Sprattus antipodum, S. muelleri
The sprat is a comparatively small sea fish measuring on average between 8-12cm (about 3 to 5 inches) with rare specimens measuring up to 20cm (8 inches). It is a thin fish, its body is flattened from side to side (laterally compressed) and forming a slight “V” shape tapering towards the belly. Sprats are silvery blue in colour with some specimens tending greenish and yet others tend to be slightly yellow on their undersides.
They are most abundant around the South Island particularly south of Banks Peninsula. Although not seen a great deal in Lyttelton Harbour they are plentiful in Otago Harbour almost all year round, where they are sort after by the local salmon anglers as first-class salmon bait. Dunedin anglers catch and freeze them for later use as salmon bait.
Sprats spawn during the colder winter months, particularly in the Canterbury Bight.
The sprat, according to the late Mr David H. Graham, a marine biologist appointed to the Marine Fisheries Investigation Station, Portobello, Otago, was found around the coast of Otago, either in the harbour, off Cape Saunders or in the stomachs of 17 species of fish which had been feeding on them, not only on the surface to varying depths down to 50 fathoms.
I have caught kahawai in Canterbury that have been gorging on the humble sprat. Their stomachs filled to capacity. Schools of kahawai and barracouta follow the vast schools of sprats over the summer waiting until sufficient digestion allows them to force down a few more. I have also caught red cod that have been full of sprats which is puzzling to me. I would not have thought a red cod fast enough to catch sprats.
Sprats are very oily fish. A school of sprats that has been attacked by predators leaves a distinctive oily film on the surface of the water.
I remember well one particular summer’s day I had arrived early at a Canterbury beach, just south of Banks Peninsula, only to find a huge screaming mob of seabirds constantly harassing a vast school of fish that was right in the last line of breakers: Kahawai were torpedoing through the school, whilst others picked up any stragglers. The object of all the attention was a vast school of sprats that must have numbered in the millions. The attentions of their pursuers were seemingly causing them to commit suicide. Hundreds at a time were being stranded high and dry on the beach as they tried frantically to avoid becoming breakfast. This school later moved several miles offshore where it remained for the rest of the day. Vast numbers of seabirds maintained a shuttle service to and fro from the beach to the school whilst the attention of subsurface predators continued from below.
These scenes, admittedly not as common as they once were, give some idea of the sheer numbers of sprats that make up these vast densely pack schools. It must at times reach into well into the millions, though I suspect that by the end of the summer they must be thinned out somewhat.
In Otago Harbour salmon anglers use “flasher rigs” to attract and catch their supply of sprats. These are lines of half a dozen little bait flies tied on small hooks and jigged up and down over the side of the wharf. On occasions, several fish are taken at a time.
In a study conducted by Gavin James of NIWA, on the stomach contents of trawler-caught salmon, caught off Canterbury, found they fed mainly on fish, with sprats making up 76 percent and juvenile hoki 5 percent of the diet. See: What Do Salmon Eat in the Sea? by Gavin James.
In summary, all I can say is; who’d want to be a sprat!