Use of Spray Marking as a Tool to Mark Least Chub (Iotichthys phlegethontis) for Mark-Recapture Studies

Authors: Jordan Holcomb, Keith Lawrence, Alan Ward, Jack Dudding, Weston Pearce

Year: 2021

Abstract

Knowing the number of individuals in a fish population can greatly enhance a manager’s abilities to make informed decisions for fishes. Estimation of population size is widely used for sportfish but somewhat less common in small, rare nongame fishes. The Least Chub Iotichthys phlegethontis is a small cyprinid (<65 mm total length) endemic to the Bonneville Basin. Historically, Least Chub were widespread in marshes and rivers across the Bonneville Basin but they have been widely extirpated and now only occur in a small number of isolated populations in Utah. The Utah Division of Wildlife Resources maintains experimental introduced populations for each wild population. Utah Division of Wildlife Resources annual monitoring protocols utilize collapsible mesh minnow traps to capture Least Chub at wild populations. Ratios of juveniles (< 35 mm total length) to adults (>35 mm total length) are obtained by measuring 100 fish per monitoring site. These ratios are paired with catch per unit effort measured by total number of fish per trap hour for each monitoring site to track populations through time. These metrics have not been formally evaluated to see how they translate to whole population level estimates. We sought to identify a batch marking technique that did not require handling of every fish allowing for a mark-recapture study to eventually evaluate total population size of wild populations. This would facilitate comparison of juvenile:adult ratios and catch per unit effort data with population estimates through time. Spray marking is a method that consists of spraying fluorescent dye granules at high pressure to embed in the scales and skin of fish. Spray marking is already utilized in state of Utah hatcheries to batch mark fingerling salmonids and has been used successfully on other small desert fishes (Douglas et al. 2001, The SW Naturalist 46[2]:141-150). One of the more robust experimental introduced Least Chub populations was selected to test spray marking as marking methodology. Spray marking equipment consisted of a gas powered air compressor with variable air pressure settings connected to a Porta-Blast (Porta-Blast, Singapore, China) paint sprayer. Spray marking dye granules were obtained from Fluomark (Fluomark, McCall, Idaho, USA). On the first marking occasion, we set 21 minnow traps for 1.75 hours and captured 1766 Least Chub (948 adults and 818 juveniles) and 100 fish were measured. Fish were held in 8 aerated buckets with frozen water bottles. We sprayed 100 fish with dry dye granules at 120 and 100 psi, respectively. Mortality was 100% and immediate at both pressures. Next, we sprayed about 80 fish at 100 psi with dye granules in water (wet dye) at 45% dye concentration and only juveniles experienced immediate mortality while adults survived. Fish were then visually sorted by adults and juveniles and all but 50 adults were sprayed at 100 psi with 45% wet dye concentration. The 50 adults not sprayed (control) and 50 spray marked adults were held in separate minnow traps placed in the refuge pond for 48 hours to evaluate post spray mortality and marking success. Mortality was 19.6% and 53.1% in control and spray marked groups, respectively. Mark retention was 98% and marks were evaluated with a uvBeast 365nm UV flash light (uvBeast, Portland, Oregon, USA), a different 365 nm black light flashlight, and 400 nm blue light flashlight with UV blocking glasses. A pop-up ice fishing hut used as a dark room. All lights fluoresced the marks and the uvBeast was the brightest. On the first recapture (second marking session) we set 22 traps for 1.75 hours and captured 1075 Least Chub (447 adults 628 juveniles). Fish were held in 4 aerated buckets with frozen water bottles and sorted and counted into juvenile and adult cohorts and juveniles were released. Adults were evaluated for marks with the uvBeast flashlight and black light flashlight and 34 fish were previously marked. Due to higher than expected mortality in the control on the initial marking occasion, we sought to also use a larger enclosure to minimize potential mortality associated with the small volume of the minnow traps and evaluate if that had an impact on overall mortality. Larger enclosures (.25m2 x .75 m high) with PVC frames and 3.175 mm cloth mesh were used. Fifty fish were sprayed using the same settings as the first occasion, 50 fish were not sprayed, and the remaining fish were sprayed at 80 psi and 25% wet dye concentration. The 50 not sprayed fish, 50 sprayed at 100 psi 45% wet dye, and 50 fish sprayed at 80 psi 25% wet dye went into respective enclosures for 48 hours. Post spray marking mortality was 40, 65.9, and 81.2% in control, 100 psi 45% wet dye, and 80 psi 25% wet dye groups, respectively. Mark retention was 95.4 and 100% in the 100 and 80 psi treatments, respectively. Given that the reduced spray pressure resulted in 100% marking success, it is possible that lower pressures and concentrations of dye could reduce mortality and still result in sufficient marking success. Handling stress should be minimized and even with many aerated buckets, we observed high mortality in unmarked controls. A subsample of fish could be evaluated for marks instead of handling every fish to further reduce handling. Spray marking proved to be very effective as a marking technique for Least Chub. However, mortality must be significantly lowered before spray marking can be viable for wild Least Chub populations.