Assessment of the effects of microPIT tags on the swimming performance of small-bodied and juvenile fish

Abstract

Monitoring the movements of fish enables management decisions to be based on the ecological requirements of the species in question. PIT tags are a cost effective, long-term method of tracking large numbers of fish. As this technology has improved, the size of PIT tags has decreased, enabling smaller species, and younger fish to be tagged and tracked. There is limited information available on the possible effects that these tags have on the survival rates and subsequent health of small fish. Swimming performance is a physiological measure that is easy to quantify in the lab and directly relates to an individual fishes health. We used swimming performance to assess the effect of microPIT tags (8.4 mm) on five small-bodied, and juveniles of five large growing native Australian fish species. In an initial trial to assess suitability for microPIT tagging, two of the small-bodied species had high mortality and were categorised as unsuitable for tagging. Sample sizes were increased for the remaining eight species to quantify potential effects of microPIT tags on swimming performance. For these eight species we found no significant effects of microPIT tags on their swimming performance.

Introduction

Monitoring large and small-scale fish movements is vital for assessing fish population dynamics, their utilisation of available resources, and to give insights into how human activities have fragmented or restored connectivity of aquatic systems (Hussey et al., 2015). Temporal and spatial data on fish movement allows fisheries management to reflect the specific requirements of different ecosystems (Cooke et al., 2016). Knowing where different species are found, and when and if they move between locations and habitats is key information for fisheries managers and research. A multitude of methods exist for collecting fish movement data (see review by Cooke et al., 2013), the cost of which generally trades negatively with the complexity of information acquired. Fish may simply be tagged with external metal or plastic tags (Harris, 1988), have their otoliths chemically labelled (Baumgartner, 2016; Cameron et al., 2016, 2011), or be tattooed with a visible implantable elastomer (VIE). These methods all provide cost-effective mark and re-capture data (Bangs et al., 2013; Hanson and Barron, 2017). At a larger cost, radio (Koehn et al., 2009; Meyer, 2017), acoustic, GPS and satellite tagging provides greater resolution of individual fish movements (Brown et al., 2010; Liss et al., 2016; Lyon et al., 2017), with tags often lasting the lifetime of the fish. Likewise, passive integrated transponders (PIT) tags (Fraiola and Carlson, 2016) provide a life-long tagging method, with low mortality rates (Grieve et al., 2018; Huusko et al., 2016; Wilder et al., 2016), and are relatively inexpensive for large scale monitoring programs.

PIT tags are a widely used method for monitoring fish movements within and between waterways (Meyer, 2017; Musselman et al., 2017; Smyth and Nebel, 2013). There are two types of transmission and tag types, full-duplex (FDX) and half-duplex (HDX). HDX systems operate in an alternating on-off read pulse. When a HDX antenna is on, the magnetic field charges the capacitator in any HDX PIT tags within range. Then when the antenna is off, or in read mode, it detects the signal emitted by the charged PIT tag. This stored energy gives HDX systems greater range than FDX systems, which transmit continuously and do not contain a capacitator in the PIT tag. This lack of capacitator enables FDX tags to be smaller than HDX tags, allowing small-bodied and juvenile fish to be tagged.

Fish species suitable for PIT tagging are limited by the ratio of tag size to body mass; this ratio is called the ‘tag burden’. The tag burden that a fish can carry without its health being compromised is species-specific (Brown et al., 1999; Jørgensen et al., 2017; Schreck et al., 2004). Generally having a dry tag mass of less than 2% of fish body mass is considered standard (Hanson and Barron, 2017; Childs et al., 2011; Winter, 1983), but tag burdens of up to 12% have been reported (see Clark, 2016 and references within). Most commonly utilised PIT tags for fish tagging are 12, 23 or 32 mm in length (0.1 - 0.5 g). Consequently, PIT tagging has been limited to fish of body masses greater than 5 g. Unfortunately, this means that a large number of small bodied fish species and juveniles of larger bodied species are often excluded from tagging studies and this represents a significant gap in our understanding of fish movement and behaviour. The relatively recent advances in the development of small (< 12 mm) microPIT tags enables us to include small-bodied (< 100 mm, 5 g) species, and juveniles of larger growing species. While microPIT tags offer the potential to study movement behaviours in small bodied and juvenile fish, there is a significant trade-off between PIT tag size and tag detection range (Burnett et al., 2013). Tag detection range is the maximum distance between the tag and the detecting antenna. Currently, the smallest commercially available PIT tags are 6 x 1 mm in size (Nonatek RFID, Lutronic International, Rodange, Luxembourge, www.nonatec.net), but are reported to have a limited detection range of 1 cm (Cousin et al., 2012). By comparison, standard 12 and 23 mm HDX PIT tags have a detection distance of up to 1.5 and 3 m, respectively (Baker et al. 2017). Intermediate sized tags (8.4 mm, 0.03 g, Biomark) offer an appropriate compromise, with detection ranges of up to 30 cm.

Although the development of microPIT tags has the potential to improve our understanding of a movement behaviours of greater range of species and juvenile life history stages, their long-term impact on the swimming performance of small (< 10 g) fish has not been well assessed. Survival and tag retention rates are commonly reported in the literature (e.g. Baras et al., 2000; Dixon and Mesa, 2011; Richard et al., 2013; Wagner et al., 2007; Ward et al., 2015; Weber and Flammang, 2017; Wilder et al., 2016), but impacts on swimming performance are scarce (Clark, 2016; Collins et al., 2013; Knaepkens et al., 2007). There was no significant effect of 12 mm PIT tags on the swimming performance of the Bullhead (Cottus gobio), which carried a tag burden close to 5% (Knaepkens et al., 2007). Likewise, there was no effect of either 8 or 12 mm PIT tags on the swimming performance of the Blackspotted topminnow (Fundulus olivaceus) (Clark, 2016), while Sockeye salmon (Oncorhynchus nerka) only showed a reduced swim performance with tag burdens of greater than 8% (Collins et al., 2013). This leads us to hypothesise that the 8.4 mm microPIT tags will not affect the survival rate or swimming performance of the small bodied, and juvenile fish of species we test.

To assess the utility of microPIT tags for fish tracking studies of small-bodied species and juveniles of larger growing species of fish, we quantified the effects of implanted microPIT tags on both sustained and burst swimming performance (Ucrit and Usprint, respectively) of ten native Australian fish species. We hypothesised that the greatest impacts associated with the implantation and retention of the microPIT in the coeliac cavity of the fish would be evident early on, and that there would be no effect of the tags on the swimming performance once the fish were fully recovered from the procedure.