This paper examines the hypotheses (1) that Sardinops sagax and Engraulis australis are spatially segregated and do not interact directly, and (2) that recent mass mortalities of S. sagax have facilitated an expansion in the distribution and abundance of E. australis. In South Australian waters, S. sagax and E. australis both spawn during summer and autumn. Eggs and larvae of both species occur over the continental shelf, and are abundant in areas where upwelling occurs (e.g. off the Coffin Bay Peninsula and the western tip of Kangaroo Island) and frontal systems form (e.g. in Investigator Strait and the entrance of Spencer Gulf). After the mass mortality events in 1995 and 1998, eggs and larvae of S. sagax were confined mainly to these areas, and estimates of the total abundance of S. sagax eggs and larvae in South Australian waters fell by between 48 and 83% respectively. Between 1996 and 1999, densities of E. australis eggs and larvae increased in both key spawning areas and the central and eastern Great Australian Bight, and total abundance of eggs and larvae increased by over 215 and 285% respectively. These results indicate that (1) S. sagax and E. australis are not spatially segregated and may interact directly, and (2) the mass mortalities of S. sagax may have facilitated an expansion in the distribution and abundance of E. australis. Hence, fluctuations in the relative abundance of S. sagax and Engraulis spp. observed in the world's productive boundary current systems may also be possible in Australian waters.
The reproductive biology of the King George whiting, Sillaginodes punctata, was investigated for populations at two localities in South Australia. Analyses involved determination of gonadosomatic indices, macroscopic staging of gonads, size frequencies of oocytes, spawning fractions of females, and batch fecundities that were related to fish size and age. Size–frequency distributions of oocytes, measured from histological slides and whole preserved oocytes, were multi-modal but were continuous amongst development stages. All development stages of oocytes co-occurred in some gonads, and hydrated oocytes and post-ovulatory follicles also co-occurred. Such characteristics suggest that the King George whiting is a multiple batch spawner with asynchronous development and indeterminate fecundity. Spawning occurred at least through the months of March–May, with a peak in April. On one occasion spawning occurred in the early afternoon on a flooding tide. Fish ranged in age from 3 to 17 years, and most, regardless of sex or size, showed active gonad development. Estimates of batch fecundity ranged from 5250 to 152191 hydrated oocytes per ovary, and were linearly related to fish weight and length, but less well to age. The linear relationships between batch fecundity and fish size differed significantly between the two localities.
The present study investigated the reproductive biology of blue mackerel (Scomber australasicus) off southern and eastern Australia and assessed the suitability of the Daily Egg Production Method (DEPM) for future stock assessment. This analysis revealed that S. australasicus is a serial spawner with asynchronous oocyte development and indeterminate fecundity. S. australasicus spawns between November and April off southern Australia and between July and October off eastern Australia. In southern Australia, ∼50% of males and females were mature at 236.5 and 286.8 mm fork length (FL), respectively. Size at ∼50% maturity could not be estimated reliably for eastern Australia owing to the smaller proportion of mature fish in samples. Mean spawning frequencies ranged from 2 to 11 days off southern Australia. Batch fecundity was related to fish size and mean batch size was 69 894 ± 4361 oocytes per batch and 134 oocytes per g of weight. The timing and duration of the spawning season, size at maturity, spawning frequency and batch fecundity of S. australasicus off southern Australia were consistent with those of S. japonicus in the northern Pacific Ocean. The present study's estimates of adult reproductive parameters of S. australasicus off southern Australia were suitable for the application of the DEPM for estimating spawning biomass. Collecting representative samples of mature fish from waters off eastern Australia during the spawning season is a high priority for future stock assessment of this species.
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