Commercial catch rates for the North West Shelf trap fishery, at up to 10 kg trap-1 h-1, are considerably greater than those reported for similar trap fisheries elsewhere. Two fish-trapping experiments were conducted off Lowendal Island (north-western Australia) to study the catch rates of four trap types (O-, S- and Z-shaped and rotational) and two bait types (pilchard, Sardinops neopilchardus, and emperor, Lethrinus choerorynchus) over various soak times (ranging from 1 to 24 h). Total catches varied significantly with soak time. Catches increased rapidly for the first 3 h, levelled off, and then decreased steadily to 16 h; there was a slight increase at 24 h. The species composition of the catch did not vary significantly with soak time. There was no significant difference in catch rates between trap types. The traps did not appear to cause fish to die by being confined in the traps for extended periods. A fairly large amount of suitable bait (about 4 kg) was necessary for effective trapping of fish. The catch rates of the pilchard bait were significantly higher than those of the emperor bait. Over extended soak times (more than 3 h), pilchard bait was lost at a rate of about 6% h-1. whereas there was no detectable loss of emperor bait.
Samples of N. peronii collected at depths of less than about 100 m showed a slight positive correlation between fish size and depth, but no correlation between abundance and depth and no die1 change in catch rate. Feeding occurred during the day, with crustaceans (Brachyura, Natantia, Mysidacea and Stomatopoda in particular) occurring in 70% of feeding individuals and fish occurring in 17%. Individuals containing ripe eggs were found throughout the year, although the proportion of ripe fish was highest in November and December. Sexual maturity in 50% of females was attained at a fork length of about 15 cm and at an age of 1 year. Zonation on the otoliths was shown to be annual and estimates of the parameters of the von Bertalanfi growth curve were L∞ = 41.9 cm, K= 0.25, t0 = 0.74 years over the age range 0.5-5 years. The growth parameters for sexually differentiated animals differed significantly, but the age-length trajectory of each was very similar over the observed age range. The instantaneous rate of total annual mortality was 1.85 and did not differ between sexes.
The shedding rate of FD-67 anchor tags from L. carponotatus was significantly lower than that of FT-2 dart tags. Mortality in fish tagged with either the FT-2 dart or the FD-67 anchor tags was significantly higher than in the untagged controls. There was no significant difference between mortalities caused by the two types of tag. Combining tag shedding and tag-induced mortality, the FT-2 dart tag produced a significantly greater rate of disappearance of tagged fish from a tagged population. Tag-induced mortality was highest during the first week after tagging and greatly reduced thereafter.
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