Movement, habitat utilisation and population status of the endangered Maugean skate and implications for fishing and aquaculture operations in Macquarie Harbour
University of Tasmania (UTAS)
Little is known about the life history of the endangered Maugean skate but it does appear to have a very restricted distributional range (Macquarie and Bathurst Harbours, the latter a marine protected area) and in all likelihood a small population size. The expansion of marine farming in Macquarie Harbour represents a key element of the salmonid industry’s plan to significantly increase Atlantic salmon and rainbow trout production in Tasmania. In considering the planning application, the MFPRP identified that research is required to more properly understand the ecological effects of the proposed farming operations, including potential impacts on the Maugean skate. Key research priorities identified by the MFPRP were the need to describe distribution, abundance and general ecology of the species, such information being a precursor to determining the nature of interactions with marine farming operations. Furthermore, the Panel noted the need to better understand the potential effects on the Maugean skate of efforts to recover escapees through fish-down using gillnets. This concern links to the more general population risk posed by gillnetting, noting that skate are taken incidentally in commercial and recreational gillnets and that gillnetting activity is strongly associated with targeting of escapees. By employing acoustic tracking and non-destructive biological sampling techniques, this project will provide the science to understand the nature of interactions between fishing and aquaculture activities and the Maugean skate as well as enhancing our knowledge about its conservation status. Through such understanding it will be possible to evaluate risk and develop strategies to manage interactions and, where necessary, implement measures to mitigate negative impacts on the skate. This proposal addresses RD&E priorities relevant to understanding the impacts of aquaculture due to expansion as well as mitigating the impacts of fishing on threatened, endangered and protected species.
1. Determine the distribution, habitat utilisation and movement of the Maugean skate in Macquarie Harbour.
2. Determine the key biological characteristics of Maugean skate, including population size, reproductive dynamics and feeding habits.
3. Describe the spatial and temporal dispersal patterns of salmonid escapees in Macquarie Harbour.
4. Assess the potential impacts of current and proposed marine farming operations on the Maugean skate population.
5. Evaluate strategies to reduce the probability of encountering Maugean skate whilst fishing (gillnetting) for escapees.
Principal Investigator: Jeremy Lyle
Key Words: Maugean Skate, Zearaja maugeana, Macquarie Harbour, telemetry, gillnet bycatch, salmonid
aquaculture, salmonid escapees.
Maugean Skate generally displayed a high degree of site fidelity, with 50% and 95% utilisation
distributions generally <3 and <10 km2 respectively. Many individuals showed an affinity for the
Liberty Point/Table Head region, which is located in the central, south western side of Macquarie
Harbour. Over half of the tagged skate left their core home range for brief periods (days to weeks)
during the study period with all but five (15%) returning to their home range. These latter individuals
relocated to new sites, often after a period of relatively extensive movement throughout the harbour.
Although several skate were detected at the entrance to Macquarie Harbour all were re-detected inside
the harbour afterwards with no evidence to suggest long-term movement out of the estuary. None of
the skate were detected at the Gordon River mouth suggesting they do not venture into the main
tributary that feeds the harbour.
Based on the number of detections, Maugean Skate spent the 85% of their time at 6–12 m depth,
although they were detected from 0.6 m to >55 m, albeit rarely, indicating they are not restricted to
their preferred depth range. Skate depth utilisation appears to be dictated by water chemistry with
shallow waters having low salinity and high temperature variability, whereas deeper waters are stable
in terms of temperature and salinity but have low concentrations of dissolved oxygen (DO) (<20%).
Low DO concentrations appear limiting for the skate and presumably their prey. Waters in their
preferred depth range tend to have relatively stable temperature (12–15 oC), salinity (18–27 ppt) and
generally retained moderate dissolved oxygen concentrations (>30%).
Maugean Skate were more active during the night and moved into shallower water, which possibly
represents nocturnal foraging behaviour. Maugean Skate have a restricted diet dominated by three
groups of epibenthic crustaceans, namely crabs, carid shrimp and mysids. Fish represented a minor
prey item. While there was no evidence of pellet feeding, this cannot be ruled out since sampling was
conducted some distance away from the farm lease sites and skate tend to have small home ranges.
Males and females matured at significantly different sizes; 50% maturity was attained at 632 mm TL
in males (based on clasper size and condition) and 662 mm TL in females (based on maximum follicle
diameter). Endocrinology generally confirmed these estimates, with females >680 mm displaying an
increased levels of testosterone and progesterone and males >620 mm displaying an increase in
testosterone. Maximum follicle diameter and hormone levels (testosterone, progesterone and 17β-
estradiol) of mature females were highly variable seasonally suggesting an asynchronous,
discontinuous reproductive cycle in which a proportion of the population is reproductively active
while the remainder are in a resting phase at any given time of year. There was, however, some
evidence that reproductive activity may be reduced during summer.
Preliminary estimates of age from sectioned vertebrae for thirteen Maugean Skate suggest that the
species is relatively short lived (maximum age observed of 11 years) but may live to about 15+ years.
Maximum age (and size) is a useful proxy for productivity and our results suggest that Maugean Skate
are probably relatively productive.
The population of Maugean Skate is possibly one of the smallest of any chondrichthyan species. The
species is only known from two Tasmanian estuaries and when considered in the context of their
preferred habitat (predominantly 5–15 m) means they probably also have one of the smallest
distributions of any chondrichthyan. The best estimate of the population size in Macquarie Harbour
was in the order of 3000 individuals. There are, however, potential biases in this assessment that
suggest it may be an underestimate and thus a feasible minimum possible population size.
Atlantic Salmon and Rainbow Trout dispersed rapidly upon release, moving widely and generally
randomly throughout Macquarie Harbour. Several Rainbow Trout and a single Atlantic Salmon
showed an affinity for regions near aquaculture leases suggesting at least some of these individuals
were feeding on aquaculture overfeed. The vast majority did not, however, survive or remain in the
harbour for longer than about two months following release. As a general observation, Rainbow Trout
tended to survive slightly longer than Atlantic Salmon. While about 25% of the escapees were
recaptured by recreational fishers (in gillnets), most are assumed to have died of natural causes
(starvation and possibly predation). About 20% of the salmonids did, however, leave the harbour
either by moving out to sea (mainly Atlantic Salmon) or entering the Gordon River (Rainbow Trout).
The fate of these individuals could not be assessed.