Distribution, abundance and reproductive biology of Pseudeuphausia latifrons and other euphausiids on the southern North West Shelf, Western Australia

We examined the distribution and abundance of euphausiids on the southern North West Shelf of Western Australia over two consecutive summers (1997–1998 and 1998–1999) using submersible light traps. These were deployed at the surface and at depth along one cross-shelf transect in the first summer, and along two cross-shelf transects and one long-shore transect in the second summer. The euphausiid component of samples was predominantly Pseudeuphausia latifrons (>98% of euphausiid catches), whereas Euphausia hemigibba,E. recurva and Thysanopoda tricuspidata were relatively rare (0.96, 0.49 and 0.0003% of euphausiid catches, respectively). There were strong depth and cross-shelf trends in catches during both summers, with the highest abundances of P. latifrons occurring in deep traps at mid-shelf stations where water depths ranged from 20 m to 100 m. E. hemigibba, E. recurva and T. tricuspidata were most abundant in deep traps at offshore stations. P. latifrons was less abundant in the first summer, which was characterised by El Niño conditions with extensive upwelling, cool water temperatures, high chlorophyll concentrations and an abundance of both meso- and macrozooplankton. In the second summer, higher catches of this species occurred during La Niña conditions of limited upwelling, warm water temperatures, lower chlorophyll concentrations and decreased meso- and macrozooplankton abundances. P. latifrons was reproductively active throughout the survey period, with higher proportions of ovigerous females (mean ± SE 29.0% ±5.0%, n=12) in samples at the beginning of the second summer than in later months (mean ± SE 10.9% ±2.5%, n=19) or during the first summer (mean ± SE 10.6% ±1.9%, n=19). Ovigerous females were more abundant in catches from deep (17.4% ±3.1%, n=27) than from shallow traps (13.2% ±2.7%, n=20) over the two summers. Histological examination of the gonads of P. latifrons showed a pattern of continuous oogenesis and spermatogenesis throughout the second summer, suggesting the production of multiple broods. Brood sizes of this species averaged 83.1 (±3.0 SE, n=93) eggs, and mean egg size was 0.286 mm (±0.001 mm SE, n=347) and spermatozoa diameters averaged 5.45 µm (±0.05 µm SE, n=70) in the vas deferens and 5.37 µm (±0.05 µm SE, n=70) in the spermatophore sac.