Our results suggest that PICO PIT-tags, VIE tags, and freeze branding are all effective methods for the long-term marking of small bodied fishes like Oregon chub. Tag retention was similar among the VIE, freeze brand, and PICO-tag treatments over 150 days. However, because we observed some scar tissue regeneration, fragmentation or loss of VIE, and shedding of PICO tags throughout the study, we do not assume perfect detection of marks after 150 days. PICO tags offer the greatest power to monitor fish, allowing unique identification of large numbers of individuals and allowing assessment of individual growth and time extant. Passive monitoring stations can be effective for PIT-tag interrogation and the greater detection range associated with larger tags enable the use of larger antennas that can span a greater area
. However, for use with Oregon chub in large, complex, and dynamic floodplain habitats, the utility of PICO tags may be limited due to the limited read range of the tag versus the size of antenna needed to adequately cover the area through which fish are moving. Similarly, to allow individual identification, each PICO-tagged fish must be captured and manually scanned, which could be cumbersome when handling large numbers of fish in the field. In contrast, VIE and freeze brands are highly visible during handling, and should allow rapid identification of marked individuals in the field. Unique identification with VIE and freeze brands is limited by the number of body locations available to mark, availability of colors or brand designs, and longevity. Because of these limitations, VIE or freeze brands appear to be the most feasible methods for group marking of large numbers of small-bodied fish. Depending on the objectives and study design, cost may factor in the decision to choose a particular tagging technique. Although PIT tags offer a number of advantages relative to VIE tags and freeze brands, the cost of the tag may be prohibitive for some studies.
The implantation technique and the relationship between tag weight and fish size are important factors in determining the effect of the tag on fish survival
[2, 10]. Ombredane suggested that survival after PIT tag implantation was influenced more by fish handling and tagging time than by the tag itself
. We observed no significant difference in survival between the sham treatments and the control treatments; however, survival differed significantly between the two sizes of PIT tags used in the current study. Although there were slight differences in the gauge of the needle used to implant these two tag types, the effect of needle size is likely negligible given that we did not insert the needle into the peritoneal cavity. The results are in contrast to previous studies which compared PIT tag implantation techniques in small-bodied fish and concluded that survival was higher using an incision technique
[4, 5]. However, as noted earlier, our method for implanting PIT tags allows us to minimize the chance of needle overinsertion, which has been attributed to internal organ damage and increased mortality with small fish
. McCormick and Smith used a similar technique to implant 11.5 × 2.1 mm PIT tags into marine damselfish (Pomacentrus amboinensis) as small as 5.2 g
. We urge researchers to assess the effect of PIT tag implantation method on survival of their target species prior to their implementation in field studies, as success appears to be species and/or size dependent and is influenced by the implantation technique.
We demonstrated that fish as small as 44 mm TL or 0.9 g can be PIT-tagged with smaller-sized PIT tags (PICO-tag) with 90% survival over 150 days, which is similar to other studies that assessed survival thresholds to determine the minimum size for PIT-tag implantation
[5, 8]. In one study, 95% survival was predicted at 52 mm for juvenile brown trout Salmo trutta (41–70 mm FL) implanted with 11.5 × 2.1 mm PIT tags and reared for four weeks
. In another study, 90% survival was predicted at 63 mm for adult Rio Grande silvery minnow Hybognathus amarus (46–89 mm standard length) implanted with 12.5 × 2.07 mm PIT tags and reared for 32 days
. Several studies have reported high survival when implanting 11 mm PIT tags in small fish of a similar weight as adult Oregon chub. For example, survival of 2–3 g juvenile gilthead seabream Sparus auratus was 85.7% over 66 days
, survival of 2–3 g Nile tilapia Oreochromis niloticus was 83% over 49 days
, and survival of 2.5–3 g Eurasian perch Perca fluviatilis was 93% over 126 days
. Moapa White River Springfish Crenichthys baileyi as small as 40 mm TL (1 g) implanted with 9 × 2 mm PIT tags had high survival (95.6%) and 100% retention over 41 days
. We demonstrated the ability to tag fish substantially smaller than in previous studies; however, we recognize that the two smaller PIT tags evaluated have a low read range, limiting their ability to be utilized in passive interrogation.
We tagged and reared fish in a laboratory environment where temperatures were cooler than temperatures typically encountered during the spring and summer months when Oregon chub are studied in the field. Survival and tag retention is often inversely correlated with water temperature for PIT-tagged fish. For example, hybrid striped bass Morone saxitilis × Morone chrysops and bluegills Lepomis macrochirus that were tagged and reared in warmer water had significantly lower survival than those reared in cooler water
[29, 30]. However, exposure to a thermal regime similar to that experienced in the wild had no effect on long-term survival or tag retention in Oregon chub following PIT tagging in the present study. This suggests that our estimates of long-term survival and tag retention are applicable to the field. We suspect that the higher initial mortality in the PIT-tag + NTR group was due to stress associated with rapid temperature change following tagging (i.e., from 12.8°C to between 12.8°C and 20°C). Despite our attempt to mimic natural temperature fluctuations, we did not simulate other factors that may also affect survival and tag retention in the field conditions (e.g., pathogens, predators) so our estimates of survival and tag retention should be viewed as maximum estimates.
Our results with VIE tags in Oregon chub are consistent with other studies demonstrating high survival, tag retention, and visibility in small-bodied fish (e.g.,
[31–34]). However, success with VIE tags appears to vary with body location and species. For example, barbel Barbus barbus tagged at the base of the anal fin had high retention (82.6%) over two months
, yet retention rates were low in the anal fin of largemouth bass Micropterus salmoides, blacktail shiner Cyprinella venusta, channel catfish Ictalurus punctatus
, Colorado squawfish Ptycholcheilus lucius, and razorback suckers Xyrauchen texanus
. In Oregon chub, the tissue around the base of the anal fin is relatively thick, unpigmented, and translucent, with little chance of damaging organs by overinserting the tagging needle. Our marks were highly visible in and out of the water at 150 days (approximately five months). Furthermore, we have since observed these tags in the field 407 days after marking (unpublished observation, B. Bangs). VIE tag retention of juvenile brook trout Salvelinus fontinalis reared in an indoor hatchery and lake environment was 100% after 970 days
. The visibility of the tag in hatchery reared fish was >95% through 585 days, but decreased to 55–70% between 700 and 900 days. In the lake environment, visibility of the VIE tag was 50–70% through 400 days, and 0% at 959 days. Josephson et al. used blue filtered light and amber glasses in dark conditions to increase tag visibility, and observed 100% retention for lake reared fish at 959 days and 75% retention in hatchery fish at 970 days
. This suggests that UV illumination may increase tag visibility and recognition in long-term field studies.
Because VIE tags often make fish highly visible, several studies have evaluated the relationship between brightly colored marks and vulnerability of small-bodied fish to predation. No significant differences in predation between marked and unmarked small bodied or juvenile fish have been reported
[1, 19, 35, 36]. However, juvenile bluegill marked with highly visible fluorescent photonic dyes had significantly higher susceptibility to predation by largemouth bass than cryptically marked fish in a controlled laboratory environment
. The vulnerability to increased predation of VIE-marked fish should be considered prior to implementation of this tagging method in field studies.
The results from studies of survival, retention, and recognition of freeze brands used with small fish are inconsistent. Fingerling walleye (50–170 mm TL) had high survival and 95% freeze brand retention at 5 months in a rearing pond, and brands were observed in wild fish after 40 months
. Juvenile (65–160 mm FL) Chinook Oncorhynchus tshawytscha, Coho Oncorhynchus kisutch, and sockeye salmon Oncorhynchus nerka had high survival and tag retention at 14 months
; however, the author noted that retention in fish <55 mm (FL) was less than 3 weeks. Chinook, Coho, and sockeye salmon have high survival with freeze brands, but marks become faded and difficult to discern within four months
. In contrast, another study reported that initial mortality was high in Coho (8.3%), although survival was not significantly different from control fish and freeze brands became unrecognizable after 6 weeks
. Furthermore, in Oregon chub marked with two marks (i.e., VIE and freeze brand) in the field, we have observed high retention of freeze brands after 172 days, and poor retention after 407 days (unpublished observation, B. Bangs). Here, we inadvertently found that body placement affected the recognition of freeze brands on Oregon chub. Brands placed below the lateral line were more difficult to recognize than brands above the lateral line, because there were slight variations of freeze brand placement on each fish, and because of the differential Oregon chub pigment patterns on lateral surfaces. Even though we noted 98% retention through 150 days, 19% of these fish had poor quality brands. When brands were placed below the lateral line, the horizontal element of the “L” shape was sometimes absent. This may have been caused by inadequate pressure or duration against the branding terminal, build-up of ice on the iron, or a combination of these factors.