This study investigated lanternfish ecology in three oceanic regions: waters of the Subantarctic Front around Macquarie Island, waters of the Subtropical Convergence in the southern Tasman Sea abyssal basin, and tropical waters of the Coral Sea. Investigations were made at the organism-level, the species-level, and the assemblage-level. These ecological studies were drawn together and combined with historical records of lanternfish distribution in a zoogeographic analysis for oceanic waters off eastern and southeastern Australia.
At Macquarie Island, lanternfishes, which are important prey for demersal and diving predators, have spatial patterns of distribution over the Macquarie Ridge that suggest biomass is enhanced where the Subantarctic Front (SAF) interacts with a break in the topography of the Ridge (the ‘Ridge Gap’). The summertime lanternfish assemblage, documented for the first time, comprised 23 taxa, dominated by Krefftichthys anderssoni and Gymnoscopelus braueri. Mean lanternfish biomass was highest (3.13 g 1000 m–3) in the Ridge Gap habitat. Lowest mean biomass (0.71 g 1000 m–3) was recorded up-current of the SAF over the Abyssal Plain and intermediate biomass (1.26 g 1000 m–3) was recorded over the Macquarie Ridge. At Ridge Gap, a high abundance of K. anderssoni was recorded in the shallowest stratum (0–250 m) during the day. I hypothesise that the oceanographic–topographic interaction between the SAF and Ridge Gap creates eddy systems and productivity fronts that passively entrain and/or actively attract lanternfishes to the Ridge Gap area. This oceanographic–topographic interaction depends on the spatial stability of the SAF in relation to the Macquarie Ridge and Ridge Gap and is vulnerable to climate-mediated change that may have flow-on effects to predators with commercial and conservation significance.
A cross-basin (longitudinal) study of lanternfishes in the southern Tasman Sea abyssal basin between Tasmania and New Zealand during the austral winter found that an east to west pattern in biomass exists that corresponds with the cross-basin pattern in production and indices of water mass energetics. Vertically stratified sampling revealed that a portion of the population, dominated by large individuals, did not vertically migrate at night. The dominant lanternfish species in the abyssal basin differed from those on the adjacent Australian continental margin. Winter δ13C and δ15N isotopic signatures in lanternfishes and macrocrustacea indicated potential differences in foraging habitat among species and a difference in 15N regimes from east to west across the basin. Trophic level estimates indicated feeding by lanternfish species through a continuum of niches through the third trophic level, and niche differentiation among species was most pronounced in the Eastern and Central sectors of the basin. The results suggest that pelagic habitats and processes may differ between the Western and Eastern sectors of the basin, which has implications for trophodynamic modelling. The study identifies scales of spatio-temporal heterogeneity that need to be considered for monitoring studies in this oceanographically complex region, and highlights the challenges associated with developing an understanding of pelagic ecosystem function at the basin scale.
In the Coral Sea, a lanternfish spawning aggregation was sampled directly for the first time. The Dana lanternfish (Diaphus danae) is the only species forming the aggregation sampled in the austral summer of 2010. The discovery of this species in the Coral Sea expands the published northern-most occurrence of the species of northern Australia by approximately some 2,000 km. Male and female D. danae in the aggregation occurred at a ratio of 23 to 1 and occupied in two non-overlapping size classes (males 71.2–95.1 mm SL, females 99.0–121.4 mm SL). Males in the aggregation were estimated to be in the 1+ to 2+ year age-classes and females in the 2+ to 3+ year age-classes. Hydrated oocytes with oil droplets, that indicated imminent spawning by females, were in higher abundance in the first trawl made through the aggregation (21:34–22:34 hrs) than in the final trawl of the evening (03:24–04:24 hrs). Maximum estimated female D. danae fecundity (25,803) and gonadosomatic index (34.01) are higher than any other lanternfish species recorded. Aggregations of spawning yellowfin and bigeye tuna feed intensively on the Diaphus danae spawning aggregation. The number of D. danae in bigeye tuna stomachs retrieved from 2009 aggregations, estimated from intact specimens and otoliths, ranged from 75 to 319. Data on bigeye tuna landings and commercial fishing vessel movements are examined to provide indicators of the spatio-temporal distribution of lanternfish-tuna aggregations. The annual Coral Sea D. danae aggregation is the only confirmed lanternfish spawning aggregation in Australian waters. Furthermore, the annual aggregations of yellowfin and bigeye tuna that feed intensively on the D. danae aggregation are the only known tuna spawning aggregations in the Eastern Tuna and Billfish Fishery. The lanternfish-tuna aggregation represents a Key Pelagic Ecological Feature and may play an important role in the life cycle of tunas and other pelagic predators, and replenishment of down-current populations.
This study derives a lanternfish zoogeographic scheme for oceanic waters off eastern and southeastern Australia for the first time. Lanternfish species records (spanning a period from 1928 to 2010) were compiled from tropical to subantarctic waters (10˚S to 57˚S). Two modelling methods were used to deal with the two types of data available: depth-stratified ‘horizontal’ trawls (presence-absence data) and ‘oblique’ trawls (presence-only data). First, binomial Generalised Additive Models (GAMs) were employed to investigate species-habitat relationships and depth interactions of environmental covariates in three species (Electrona risso, Diaphus mollis and Diaphus leutkeni) for which presence-absence data were available from horizontal trawls. This step refined the selection of environmental covariates for modelling of all species over the full study area required presence-only modelling (MAXENT) that combined species presence records from horizontal and oblique trawls. A four-region zoogeographic scheme is hypothesised: Coral Sea region, Subtropical Lower Water region, Subtropical Convergence/South Tasman region and Subantarctic region. Regions are placed into context of two prevailing biogeographic schema for the pelagic region that are derived from physicochemical parameters. Lanternfish zoogeographic regions are congruent with some aspects of both existing physicochemical schema, but neither existing schema alone fully encapsulates the proposed lanternfish regions. The major frontal systems of the Tasman Front, Subtropical Convergence and Subantarctic Front represented zoogeographic boundaries. An additional boundary was identified at approximately 25˚S (coined the ‘Capricorn’ boundary herein) that is congruent with a previously defined water-mass discontinuity and biogeographic boundaries in other marine groups.