In 2018 we completed the fourth round of annual performance assessment surveys for spotted handfish across the 9 Derwent Estuary study sites. This provides 62 data points in total, with 36 points from our NESP work building on our 26 data points from previous studies. Two results of interest from this year were the discovery of fish, after a year’s absence, at the Ralphs Bay site and a continued decline in numbers at Mary Anne Bay. For the Ralphs Bay site the fish are still in very low numbers, which is consistent with what we have observed since 2015. This suggests that when densities of fish dip below 3-5 fish per hectare then the monitoring program does not have the sensitivity to reliably detect their presence. For the Mary Anne Bay site the recent trend has seen a decline in numbers back to levels that are more commonly seen at numerous other sites. Several student projects this year provided important context to both this results and conservation of the species in general.
The first of these were some preliminary results from Mr Alex Hormanns’ UTAS Masters project. This involved the planting of 5000 Artificial Spawning Habitats (ASH) into 5 site based arrays, made up of 50:50 mixes of plastic and ceramic ASH. When we surveyed the arrays numerous eggs masses were being guarded by fish who had spawned onto ASH, with the fish showing a preference for ceramic over plastic ASH. The relative use of ceramic ASH was also higher than the raw numbers suggested as ceramics had a lower survival rate than plastic, with 1964 plastic and 1524 ceramic ASH or 3488 out of the 5000 remaining at the end of the surveys. One variable that seems to explain the amount of ASH used at a particular site was the densities of stalked ascidian which provide natural spawning habitat. Sites that had ascidian densities larger than 0.05 per m2 saw very low ASH use. The sites that saw the most ASH use had relatively low densities of both ascidians and spotted handfish. We have now incorporated ascidian counts into the monitoring program and will use these data to plan further ASH planting.
The second piece of information was from Mr Tyson Bessell’s UTAS honours project (first class), ‘Biological Parameters of the Spotted Handfish’. This work demonstrated the accuracy of the I3S autonomous pattern recognition program for identifying spotted handfish – this allowed us to exclude technical issues as a reason for low recapture rates. Tyson then used both recapture data of repeated length measurements of wild fish in combination with a small number of opportunistically collected otoliths to model the age of the fish. The oldest fish in the population appear to be 10 years old, however based on length frequency data only 10% of the fish within the sample population are older than 5 years. Sexual maturity occurs around 2 years of age so most fish will have a 1-3 year window of opportunity to reproduce. Important caveats to this work are that it is based on a limited number of both recaptures (n=13) and otoliths (n = 7) and only one otolith was from a female.
From our new insights into handfish biology and conservation we are starting to be able to develop an understanding of the species local population dynamics. Aspect of our monitoring program that have previously remained un-explained were: trends of declines across years, variability between years and increases in densities between the current and historic data, as well as low recapture rates. These site population dynamics and re-capture rates may now be explainable based on the relatively short lifespan of the species and spawning success related to the availability of natural or artificial spawning habitat. While our monitoring program can provide density estimates, the likelihood of recapturing an individual with is low due to rapid natural mortality (90% dead two years after first capture) when placed in the context of annual sampling. Also, if stalked ascidians density are by nature stochastic then there will be problems with recruitment, especially if stochasticity increases or there is a long-term average decrease in ascidians or other natural spawning habitat due to grazing by introduced marine pests or changes to catchment processes. If spawning fails or is reduced then declines over time periods 1-4 years will occur as cohorts pass through their breeding period and natural mortality cumulatively removes most individuals from local populations.
This relatively short lifespan is also of importance to efforts to captive breed the species. While animals bred in captivity in 2017, none did in 2018. Clearly solutions to enhance breeding will need to be found quite quickly before the brood stock and captive bred fish die.