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Larval fish from the Bothidae family. Image credit: J. Uribe-Palomino, CSIRO-IMOS.

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Results from the National Ichthyoplankton Monitoring & Observation (NIMO) project

The seasonal occurrence of larval fish (phenology) is a useful metric for assessing the rapidly changing marine climate. Phenology of ichthyoplankton is a tangible outcome of changing fish reproduction and species distributions.

In 2015 the Australian Fisheries Management Authority (AFMA) funded a project to examine if useful trends can be gleaned from long-term observing of larval fish communities. We were tasked to assemble the historical data on larval fish collections; and to begin collecting larval fish assemblages on the monthly trips to the IMOS National Reference Stations. The existing zooplankton samples from the NRS use a smaller net with finer mesh.  When fish larval concentrations are only 1 to 2 m-3, a bigger net was needed to sample greater volumes, without compromising the logistics of the small NRS boats.

We held a workshop in Hobart in December 2015, to determine a standard list of 218 distinctive or important taxa (from 144 families) for temperate Australia (there are over 5,000 species of fish around Australia). Valuable input came from a Fisheries Research and Development Corporation (FRDC) project of larval fish distributions (Bradford & Bruce 2001), although that concentrated on 40 commercial species. 

Historical data.  We have now assembled 11 high-quality data sets from temperate Australia (see Figure 1), recording 480,000 larvae from as early as 1983, with each data set sharing the same standard species list. There were clear ‘indicator species’ across latitude along the east coast, including lutjanid snapper in the north of NSW and QLD, silver trevally and redfish at mid latitudes of NSW, and jack mackerel near TAS. A key gap in this historical data is regular seasonal sampling, to detect inter-annual trends in phenology, which is why the NRS sampling is so important.

NIMO-NRS sampling.  At five of the IMOS National Reference Stations we use an 85 cm diameter net with 0.5 mm mesh, sampling 300-500 m3 over a 10-minute tow at 3 knots.  At each NRS, three samples are collected – an offshore and nearshore sample; and a second offshore sample preserved in ethanol for future genetic analyses.

Initial analyses for just the east coast revealed that the North Stradbroke Island (NSI), Port Hacking (PH), and Maria Island (MAI) NRS have different larval fish communities than what was historically present. In particular, MAI has become more similar to northern and mid latitudes (NSI, PH), and there was evidence from MAI of a southward shift in the spawning of some temperate fish taxa: namely the appearance of larval sardine and wrasse, and an increased abundance of anchovy. Such shifts in species composition may help interpret causes of changes in adult fish communities (such as distinguishing environmental and fisheries-related causes).

Recently an intern at UNSW, Karen Pendleton submitted her report on the eastern NRS data from late 2014, 2015 and 2016. A total of 7,961 individual larvae were identified comprising 131 taxa (97 families and 1 order) across all years and sites from Sep 2014 to Dec 2016 (see Figure 2).

Karen observed a clear latitudinal pattern in abundance, species richness and diversity, with higher abundance, species richness and diversity observed at northern and mid latitudes of NSI and PH, compared with MAI. Clearly, there is a useful ecological signal, even when only sampling once per month at the three eastern NRS (see Figure 3).  Distinct larval fish assemblages were associated with each NRS region and distinctive taxa were found inshore (PH50) and offshore (PH100). Sampling nearshore and offshore stations at each NRS improves the assemblage representation each month.

Analysis also revealed assemblages associated with seasons – this is a key step that the historical data needs.  Without understanding the seasonal variation, it is impossible to estimate the significance of long-term trends. An exciting outcome will be a latitude by season matrix of some key taxa, showing the potential for a signal in larval communities to be detected as fish reproduction shifts in time and space.

Acknowledgements.  We are especially grateful for the enthusiasm and can-do attitude of the boat crews working the monthly NRS trips, led by Frank Coman (NSI); Tim Ingleton (PH); Claire Davies (MAI).  We acknowledge the financial support of AFMA and the many volunteers including Micheli Costa, Evan Leonard, Thomas Males, Harshitha Sriramachandra Kumar, Derrick Cruz and Anna Burke who have sorted and help identify samples over the past three years.

Written by Iain Suthers, James Smith, Tony Miskiewicz, Anthony Richardson, Francisco Neira and Ana Lara-Lopez

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Figure 1 Summary of historical larval fish data sets from temperate Australia

Figure 2 a) Total larvae and

Figure 2 b) total taxa from east coast NIMO, for those sorted-ID'd samples from Sept 2014 to Dec 2016 at North Stradbroke Island (NSI); Port Hacking 100 and 50 m stations (PH); Maria Island (MAI).

Figure 3 A multivariate representation of more abundant NIMO taxa (MDS) showing the difference among NRS.