Yellowfin whiting

The yellowfin whiting is a benthic predator, using its well developed eyesight and a downward pointing jaws that can be protruded to 'suck up' and capture its prey from the seafloor.[12] Studies on the mouth morphology of a range of sillaginids concluded that the mouth morphology of most species has little influence on the type of prey caught, with differences in diet more likely due to variations in the foraging behavior.[13] Studies conducted in southwest Western Australia found the species predominantly takes polychaetes as its main prey item, although crustaceans, particularly amphipods and penaeids also make up a large portion of its diet. Items taken infrequently or in lesser amounts include small teleost fish, bivalve molluscs, marine plant material and other crustaceans such as tanaids, decapods and cumaceans.[12][13] Modeling with the stable isotopes ¹³C and ¹⁵N indicate that seagrass and epiphytic algae were the primary initial sources of energy and nutrition that flow into the fish via detritivores which the species preys on, with salt marsh plants and macroalgae contributing in some settings.[14]

The species is known to undergo a dietary shift as it grows to adulthood. Young individuals less than 10 cm in length consume considerable volumes of copepods and minor amounts of polychaetes, but once they reach around 10 cm, the diet shifts to a polychaete-dominated one. Further growth sees an increase in the volume of amphipods,[13] small fish and oligochaetes taken. These changes appear to be unrelated to mouth morphology, instead the fish becomes more mobile and possesses a larger mouth to be able to take these prey.[12] The yellowfin whitings' diet also varies spatially and temporally, which appears to be a function of prey availability at different sites and periods of the year.[12] There is little resource partitioning between the small individuals of sillaginids which inhabit shallow inshore habitats, with Sillago vittata, S. burrus and Sillaginodes punctatus also taking copepods as their primary prey in southwest Western Australia. This changes as each species grows, with S. vittata and S burrus moving to deeper waters to avoid interspecific competition, whilst the remaining S. schomburgkii and Sillaginodes punctatus partition the resources, with S. punctatus consuming more decapods and shrimps.[13]

Yellowfin whiting reach sexual maturity at a length of 200 mm in males and 230 mm in females, with the majority of both sexes reaching this length by the end of their second year of life.[15] The timing of spawning varies over the species range, a trait found in a number of other sillaginids. In the northernmost part of its range in Shark Bay, spawning occurs between August and December,[16] while further south off southern Western Australia, spawning occurs between December and February. The South Australian population also spawns between December and February, with this event preceded by the movement of fish into shallow tidal and estuarine regions where spawning occurs. Ripe fish then break off the main schooling body to form smaller spawning schools, where the eggs are shed.[17] There is conflicting data concerning the pattern of spawning in yellowfin whiting, with an earlier Shark Bay study finding the species to be a single spawner,[18] whilst recent studies in the south indicate they are multiple spawners.[15] In South Australia, once spawning has occurred the fish disperse and move further offshore once again.

Females release between 170,000 and 217,000 eggs per season, with these eggs being pelagic and spherical, with a diameter of 0.6 mm.[10] The development and morphology of the newly hatched larvae has been extensively described in the ichthyological literature. By the time they reach 2.7 mm, the mouth and gut are functional, the eyes are pigmented, a gas bladder is present, and yolk absorption is complete. The larvae are elongate, having 36 to 38 myomeres, with flexion occurring by 4.8 mm.[19] The juveniles appear in Western Australian estuaries during March, with subsequent growth being fairly rapid. However, the yellowfin whiting is one of the slower growing sillaginids, but reaches a much larger maximum size than most of its relatives, with a known maximum length of 42 cm.[15] On average, individuals measure 8 cm after their first year and weigh between 60 and 190 g, while by the end of their second year they have attained 24 cm, with females thought to grow slightly faster than males.[18] The oldest known individual taken was a 12-year-old female that measured 35 cm, while the oldest known male was at least 7 years old and measured 34.8 mm.[15]

There are two major fisheries in operation for yellowfin whiting; one in the Gulfs of South Australia and another in Shark Bay, Western Australia. There are also up to four minor fisheries in Western Australia, centered in the Blackwood River estuary, Geographe Bay, the Leschenault Estuary and Cockburn Sound.[10] Fishing methods employed commonly include beach seines, bottom set gill nets along creek edges, ring nets, and long gill nets set on sand banks. Nets are usually set in the mornings in South Australia, when fish often move from the tidal flats back into the deeper channels. The two major fishery areas have produced very large numbers of fish in the past, with the South Australian catch representing up to 65% of the entire catch from the gulfs in the late 1970s.[10] In Shark Bay, the largest known annual catch was 204 tonnes during 1961. This has dropped substantially, and current Shark Bay catches have fluctuated between 100 and 130 tonnes per year since 1990, with the 2003 catch around 110 tonnes.[20] Including the minor fisheries catches of Western Australia, the total WA yellowfin whiting catch was 131.4 tonnes, and made up over 95% of the total whiting catch in the state.[21] South Australia has similar modern catch numbers, with the fishery recovering from a major downturn in the late 1980s, when the catch dropped below 20 tonnes per year for an unknown reason. Concerns were raised about previous over-exploitation, but catch numbers have since recovered to levels of around 150–170 tonnes per year.[22]

Yellowfin whiting have become a major target for anglers in both South and Western Australia for a number of reasons: they are very good table fish, they provide good sport on light line, and are easily accessible from beaches and jetties, with a boat not necessary for their capture. Yellowfin whiting are actually most commonly targeted from beaches, estuaries and jetties constructed over shallow waters, with good catches often made on the ingoing and outgoing period of the tide. Due to their easily spooked nature, tackle used to capture the fish is usually very light, with lines kept below 6 kg, hooks below size 4, and sinkers to an absolute minimum as heavy lines and sinkers often scare away the fish.[23] Specialist whiting fishermen often attach a red bead or piece of tubing directly above the hook to attract the fish, although the usefulness of this is debated. The most common bait used is 'beach worms', which may be from a variety of families, with prawns, cockles and squid occasionally taking good catches also. Lure and fly fishing for the species is poorly developed, with their shy nature preventing these methods from being effectively used.[23]

The recreational catch often is greater than the commercial catch in some areas, with a survey carried out in Blackwood River indicating 120 700 fish were taken in a year by anglers.[10] A similar survey conducted in the South Australian Gulfs found recreational fishermen accounted for 28% of the entire yellowfin whiting taken during the 2000/2001 period, representing over 50 tonnes of fish.[22] Recreational bag limits have been put in place to prevent over-exploitation by anglers in both states, with South Australia imposing a minimum size limit of 24 cm and a bag limit of 20 fish on anglers.[24] In Western Australia, there is no minimum size limit, but a bag limit of 40 fish in combination with school whitings (Sillago bassensis and Sillago vittata).[25]