Biological Oceanography (led by Iain Suthers)

IMOS larval fish monitoring project

(formerly called NIMO – national ichthyoplankton monitoring & observing)

Larval fish are sensitive to environmental changes, with many oceanographic processes influencing their distribution, abundance and survival. Most species have eggs and larvae that can be sampled with simple plankton nets in the upper mixed layer of the water column resulting in the capture of a broad suite of larval fishes (Koslow & Wright, 2016). The sensitivity to ocean oceanographic processes, and the ease of capture, make larval fish useful indicators of change. IMOS provides a national oceanographic monitoring framework, although there are many gaps in different parts of the ecological system, particularly fish reproduction.

Ichthyoplankton monitoring began at 3 east coast National References Stations (NRS) – North Stradbroke Island (NSI), Port Hacking (PH) and Maria Island (MAI) – and subsequently including Kangaroo Island (KAI) and Rottnest Island (ROT). Trials are commencing at the north Queensland Yongala site.  The initial two years of data and over 3 decades of historical larval fish data were integrated and recently published (Smith et al. 2018). Our key advance was to recognise 218 standard, distinctive and abundant larval fish taxa, and to determine indicator taxa for the eastern seaboard. However, the power to detect temporal trends in these taxa was compromised by the lack of seasonal sampling and the Tasmanian waters were not well represented in the post 1997/98 ENSO event years. Therefore, there is a need to continue, evaluate and consider on-going larval fish monitoring at the NRS, which provides a temporal backbone to the variety of broad spatial but intensive surveys undertaken by other government and university research groups. In early 2019 the IMOS-Larval Fish Monitoring distributed new 61 cm diameter bongo nets with the same mouth area and mesh size as before, and a superior flow meter (a TSK). Our vision is for larval fish monitoring to continue and investigate alternative technologies such as DNA barcoding.

Asch, R. G. 2015. Climate change and decadal shifts in the phenology of larval fishes in the California Current ecosystem. Proc. Natl. Acad. Sci. USA 112, E4065-E4074.

Koslow, J. A. & Wright, M. 2016. Ichthyoplankton sampling design to monitor marine fish populations and communities. Mar. Policy 68, 55-64.

Peabody et a. 2018.  Decadal regime shifts in southern California’s ichthyoplankton assemblage.  MEPS 607: 71-83

Smith J. A., …(20 others).. and I. M. Suthers. 2018. A database of marine larval fish assemblages in Australian temperate and subtropical waters. Sci. Data. 5:180207

Voyages on RV Investigator:

September 2017

“The whole enchilada: From production to predation in the western Tasman Sea”

The dynamic ocean habitats are plain to see by satellite and some fisheries are managed by Sea Surface Temperature (SST). Our goal was to convert this physical view to an ecosystem one, by sampling four characteristic oceanographic habitats off the New South Wales coast, and converting the biological samples into a size-based ecosystem.  Fisheries productivity is related to seasonal and environmental conditions including climate regimes. At present we have only satellite derived environmental variables of the sea surface for real-time management of fisheries such as lobster and tuna permits. With further analysis, this voyage will deliver a pragmatic, size-based ecological basis to managing our marine estate. This was a voyage of discovery, sampling 4 eddies off eastern Australia for the first time, with 5 gear types, in day and night, and using the full bioacoustics capability of the vessel. A voyage highlight was the discovery of many larval lobster in an old warm core eddy of eastern Australia.  We developed data assimilation techniques and observation directly applicable to the national ocean modelling effort. Analyses are continuing but several new range extensions for fish and squid were made.

Our findings support policymakers and the marine industry through the Integrated Marine Observing System (IMOS) and integrating disparate marine scientists. For genuine integration, in an ecosystem sense, we can now develop more realistic, ecological models that incorporate the IMOS data streams from this voyage’s size-structured approach.

June 2015

“Submesoscale processes – billows and eddies – along the productive shelf by the East Australian Current”

Frontal eddies or “billows” are ubiquitous, small cyclonic eddies <100 km in diameter, and regularly characterise the continental side of all ocean boundary currents.  They occur approximately weekly, and last up to 3 weeks which is sufficient for the early life history of fish.  The physics and biology of these ubiquitous eddies are not understood.  They are not resolved by present-day surface altimetry, but are evident along the East Australian Current (EAC) in SST or in real-time surface currents from the Coffs Harbour HF Radar (30.5°S).  We will determine if uplift within the eddy nurtures plankton in comparison to the inner shelf water; and in comparison to similar eddies offshore around the EAC retroflection.  We expect entrainment of adjacent shelf water is pre-conditioned to sustain larval fish, compared to entrainment of Tasman Sea water.  We expect the condition and size distribution (survival) of larval fish will be greater in frontal eddies than in source water on the shelf or in the EAC.  Frontal eddies may be a general mechanism for recruitment to coastal fisheries, such as for the Kuroshio Current, Gulf Stream, Agulhas Current.

Australian Nuclear Science and Technology Organisation (ANSTO) collaboration.

“From production to predation in the western Tasman Sea – assessing the trophic status of the size-structured ecosystem using stable isotope analysis”

(Suthers, Everett et al. and Dr Debashish Mazunder-ANSTO, supporting the analyses and with AINSE scholarships both Lucas Kas (honours 2018) and Peter Garside (MSc, 2018-2019)

The size-structure of aquatic systems provides a powerful means for understanding how energy flows through the ecosystem to shape the community (Treblico et al. 2016). Nitrogen stable isotopes are widely used to evaluate these predator-prey relationships and trophic structure (Fry 2006). Investigations of food-webs using stable isotopes have shown that trophic position of an individual scaled positively with body-size (Robinson and Baum 2016) (Jennings and van der Molen 2015). Relating stable isotopes to body-size provides estimates of the predator-prey mass-ratio, food-chain length and trophic transfer efficiency (Reum et al. 2015; Jennings and van der Molen 2015). These rates of pelagic energy transfer through the plankton and fish communities are relatively unknown for the Tasman Sea. A range of oceanographic habitats, used to dynamically manage pelagic fisheries (Hobday et al. 2011), especially of the valuable southern bluefin tuna (Hobday & Hartmann 2006), yet the food-web structure of these oceanographic habitats are largely unknown. By combining size spectra theory (Blanchard et al. 2017) with empirical estimates for trophic structure, we can quantify the energy flow through the pelagic ecosystem of the western Tasman Sea, and hence reduce the uncertainties around fisheries biomass..

Our overall goal is to resolve the community relationship between trophic position and body size across multiple oceanographic habitats in the western Tasman Sea. Our specific goals are:

1)            To quantify the community-level relationship between trophic position (delta 15N) and body size for the size-structured plankton and fish communities in four oceanographic habitats of the western Tasman Sea; and

2)            To determine the predator-prey relationships (community predator-prey mass ratio) of size-structured communities in the western Tasman Sea using constant and scaled fractionation relationships.

The overall goal of our voyage on RV Investigator was be to investigate the size-structure and trophic ecology of the planktonic and fish communities of the Tasman Sea. We sailed from Sydney to Brisbane and biologically characterised a range of oceanographic habitats including the East Australian Current, the Tasman Sea, cyclonic and anti-cyclonic eddies and the continental shelf.

This stable isotope analysis allows us to develop community-level body-size and trophic-level relationships for the Tasman Sea. By using stable isotopes to understand the trophic structure of the ecosystem, we can extract additional information from our samples, including estimates of food-chain length and trophic transfer efficiency (Reum et al. 2015; Jennings and van der Molen 2015).

We captured myctophids and also bathylagids; on board was Dr Andrew Stewart from the Te Papa museum in Wellington; we also found sternoptychids, Gonostomatidae/Phosichthyidae, Chauliodus, Idiacanthus, or Nemichthyidae. The composition depended on the time samples and what the scattering layer is doing.

We found that the fish from some habitats (warm core eddies or East Australian Current) will exhibit higher and less variable 15N as a result of greater carnivory, greater food chain length and greater trophic transfer efficiency, than more productive cold core eddies (e.g. Henschke et al. 2015).

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