This study provided the first insights into the movement ecology and novel use of pop-up satellite archival tags in any member of the sawshark family. While the low sample size represented in this study limits our conclusions on a broader scale, it provides evidence for previously undescribed vertical behaviour of the common sawshark and more importantly, the applicability of PSAT technology in sawsharks.
The common sawshark Pristiophorus cirratus displayed a considerable ability to move. The first transmission date of the tags when paired with archived tag data allowed us to ascertain a conservative distance of animal movement over time. This track displayed a minimum movement of 70 kms in three weeks, or 3 km day− 1 in one individual. Previous research on other small benthic sharks, such as the Port Jackson shark Heterodontus portusjacksoni, found rates of movement around 1.8 km day− 1 (42) and 6.5 km day− 1 (43). Furthermore, a recent study investigating long-term migrations of Port Jackson sharks found that they can move up to 19.5 km day− 1 and move distances greater than 600 km in migratory events (44). Therefore, future research into sawsharks should aim to monitor individuals for evidence of philopatry or migratory events with a longer-term tracking study.
Diel vertical movement is a common phenomenon observed across a broad range of marine taxa (45–52). A number of shark species display diel vertical movements correlated to ascension at night and a return to depth at day (53–57). Our data suggest that sawsharks may employ a similar pattern of movement. One of our tagged individuals displayed regular vertical movements in the water column by ascending during the night in approximately 12- and 24-hour cyclical patterns and returning to what we assume is the sea floor at night. Similar diel movements were observed in another individual. However, the third individual displayed a very different vertical pattern of movement. Diel mediated vertical movement patterns are common in large epipelagic fishes (50, 51), however, this phenomena is not well documented in small, benthic-associated fishes (45). Current literature suggests the common sawshark feeds primarily on benthic primary consumers (26), so it is plausible the observed vertical movements are predatory events following the well documented diel movements of primary consumers (58–60). Furthermore, similar ‘yo-yo’ vertical movements where the animal makes regular rapid vertical ascents then descents have been linked in other shark species for prey detection (51). Our present dataset, however, is lacking the resolution required to unequivocally link a driving factor for such movement and should be a focus of future research for sawsharks.
The impacts of sawsharks employing vertical movements may have greater implications for the movement of nutrients across their distribution. Research has shown that predators are capable of rapidly altering nutrient cycling rates through direct excretion and through nutrient translocation (61). The consistency of ascension to five meters depth by of one of our tagged individuals suggests that common sawsharks may move from richer, shallower coastal shelf waters to the deeper continental slope benthos may play an important role in nutrient cycling of these ecosystems (61, 62). Clearly it is difficult to make too many assumptions on such a limited dataset and more tagging is required to understand the importance of this behaviour to the ecology of this species, sawsharks and their respective ecosystems.
The behavioural changes observed in our tagged individuals post-release suggested a potential impact of the tagging event on the behaviour on sawsharks. All three individuals displayed limited vertical movement during this period and a progressive movement towards deeper water, potentially as a post-release response to capture. Presumably, this could be a defence mechanism where moving to deeper waters may provide increased protection against visual predators potentially due to lower light levels (50, 63). This response may also be related to behavioural, physiological and biochemical changes such as those observed in a range of species in relation to capture induced stress (64–67). These include blood chemistry parameters such as lactate or pH, which have been correlated to irregular behaviour and even to moribund fish (48, 68, 69). Furthermore, these effects have been observed to have lasting sublethal effects that may affect the fitness of released fish (65, 66). Capture induced stress and subsequent effects are species specific and likely mediated by basic biological functions, allowing for better adaptions to capture (i.e. buccal pumping allowing for oxygenation when movement is limited by fishing gear). However, it is still unclear what effect fishing has on the physiology on sawsharks and their resilience to capture and subsequent release. Areas of future study could include monitoring and comparison of blood physiology parameters under different fishing techniques to allow a better understanding of technique specific responses, which could then allow for better understanding of survivability post-release.
It is possible that one of our sharks perished during the monitored period. Mortality events are an inherent risk of studying sharks that first must be captured for tags to be fitted. One study estimated that sawsharks in the gillnet fisheries have an approximate 50% mortality rate post-release, thought to be due to physical damage received in the gillnet (70). Mitigation of capture stress to promote survivability of fish post tagging has seen increasing attention (40, 71). Novel techniques such as releasing tagged sharks in cages where the door is pressure released on reaching the seabed has seen some success by providing animals shelter while they recover (40). However, our current understanding of sawshark biology and ecology is lacking the fundamental information to understand what the main drivers of stress and eventual mortality in released sawsharks are.