This study demonstrated that a number of movement patterns are exhibited by tagged blue-spotted flathead (Platycephalus caeruleopunctatus) found on soft sediments in Jervis Bay. Over a daily timescale, all fish in our study used small relatively compact areas each day when actively tracked across daylight hours. Over periods of up to 60 days, blue-spotted flathead in our study showed two broad movement patterns; three out of five tagged fish showed strong site attachment and were detected on each day of tracking within the Hare Bay no-take sanctuary zone. The remaining two fish appear to have moved much larger distances of more than 3 km away from tagging location. Given the perception that soft sediment fishes are unlikely to show site attachment [7], and observations that blue-spotted flathead can be strong active swimmers (Fetterplace personal observation from baited underwater video; see data and materials section), it is particularly interesting that the majority of tagged fish in our study showed such strong site attachment. The ability of blue-spotted flathead to target many types of prey [19] coupled with the expected ambush predation by flathead species in general [20] could explain why blue-spotted flathead generally utilise relatively small areas over a day. Why some individuals continue to show this compact space use over periods of 60 days and others move away is not clear.
Intriguingly, the movement patterns of the oceanic blue-spotted flathead assessed in this study are consistent with those for estuarine dusky flathead (Platycephalus fuscus) found in southern Australia [21]. Dusky flathead were found to be largely sedentary, often remaining in one section of Gippsland Lakes for months. A small number of dusky flathead, however, were recorded moving up to 30 km over a few days. The use of active tracking in our study provided high-resolution movement and space-use patterns over a much smaller scale (10–100s of metres). Unexpectedly, and contrary to suggestions that fish on soft sediments would likely move over larger distances than those on hard substrata [7], blue-spotted flathead in our study also exhibited short-term site attachment comparable to many temperate reef fishes (e.g. [11, 22]). In addition, blue-spotted flathead MAI of 22.11–44.96 m h−1 (mean ± SE = 29.34 ± 4.15) is much lower than the reef-associated luderick (Girella tricuspidata, 165.4 ± 74.87 m h−1; mean ± SE) assessed within the same embayment and with the same tracking technique [23].
Two fish were lost from the study after 4 and 25 days. This was despite extensive searches of at least 3 km from their last recorded positions. The underlying reason for this is unclear but could conceivably include capture, tag failure, predation, or movement out of the study site. Our observations suggest that blue-spotted flathead are robust and survive surgery well; they recover readily from anaesthetic and, lacking a swim-bladder, are unaffected by barotrauma. Previous tagging effects studies have indicated that ‘tagging-induced’ mortality tends to occur within the first 24 h after release [24]. Four out of five of our tagged fish were detected moving up to 25 days after surgery. This suggests that mortality from surgery in our study was unlikely. We would argue instead that the two fish that were not detected for the entire study simply moved out of the study area. Capture is unlikely, at least in the study area, due to the study area being in a no-take sanctuary zone. As these two fish may in fact have travelled outside of tracking range, it follows that some part of the population moves much greater distances than the averages estimated here. Why they moved remains unclear and as our study is preliminary with a small sample size it not possible to estimate exactly what portion of the blue-spotted flathead population makes these larger-scale movements or how large these movements may be.
The larger-scale movements shown by two fish do not appear to be driven by size, as both small and larger fish left the study area and conversely both small and larger fish also showed site attachment. As it is not possible to distinguish the sexes of blue-spotted flathead based on markings or size (they are not known to show sexual size dimorphism), it is more difficult to assess whether these movements may be related to the sex of the fish. Many fish make seasonal migrations at specific times of year (e.g. [25]), and the closely related dusky flathead have been reported to seasonally migrate in order to spawn, based on indirect evidence such as aggregation sightings and the capture of spawning females around the mouths of estuaries [28]. While blue-spotted flathead are thought to spawn year round [26], there are no published evidence to support this and no evidence of migration movements to date. Further investigation is required to determine whether or not the larger movements shown by some of our tagged fish are just roaming movements over scales greater than our study size or are linked to spawning movements.
We did not catch any blue-spotted flathead on, or detect tagged fish blue-spotted flathead moving onto seagrass or surrounding reef, suggesting that they are exclusively soft sediment fish. Our movement data supports findings of recent baited remote underwater video (BRUV) studies where no blue-spotted flathead were recorded on reef within Jervis Bay (Rees, Davis and Knott, unpublished data and Coleman et al. [27]). However, other BRUV studies have found very small numbers of blue-spotted flathead on reef habitat; for example in Batemans Marine Park, Kelaher et al. [28] recorded blue-spotted flathead on five out of 384 drops over 5 years; this raises the possibility that blue-spotted flathead occasionally venture into edge areas of reef and seagrass habitats or reside there in very low numbers.
Many studies on the effectiveness or impacts of MPAs have focused on changes in abundances and diversity, without taking into account critical information on movement patterns of the species within them [2, 29]. This is often because this information is not available or because while potentially very useful, quantifying the movement patterns and observing the natural behaviour of marine fish in the field is difficult to achieve. Without knowledge of the basic movement patterns of a species, it is difficult to predict effectiveness of spatial protection measures such as MPAs [6]. Our study indicates that no-take sanctuary zones protecting soft sediment habitats in JBMP appear large enough to adequately encompass the expected short-term movement of blue-spotted flathead exhibiting site attachment. However, our data suggest that two movement patterns are likely to exist within the population, one that is highly site attached, and thus would potentially benefit from MPAs, and one that tends to roam, and thus may not benefit as much. If these preliminary data are found to be representative of longer-term patterns of movement and activity space use by a large part of the blue-spotted flathead population, then it is likely that the Hare Bay no-take sanctuary zone is sufficiently large to provide protection for a large number of blue-spotted flathead. If this is the case, we would suggest that comparably sized zones on soft sediments in other areas of temperate Australia may also be appropriate. Though it is beyond the scope of this study, investigating what portion of the blue-spotted flathead population would need to show site attachment for spatial closures like MPAs to be effective will require tagging of much larger numbers of fish and deserves further attention.
As this investigation was a preliminary assessment for movement of blue-spotted flathead with a view to expanding the duration and area of coverage, the current study has a number of implications for design of a large-scale tracking array. As a large tracking array can be expensive and time-consuming to install, our data provide guidance to best place passive receivers to cover this movement most efficiently. Our results indicate that using a tightly spaced passive acoustic array for investigation of the movement of this species is feasible and would yield meaningful results. However, given the potential wider ranging movements of this species, using multiple approaches would be useful to provide a more comprehensive understanding of their movement patterns. At the current study site, the entrance to Jervis Bay has now been gated and an array of receivers placed around the edge of the bay. These extra receivers (also part of other ongoing studies) should provide a good idea of visitation to other sections of Jervis Bay and also detect if fish leave Jervis Bay.