during a cold winter, they will be heavily concentrated in the deeper warmer parts of the cold  northern gulfs. Whilst they are seeking cooler waters to slow their metabolism, they still avoid extreme low temperatures. They will often be out of reach of the land based recreational fisher. I have talked with professional fishermen at Pt Parham and Pt. Wakefield. Their most productive catch days are after a few very cold frosty mornings. When the shallow water is the coldest. This is when the YFW retreat from the icy shallow waters and are in concentrated schools in the deeper warmer pockets of water. This aggregation makes them easy targets for the professional’s nets. Hence as the Figures 1 & 2 show, the majority of the professional catch is taken in winter in the northern ends of the gulfs. If you are wading these areas in winter you can see and hear the professionals banging their boats, rounding up the fish into tighter schools prior to netting them.

 

There are land based spots that are connected to deep water where you can burley in some YFW in winter. This is the only time I ever consider using burley for YFW. Other times of the year you attract too many nuisance fish rather than YFW. In winter other than Salmon trout, there are less nuisance fish about. However with a lower metabolic rate, the YFW are very lethargic in winter. Being a poikilothermic animal they wind up in a state of torpor when water temps fall below 12c. Their bites are timid. Often just sucking baits. Along with burley, smaller hook sizes and the lightest gear is more effective. I normally use a Size 4 hook but drop down to a size 6 even 8, in these months. Keep weights to a minimum. Often it is only on the bigger incoming night tides that you will find them. That is when the deeper warmer water is lifted up and onto the beach, bringing with it the fish it holds. The bloodworm runs in winter are certainly a good time for the wading fisherman to catch a few YFW. They are stimulated into feeding more. My fishing logs show, it is an exceptional winter YFW season when the water temperatures are over 13C during the bloodworm run.

There is a population of YFW that remains down south in the Onkaparinga River estuary over winter. This again is due to the fact that bloodworms inhabit that waterway. I know of fishermen that regularly catch bags of YFW in the Onkaparinga Estuary in winter. Using frozen bloodworms for bait. Little wonder that the most common early season sighting of YFW on the suburban beach areas is around the mouth of the Onkaparinga river. There are similar enclosed water bodies, such as Franklin Harbour, in the Spencers Gulf that consistently hold YFW in winter. These areas provide cooler water for a lowered metabolic rate along with a reliable source of food.

Other areas that are consistent in winter tend to hold a bit warmer water. The Torrens Island power station hot water outlet always has a few YFW in winter. Spots in the Southern Spencers Gulf like Pt Lincoln can be affected by the warm water flows at the tail end of the Leeuwin Current which will attract and hold the YFW. All the enclosed water bodies that are productive in winter, end up being too hot in summer for the YFW and their food.

 

As the water temps starts to rise, it sees the YFW more energetic and active. These Poikilothermic animals, warm up, increase their metabolic rate, and start to vigorously search for food when water temps get past 14C. Water temperatures soon rise to the optimal temperature range for the growth and reproduction of worms, cockles and clickers on southern sand flats and beaches. These now become the main food source for YFW. Bloodworms are now less available, having settled deeper into the mud for summer. And the seaweed worm areas are less disturbed and less available. It will take the water temperature to get above 22c before it is too hot for the food sources in the northern gulf. Again these benthic organisms take shelter from extreme conditions by burying themselves deeper, reducing the availability of food. However, in summer the relatively cooler southern sections of the gulf now have the optimal water temperatures for the growth and reproduction of their food sources. The main population of YFW move south into the cooler southern areas, spread across the suburban beaches, which now holds more food. In the warmer months I have found them as far south as Normanville and Coobowie in the St Vincent Gulf. Tumby Bay and Hardwicke Bay in the Spencers Gulf.

 

A number remain in the northern gulf waters. They tend to be a polarised population of very large and small fish, I find. The larger fish I think become more specialist feeders on alternative food sources like shrimp and juvenile prawns that now are more numerous in the warmer Northern waters. It is common resource partioning within a species where they do not all compete for the same food source. Shrimp are just the thing our YFW surface lures are designed to mimic. Casting surface lures for YFW in winter only seems to attract Salmon. A species that has a highly developed circulatory system as seen in their blood content. Is a bit more of a homeotherm with a higher metabolic rate through out winter. It needs a higher metabolism to chase it’s primary food source of small fish.  Whereas the Poikilothermic YFW need the higher water temperatures of summer to raise their metabolic rates before they become aggressive feeders of food on the surface and chase our lures.

 

Cheers, Des

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Cheers Des, very interesting (no really 👍) I love me some proper research based on factual sources, for what it matters it explains a lot of my observational findings, in particular why at one particular location mid Spencer you catch them all year round but target them differently, ie shallows in summer, deeper 5-6m "pockets" in winter.

Also the bloodworm taste thing, so true especially for September KGs and red mullet, makes perfect sense now.

Keep these coming as much as you want, I'll read them for sure.

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