**4.2.1 Tagging of animals**

Using the jigging fishing method (Figure 7), squid at depths >60 m can only be caught at night, using powerful lights to attract them to the surface. For the manual tracking study, squid were caught from an 8 m inflatable boat anchored next to a chokka boat. The two boats were close enough for the chokka boat lights to attract squid to the area around the smaller boat. Two squid were caught in this manner, on separate nights, and tagged with V9TP-6L continuous sensor transmitters. Details of the transmitters used are given in Table 2. Animals were tracked (Figure 11) from the time of tagging to shortly after sunrise. The tagging method and instrumentation used was the same as that described for the VR2 and VRAP studies.


Table 2. Details of acoustic transmitters used in the VR100 tracking study

#### **4.2.2 VR100 data analysis**

The VR100 data was manually examined, using Microsoft Excel, for erroneous depth and/or temperature data. Erroneous data were identified by their large difference from previous and successive values, whereas these were similar. Those data entries containing errors

The Use of Acoustic Telemetry in South African Squid Research (2003-2010) 435

Each of the systems described here (VR2 receiver arrays, VRAP system and VR100 manual tracking system) have various advantages and disadvantages. VR2 receiver arrays are ideal for studying movement and behaviour on a spawning site (Downey et al. 2010), homing behaviour (Mitamura et al., 2005), movement in a river (Carr et al., 1997) or straight (Welch et al., 2004) and movement within a marine reserve (Egli & Babcock, 2004), to name a few examples. These receivers allow researchers to monitor a large area (depending on the number of receivers used) continuously and for long periods of time. Depending on the study area, the geometry of the array can be selected to maximize coverage in critical sites, providing information on the entering and exiting of a specific area (Egli & Babcock, 2004). Range tests can be used to determine the maximum and minimum receiver ranges at a specific location and using specific transmitters (Singh et al., 2009). Placing the VR2 receivers in such a way that the receiver ranges of individual VR2s overlap, maximises the likelihood of a tagged animal being detected when in the area. VR2 receivers can be used to determine direction of animal movement to a certain degree, depending on the design of the array and the study site itself. These receivers are however, more often used to collect presenceabsence data and it is not known where in the array the animal is situated. As the VR2 receiver is programmed to work on a single frequency, there is a limit to the number of transmitters that can be introduced into the system at one time. As previously mentioned and as described by Singh et al., (2009), transmitters send out a series of pulses known as a 'pulse train'. Only when all the pings are recognised in sequence by the receiver, is the pulse recorded as a signal detection. The overlapping of 'pulse trains' from two or more transmitters results in no signals being detected. As the number of transmitters in a system increases, so it is possible for the number of successful detections to decrease. However, as the data can only be downloaded once the receiver is retrieved, it is not possible to discern how many transmitters are present in the area using the VR2 receivers. It is therefore necessary to use a VR100 to monitor 'system saturation' (Singh et al., 2009) before introducing more tagged animals into the system. Another method to reduce the number of signal collisions is to programme transmitters with longer off times. However, the speed with which the study species moves needs to be taken into consideration, to prevent an

**5. Comparison of the various telemetry systems** 

animal moving through an array too quickly to be detected.

The VRAP system differs from the VR2 receiver array in that data recorded is transmitted to a land-based station and the movement of tagged animals in the study area can be observed in real-time. In addition, the direction of movement and location of a tagged animal within the array can be monitored and recorded. One major disadvantage of the VRAP system when compared to the VR2 receiver array is the size of the area that can be monitored. In the study discussed here, the 300 m equilateral triangular configuration resulted in the buoy triangle covering an area of ~ 400 m2. As previously mentioned, accuracy decreases outside of the buoy triangle. In addition, when a transmitter is directly behind a buoy, no position can be calculated (Aitken et al., 2005). Shadow zones (areas along parabolas behind each buoy) also exist. Two positions are calculated for transmitters in this area. The VRAP software assumes the calculated position closest to the last valid position fix is correct and this value is plotted. As for the VR2 receiver arrays, it is also possible for 'system saturation' to result in a decreased number of successfully detected signals. As the VRAP system is used to monitor tagged individuals in real-time however, the number of tagged animals present within the area can be observed before introducing more tagged individuals. The

were removed before plotting. Depth and temperature data were plotted against date and time, allowing for analysis of the vertical movement of squid on the deep spawning grounds. Depending on the strength of received signals, a strong signal indicating the tagged animal to be in close proximity, VR100 GPS coordinates were integrated into Arcview GIS. This allowed for an analysis of the horizontal movement of tagged squid on the deep spawning grounds.

As this is an ongoing study, only initial findings are discussed here. The large male remained in the upper 40 m of water from the time of release until just before sunrise. As the sky turned pink in the east (dawn) the squid quickly moved to the bottom, where it remained until tracking was terminated. Similarly the sneaker male remained at depths 40 to 80 m from the time of release until dawn when it too moved to the bottom, remaining there until the termination of tracking. Both animals remained on the midshelf, directly off Cape St Francis point (Figure 1), with the large male covering an area ~ 3.311 km2 and the sneaker male an area of ~ 1.29 km2. Both animals moved continuously until settling on the bottom at sunrise, where they remained fairly still. During these movements the tagged squid were exposed to water temperatures of 15 to 19 °C, and 11 °C when settling on or near the bottom.

Fig. 11. Active tracking using a VH110 directional hydrophone, held in the water, and a VR100 receiver
