Using an historic dataset of 8 years of fish tagging by Florida Fish and Wildlife Conservation Commission scientists in the Florida Keys, we examined factors a posteriori that influence probability of presence of internally tagged fish. At 6 days after tagging, Kaplan–Meier survival curves demonstrated that fish tagged in situ had a 25% higher probability of detection within the acoustic array (i.e., 75% lower probability of disappearing) compared to fish tagged on the boat. Tagging method was the only factor affecting the risk of disappearance when we examined the initial Cox proportional hazards model assessing the probability of presence of all fish.
In this study, there were several variables that could not be included in the initial hazards model because of correlation, but their inclusion and limited influence in the subsequent boat-tagged and in situ tagged models reinforced that the tagging method was the most important factor in risk of disappearance from the array. Acoustic tracking data of in situ surgically tagged fish from two other telemetry studies also demonstrate the benefit of tagging method, indicated by high survival rates (97–100%) post-surgery [4, 43]. The decrease in probability of presence of boat-tagged fish could be due to several factors. As described in Feeley et al.  and Herbig et al. , all fish tagged on the boat were returned to the seafloor via divers after they recovered from anesthesia to ensure a safe return to the reef. Nevertheless, the tagged fish may still have suffered from barotrauma, increased stress from being caught by hook and line, increased handling times (which could also increase the impacts of barotrauma), or long-lasting effects of anesthesia. All these factors could increase risk of boat-tagged fish disappearing from the array.
At the start of in situ tagging, we decided not to use anesthesia at depth because we discovered that fish held ventral side up with their eyes covered were just as calm as anesthetized fish. We also deemed it advantageous to avoid unnecessarily impairing the tagged fish’s response to predators once the fish was released into sheltering habitat after surgery. Using anesthesia during fish surgeries may not always be the best course of action in every situation, and all potential effects on the fish must be considered [8, 23]. We believe the benefits of reduced total handling time for fish tagged in situ versus fish tagged on the boat, which usually included a recovery period (mean ± SE: In situ = 9.30 ± 0.31 min, Boat = 72.17 ± 6.02 min), coupled with immediate release into protective habitat and the lack of pressure and temperature changes, explain the increased probability of presence within the acoustic array for in situ tagged fish. The increased probability of presence of fish tagged in situ compared to fish tagged on the boat indicates that any negative effects due to the lack of anesthesia are minimal and do not outweigh the benefits of in situ tagging.
There were several surprising results presented from the analysis of this data. Some of the variables that were originally believed to impact the probability of detection were either not influential in the hazards models or had the opposite effect. For example, we hypothesized that additional handling time would increase the risk of a tagged fish having an event. The models, however, suggested an increase in 1 min of handling time slightly decreased the risk of disappearance for boat-tagged fish, though the confidence intervals neared or crossed 1 in all cases, indicating little to no effect. Handling time may not have been very influential in risk of disappearance, but it is still surprising that increased time may have reduced risk, especially as handling times for fish tagged on the boat were high compared to other tagging studies [3, 39] and minimizing handling of fish has been reported as one of the most important considerations in implantation surgeries . For in situ tagging, handling time was not included in best-fitting models, but that may be because there was little variance in the handling times of fish tagged in situ.
Fish size was not a significant factor in the best-fitting models. Fish tagged in this study varied between 32 and 107 cm TL, a range large enough that differences in probability of presence because of size were expected. We hypothesized that smaller fish would be at a higher risk of disappearance because they might not have the same energy resources to aid in recovery as larger fish or they may be more susceptible to predation. However, this was not supported by the models. “Fish size” was included in the best-fitting model for in situ tagged fish at 6 days after tagging but was not significant, and a large spread in the hazard ratio confidence interval (0.82–21.84) represented high uncertainty in the hazard ratio estimate. While there are likely species-specific differences (e.g., body type, stress tolerance) that influence probability of presence of different sized fish, in our study fish size did not significantly affect probability of presence post-tagging. Also, fish less than 20 cm FL have been shown to have high survival from internal tagging , suggesting that surgery and implantation of appropriately sized tags for the body size of the fish may not impair survival, regardless of fish size.
Fish family was also not a significant covariate, although differences in tolerance to barotrauma and handling were seen among species. The high number of species in this study (which originally led to overparameterization in models) and the unevenness of species between tagging methods resulted in grouping species by family. There are physiological differences between species of the same family, and grouping them by family may have masked these differences and may help explain why family did not affect probability of presence. Although an in-depth look at the effect of tagging method on species was not possible in this study, a planned, paired study with fewer species may find that fish species influences probability of presence. For any tagging project, factors such as body type, susceptibility to barotrauma, and stress tolerance should be considered when deciding on the appropriate tagging method.
Water temperatures and differences between water and air temperatures vary throughout the year and could affect the risk of a fish disappearing from the array, but month as a proxy for temperature did not have a significant influence in either boat-tagged or in situ tagged models. Life history traits of the fish families (e.g., the time of year that they reproduce) are also linked to time of year and water temperature, and thus linked to the variable month. Although time of year was not a significant in this study it should also be considered when deciding how to maximize fish survival and probability of presence when setting up a tagging project.
Experience of the surgeon has been shown to influence tag retention and survivorship [5, 10], but this was not seen in our study. Surgery count was not in any best-fitting model, but was a covariate in the third best-fitting model for both the initial model at 4 days after tagging and the in situ model at 6 days after tagging. Although the effect was not statistically significant, both models indicated an increase in surgery count decreased risk of fish disappearance. Consistent training among surgeons may have reduced the effect of surgery count enough so that other variables had larger influences. Before the start of a tagging project, surgeons practiced the tagging procedure with guidance from an experienced team member, and initial training was provided by a professional plastic surgeon (D. Hawtof, personal communication). Additionally, less experienced surgeons were always paired with an experienced surgeon during the tagging procedure to provide guidance during both boat and in situ tagging. The training and oversight throughout the tagging procedure may have helped to reduce the effect of surgeon experience on risk of disappearance.
Hazards models were analyzed at both 4 and 6 days after tagging because there was discrepancy in the literature about how long after surgery to examine survival related to the tagging procedure. Results between these two time periods were similar (e.g., hazard ratios for tagging method in the initial model differed by only 0.015), but we felt it was informative to present both cases. Although differences between the results at both time periods were minimal, they are still interesting to note because they show how the dynamics of fish survival may subtly shift over time. The maximum time after surgery that a fish was still detected was just over 4 years (data from this fish was analyzed in ), and a possible future direction of work could include examining longer term differences in detection patterns between different tagging methods, particularly if the goal is to maximize data acquisition. Analyzing probability of presence on a longer time frame may find different factors which are more influential than those found in this study, but consideration should be given to whether disappearances from the array past 6 days post-surgery are a result of the tagging procedure or from a natural behavioral event.
This study was not originally designed to rigorously examine all factors affecting boat and in situ tagged fish. Therefore, some variables may not have proven as influential as they would in a more systematic study. Depth was expected to have more of an influence on hazard because detrimental effects of barotrauma have been shown to increase with depth of capture . In this study, depth may not have been the best representation of the severity of barotrauma. Boat-tagged fish were generally caught using hook and line and were not necessarily caught from the seafloor, which was the depth recorded, and furthermore, mean depth of capture of boat-tagged fish was significantly shallower compared to in situ tagged fish (Wilcoxon two-sample rank test, p < 0.05). Fish that were near the surface or mid-water column when caught on hook and line would have a greater depth recorded than where they were physically caught in the water column, meaning that they may have exhibited fewer signs of barotrauma than if they had actually been caught at the depth recorded. Fish tagged in situ were caught via baited traps on the seafloor, so the depth of capture was the depth of the trap and the actual depth of the fish when it entered the trap. Additionally, after the start of in situ tagging, this method was preferentially chosen over tagging on the boat at deep sites and only a few fish were captured, tagged, and released from the boat in deep water (> 30 m) and in situ tagged fish had a higher overall sample size compared to boat-tagged fish. The method of tagging was based on what was deemed best for survival of the fish and logistically made sense for each tagging event. If boat tagging data had included more fish captured from deep water, resulting in no discrepancy in mean depth per tagging method, we hypothesize depth would have had a larger effect on survival. Additionally, comparing risk of disappearance between tagging methods where boat-tagged fish were either released at the surface or brought back down to depth for release (the method used for all boat-tagged fish in this study) would further explore the effects of depth, barotrauma, and release method.
Fight time, location of hook placement on fish, whether the fish exhibited signs of barotrauma, whether a fish with barotrauma was vented before its return to depth, and overall health of the fish at time of release were not often recorded and thus were not included as covariates in the hazards models. The amount a time a fish spent in a trap could influence the fish’s health and probability of presence after tagging, but the time a trap was baited does not equal the time a fish was in a trap (a trap may have been baited and left for 12 h, but a fish may have entered the trap at hour 11). Use of cameras for identifying how long fish were in traps and recording specifics on condition upon release would provide more details on why risk of disappearance was higher in boat-tagged fish.
This study demonstrated some of the limitations of a posteriori-designed study including correlated variables and missing data (particularly during the initial stages of conducting acoustic telemetry research when it was not known what supplementary data would be beneficial to record). However, despite these caveats, we believe that this historic dataset using over 100 internally tagged fish representing 14 different species surgically tagged by 11 different scientists was robust enough to demonstrate that tagging method had an effect on risk of fish disappearance. Telemetry studies are expensive to conduct, both in terms of effort and money, and often outcomes are not known until months later when the data can be downloaded. Therefore, it is critical to have a better understanding of how tagging method influences probability of presence (which can be considered a proxy for fish survivorship). By examining the effects of tagging methods, we can inform future telemetry studies, thereby increasing the usefulness of telemetry data.
This study has shown that there were clear benefits to tagging fish in situ, though due to high correlation among variables we are unable to pinpoint the source behind decreased risk of disappearance compared to boat-tagged fish. The lack of barotrauma, shorter handling time, lack of anesthesia, and use of baited traps instead of hook and line may all have played a role in increasing an in situ tagged fish’s probability of presence. We understand that this in situ method may not always be practical. In situ tagging can increase the amount of effort spent to tag each fish because of the logistics required and the time limitations when tagging at deeper depths. Some target species may be located outside recreational diving limits, which can increase effort and cost for specialized training and dive gear. Also, in this study we used baited traps to capture fish in situ, but other methods should be considered if the species of interest is unlikely to be captured in this manner. Furthermore, studies that rely on anglers to supply the tagging subjects or focus on species found in very shallow water may not find in situ tagging feasible or worthwhile. However, we suggest that all aspects of the tagging procedure, from tagging method (e.g., on a boat vs. in situ) to use of anesthesia, method of capture, and method of release be carefully considered for each target species to reduce stress to the fish, and maximize success of the study.