The effects of brodifacoum cereal bait pellets on early life stages of the rice coral Montipora capitata

Introduction

Coral reefs are among the most biologically productive and diverse ecosystems in the world (Reaka-Kudla, Wilson & Wilson, 1997; Hoegh-Guldberg, 2014; Birkeland, 2015). They support the lives of millions of people through economic, recreational, ecological, and cultural services. In Hawai’i coral reefs are valued at '9.7 billion, including the services provided by protection of the shoreline, fisheries support, and '363.5 million in reef related tourism activities a year (Bishop et al., 2011). Corals are also culturally important in Hawai’i. According to the Hawaiian creation chant, the Kumulipo, the coral polyp is considered to be the basic unit of life (Liliuokalani, 1897).

Despite their importance, coral reefs are declining globally due to the compound effects of global stressors like ocean acidification, and local stressors including terrestrial runoff (Sweatman, Delean & Syms, 2011; Hughes et al., 2017; Greer et al., 2020). Terrestrial runoff can result in the introduction of toxicants such as pesticides to the marine environment, leading to decreased survival of early life stages of coral (Lewis et al., 2016; Flores et al., 2020). Coral reef persistence and recovery from stress depends on successful reproduction (Richmond, Tisthammer & Spies, 2018), however early life stages of many marine organisms are often the most sensitive to toxicants (Reichelt-Brushett & Harrison, 1999). Marine larvae, including coral, depend on chemical cues during substrate selection and the induction of metamorphosis (Freckelton, Nedved & Hadfield, 2017). Reef persistence and recovery is dependent in part upon dispersal and arrival of competent planula larvae from nearby reefs (Gouezo et al., 2019). Thus, pristine remote reefs, relatively free from high anthropogenic disturbance, are of particular importance since they can act as a larval repository providing seed for other areas of the ocean.

Many remote islands have suffered accidental introductions of invasive species that reduce island biodiversity, threaten persistence of critical species, and alter ecosystem function (Angel, Wanless & Cooper, 2009; Bolton et al., 2014; Shiels et al., 2014; Towns, Atkinson & Daugherty, 2006; Graham et al., 2018). Invasive rodents threaten bird populations around the world, including on Gough Island in the Southern Atlantic, multiple islands in the Southern Ocean, and Midway Atoll in the Northwestern Hawaiian Islands (Angel, Wanless & Cooper, 2009; Wanless et al., 2009; Jones et al., 2021; Work, Duhr & Flint, 2021). Introduced omnivorous rodents have wiped out native plant species and have been documented preying on bird eggs, chicks, and adult seabirds (Raine et al., 2020; Jones et al., 2021). To combat accidental introductions, rodent eradication efforts require swift removal of the invasive species, continued monitoring for remaining rodents, and prevention of reintroductions. Aerial applications of chemical rodenticides have been increasingly used since the 1960s to eradicate rodent populations on islands, most commonly in New Zealand (Towns & Broome, 2003). Rodents have been eradicated from at least 284 islands worldwide, but efforts have been less successful on tropical islands compared to temperate islands (Howald et al., 2007; Keitt et al., 2015).

Midway Atoll is located in the central North Pacific Ocean, and is part of the Papahānaumokuākea Marine National Monument. Midway Atoll is culturally important to Native Hawaiians, originally known to Hawaiians as Kuaihelani which means “backbone of the heavens”, referencing the reflection of the island in the sky (Kikiloi, 2010). Rats and mice were inadvertently brought to Midway Atoll around 1943 while it was an active military base. Rats were successfully eradicated in 1996; however, mice remain on the island. Midway Atoll is also home to the largest breeding colony of Laysan Albatross in the world (Harrison, 1990). Trail cameras have observed mice attacking adult albatross on Midway Atoll during egg incubations, which can lead to death or severe injury of the nesting birds (Work, Duhr & Flint, 2021). In light of this threat, mouse eradication efforts are proposed for Midway Atoll using three aerial applications with additional hand broadcasts of cereal bait pellets containing the rodenticide brodifacoum. Efforts are planned for the summer when fewer seabirds are present on the island, and the vegetation food source for the mice is the lowest due to the dry season (United States Fish & Wildlife Service, 2019).

Eradication efforts are expensive and require careful planning to minimize impacts on nontarget organisms. Cereal bait pellets containing chemical rodenticides are used to target specific organisms. Prior to 2007, aerial application of cereal bait pellets had been used in 76% of the areas targeted for rodent eradication, with brodifacoum being the most common active ingredient (Howald et al., 2007). Eradication efforts using aerial or hand broadcast application of chemical rodenticides over large areas have been restricted to isolated areas to minimize risk of human exposure. Brodifacoum is a second-generation anticoagulant rodenticide with highly lipophilic properties that is more potent and longer lasting than first-generation anticoagulants such as warfarin (Leck & Park, 1981; World Health Organization & International Programme on Chemical Safety, 1995; Howald et al., 2007).

While aerial application of cereal bait pellets occurs exclusively over land to target rodents, there are also unavoidable and small amounts of pellets that are blown or drift into the coastal waters. Effective operations will cover the entire island with pellets up to the water line to ensure coverage of the entire habitable area of the target species. Thorough coverage, however, also leads to inevitable contamination of pellets to the ocean through with tidal changes, as observed on Palmyra Atoll (Engeman et al., 2013). Additional runoff from land or accidental drops of pellets over water may introduce pellets to marine ecosystems. Surveys after use of pellets to eradicate rats found small amounts of brodifacoum in fish and shellfish sampled around the island briefly after application of pellets on land (Masuda, Fisher & Beaven, 2015). Brodifacoum was also detected at low levels (between 0.0034 and 0.0112 mg per kg) in fish collected in a lagoon on Wake Atoll 3 years after pellet applications (Siers et al., 2020).

Non-target mortality after applications have been noted, and often include birds and mammals (Pitt et al., 2015; Ogilvie et al., 1997). Birds may face secondary exposure threats from eating insects that consume bait pellets (Ogilvie et al., 1997; Howald et al., 2010). Insects may scavenge for bait pellets, and can contain 1 ppb brodifacoum for weeks after consumption ceases (Brooke et al., 2013). Ninety percent of slugs sampled after a brodifacoum application contained brodifacoum residue up to 0.77 mg per kg (770 ppb), highlighting a potential secondary exposure for other animals (Alomar et al., 2018). However, it is possible snails and insects will not eat the pellets, or may metabolize and excrete brodifacoum rapidly before birds can be exposed by eating them (Brooke et al., 2011). Rats were found to excrete 14–22% of the brodifacoum they ingested, highlighting another potential environmental pathway of brodifacoum to non-target organisms or the marine environment after a rainfall (Fisher, 2009). While excretion in mice has not been studied, similar life history and feeding strategies shared by rats and mice make excretion of brodifacoum by mice possible.

Extreme examples of pellets accidentally spilled into aquatic environments have been reported. A helicopter transporting pellets for an eradication effort accidentally dropped 700 kg of cereal bait pellets containing 20 parts per million (ppm) brodifacoum into a lake in New Zealand. There was no brodifacoum detected in water or soil samples after the incident (Fisher et al., 2012). The only documented case of direct introduction of cereal bait pellets containing brodifacoum to the marine environment occurred after a road transport accident resulted in twenty tons of bait pellets on the shore of a beach in southern New Zealand (Primus, Wright & Fisher, 2005). Visible residue of pellets persisted for 1 week, and it is believed that a significant portion of the brodifacoum remained adsorbed to bait particles that accumulated on the seabed. Although dead birds and seals were found in the area after the spill, no signs of brodifacoum toxicity were detected in these organisms. Mussels were found to contain 0.41 ppm brodifacoum the day after the spill, but concentrations decreased quickly in mussels after exposure. Limpets and abalone also had detectable concentrations of brodifacoum for 80 to 191 days after the spill, most likely due to continued exposure to brodifacoum through sediment consumption, and the long half-life of brodifacoum. It was determined that the spill effects were mostly contained to a 100-m² area. Brodifacoum is nearly insoluble in water, so even large numbers of pellets in the water would not result in high concentrations of brodifacoum in the water. The concentrations of brodifacoum in seawater samples were below the minimum detectable limit of 0.02 ppb by 36 h after the accidental spill. However, residues in shellfish persisted for 31 months after the incident (Primus, Wright & Fisher, 2005).

Few laboratory studies have tested the effects of brodifacoum or cereal pellets containing brodifacoum on non-target organisms (Brooke et al., 2013; Brooke et al., 2011; Alomar et al., 2018). To date, no studies have been conducted on marine invertebrates such as corals. The back reef of Midway Atoll has high coral cover, and is dominated by Montipora spp (Kenyon, Wilkinson & Aeby, 2010). Montipora capitata is a broadcast spawning species that is widely distributed through the Main Hawaiian Islands, Northwestern Hawaiian Islands, and throughout the Pacific and Indian Oceans (Vroom & Braun, 2010; Veron et al., 2016), and whose reproductive peak coincides with the planned mouse eradication project on Midway. Therefore, the potential impact of rodenticide contamination on the reproductive success and early life stages of this dominant coral species are of serious concern. To test the potential effects of rodent eradication efforts on early life stages of corals, M. capitata gametes and larvae were exposed to brodifacoum, brodifacoum treated cereal bait pellet, and inert cereal bait pellet solutions in the laboratory. We hypothesized that exposure to brodifacoum, brodifacoum cereal bait pellets, and inert cereal bait pellets would decrease fertilization success and larval survival in M. capitata.

Materials and Methods

Larval development assays

Fifteen hours after fertilization assays were started, remaining intact larvae were transferred and rinsed gently in beakers containing FSW. Larvae from these beakers were transferred to 25 mL glass bowls containing ten larvae per replicate with 10 mL of FSW and monitored for 3 days.

Larval survival assays

Extra gamete bundles collected from the same M. capitata colonies used for fertilization assays, additional gamete bundles collected from other traps, and gamete bundles collected from the ocean surface were added to a cooler containing two micron filtered seawater and allowed to fertilize. Seawater was changed three times with 0.5 micron filtered seawater 1 h after the addition of bundles to remove excess sperm. Seawater changes were completed once a day while fertilized gametes were undergoing embryogenesis and larval development.

Ten 1 week old larvae from the per replicate (n = 10 for pellet solutions) were added to 25 mL glass bowls containing 10 mL of the same treatment solutions used for the fertilization assays. Crushed pellet was added directly to the glass bowls prior to the addition of larvae with the same treatments of FSW Control, Low Brodifacoum Pellet, Medium Brodifacoum Pellet, High Brodifacoum Pellet, Low Inert Pellet, Medium Inert Pellet, and High Inert Pellet solutions. The pellet treatments were all compared to the FSW Control treatment. Larval behavior was monitored, and survival quantified daily for 3 days. Larvae were considered alive if they were swimming, had visible, moving cilia, or moved when lightly prodded with a pipette tip. Larval swimming behavior was assessed visually as either actively swimming (Yes), or not swimming (No).

Additionally, larval assays using 0.00033% DMSO Control, 1 ppb Brodifacoum, 10 ppb Brodifacoum, and 100 ppb Brodifacoum treatments were conducted (n = 5). Larval behavior was monitored, and survival quantified daily for 3 days and compared to the DMSO Control treatment. Larval survival assays are commonly conducted to evaluate the effects of putative toxicants on early life stages of corals because they are known to be more sensitive to stressors than adults (Reichelt-Brushett & Harrison, 1999; Markey et al., 2007; Flores et al., 2020).

Statistical analyses

Fertilization experiments were conducted on four separate nights with six replicates per night during both June spawning events, six replicates during the second night of the July spawning, and two replicates on the first night of the July spawning due to low output. All results were combined for a total of 20 replicates. Standard least squares models were used to analyze the data in JMP Pro 16 with fertilization success as the role variable, treatment as a fixed effect, and date as a random effect. Dunnett’s multiple comparisons test was used to compare treatments to the control (FSW for pellet treatments, DMSO for brodifacoum treatments). Separate models were used to analyze the pellet treatments and the brodifacoum only treatments. Multiple pair-wise comparisons using Tukey’s HSD procedure was used to test for differences between pellet types (brodifacoum vs inert).

A single experiment with four replicates was run to monitor survival of larvae fertilized in treatment solutions. Data were not normally distributed, so the non-parametric Kruskal-Wallis and post hoc Steel test with control were conducted. The DMSO Control treatment was used as the control in the brodifacoum only experiments, and FSW Control treatment was used as the control for the pellet experiments for statistical analyses.

A larval survival experiment was conducted using brodifacoum only solutions (n = 5). Data were not normally distributed, so the non-parametric Kruskal-Wallis and post hoc Steel test with control were conducted. Behavior and survival were compared to the DMSO Control treatment in statistical analyses.

Pellet solution larval survival assays were run twice, with five replicates in June, and five replicates in July 2021. A standard least squares models was used with larval survival as the role variable, treatment as a fixed effect, and date as a random effect. Dunnett’s multiple comparisons test was used to compare treatments to the FSW Control treatment. Multiple pair-wise comparisons using Tukey’s HSD procedure was used to test for differences between pellet types. All data were analyzed using JMP Pro 16.

Results

Fertilization assays

Brodifacoum treatments

Brodifacoum only treatments were analyzed separately from the pellet treatments. Effect screening showed an effect of treatment (p = 0.0049), but no effect of date (p = 0.4912). Fertilization success was 92.13% ± 5.58 (mean ± S.D.) in the DMSO Control (Table 2). Fertilization decreased slightly with increased concentration of brodifacoum from 91.61% (±10.08) in 1 ppb Brodifacoum, 85.03% (±26.88) in 10 ppb Brodifacoum, and 70.64% (±30.75) in 100 ppb Brodifacoum treatments (Fig. 1). There was not a significant decrease in fertilization success in 1 ppb or 10 ppb Brodifacoum treatments compared to the DMSO Control. There was a significant decrease in fertilization success in the 100 ppb Brodifacoum treatment compared to the DMSO Control (p = 0.004). Fertilization success was high in the 100 ppb Brodifacoum treatment on June spawning night one (85%), June spawning night two (90%), and July spawning night one (85%), but low during the second July spawning night (32%).

Table 2:

Average fertilization success (%) after 5 h.

Average fertilization success and standard deviation of each treatment 5 h after gametes bundles were added to treatment solutions.

Label	Average fertilization success (%)	Standard deviation
DMSO control	92.13	5.58
1 ppb Brodifacoum	91.60	10.08
10 ppb Brodifacoum	85.30	26.88
100 ppb Brodifacoum	70.64	30.75
FSW control	93.00	10.31
Low Inert Pellet	78.00	22.38
Medium Inert Pellet	52.50	25.35
High Inert Pellet	4.50	6.86
Low Brodifacoum Pellet	82.70	24.9
Medium Brodifacoum Pellet	58.16	32.67
High Brodifacoum Pellet	3.37	4.71

DOI: 10.7717/peerj.13877/table-2

Pellet fertilization assays

Effect screening showed an effect of treatment (p < 0.0001), but no effect of date (p = 0.3151) in the standard least squares model. Fertilization success was high in the FSW Control treatment at 93% ± 10.31 (mean ± S.D.) and decreased with increasing concentrations of both brodifacoum and inert pellets (Fig. 1, Table 2). Fertilization was inhibited in the high concentrations of both pellet types. Fertilization success was relatively high and not reduced in the Low Inert Pellet treatment at 78% ± 22.38, as well as the Low Brodifacoum Pellet treatment (82.70% ± 24.9). Fertilization was reduced in both the Medium Inert Pellet and Medium Brodifacoum Pellet treatments, as well as High Inert Pellet and High Brodifacoum Pellet treatments (p < 0.0001 for all). Fertilization success was 52.5% ± 25.35 in Medium Inert Pellet, and 4.5% ± 6.86 in the High Inert Pellet treatment. Fertilization success was 58.16% ± 32.67 in Medium Brodifacoum Pellet and 3.37% ± 4.71 in High Brodifacoum Pellet treatments. There were no significant differences between Low, Medium, or High treatments between pellet types (p > 0.97 for all comparisons).

Larval survival assays

Larvae fertilized in FSW, exposed to brodifacoum solutions

Larval survival was high in the DMSO Control and 1, 10, and 100 ppb Brodifacoum treatments during the 3-day exposure. There was no decrease in larval survival in any treatment (Fig. 2, Table 3).

Table 3:

Average larval survival (%) of larvae exposed to treatments for 1, 2, and 3 days.

Treatment	24 h		48 h		72 h
	Average larval survival (%)	Std Dev	Average larval survival (%)	Std Dev	Average larval survival (%)	Std Dev
DMSO control	100	0	92.00	10.95	84.00	5.48
1 ppb Brodifacoum	94.00	5.48	96.00	5.48	94.00	5.48
10 ppb Brodifacoum	94.00	5.48	88.00	13.04	86.00	11.4
100 ppb Brodifacoum	90.00	7.07	90.00	7.07	88.00	4.47
FSW control	94.00	6.99	93.00	8.23	92.00	10.33
Low Inert Pellet	99.00	3.16	95.00	7.07	92.00	6.32
Medium Inert Pellet	97.00	6.75	68.00	36.45	83.00	15.67
High Inert Pellet	82.00	28.60	54.00	18.97	49.00	18.523
Low Brodifacoum Pellet	96.00	9.67	95.00	7.07	89.00	9.94
Medium Brodifacoum Pellet	95.00	8.50	50.10	43.84	66.00	30.26
High Brodifacoum Pellet	58.00	31.90	38.00	41.85	27.00	32.34

DOI: 10.7717/peerj.13877/table-3

Larvae fertilized in FSW, exposed to pellet solutions for 3 days

Larval survival was high in all treatments after 24 and 48 h except for High Inert Pellet and High Brodifacoum Pellet treatments (Fig. 2). Larval activity was reduced in these treatments, and larvae appeared to be stuck to particles floating on the surface or at the bottom of the glass (Fig. 3). Larval survival was only significantly reduced in the High Brodifacoum Pellet treatment after 24 h (p = 0.0266). Survival was reduced to 82% ± 28.57 (mean ± S.D.) in High Inert Pellet, and 58% ± 31.9 in High Brodifacoum Pellet treatments after 24 h, then 54% ± 18.97 in High Inert Pellet and 38% ± 41.85 in High Brodifacoum Pellet treatments after 48 h. The reduction in larval survival was significant in High Inert Pellet (p = 0.0166) and High Brodifacoum Pellet (p = 0.0019) after 48 h. Larvae were counted as alive if they appeared intact and would swim when prodded lightly. Most larvae in both high pellet treatments were not moving on day 2. By day 3, most larvae in High Inert Pellet and High Brodifacoum Pellet treatments had degraded, and those remaining were not swimming. Larval survival was again only significantly reduced in the High Inert Pellet (49% ± 18.53) and High Brodifacoum Pellet (27% ± 32.34) treatments (p = 0.0013, p = 0.0024). The experiment ended after 3 days because most larvae were degraded in both the High Inert Pellet and High Brodifacoum Pellet treatments. Larvae in other treatments were not monitored daily after day 3. There were no significant differences between paired treatments of the same pellet amount between inert or brodifacoum pellet treatments.

Discussion

Fertilization success was only reduced after exposure to pure brodifacoum solutions at a high concentration of 100 ppb brodifacoum. Brodifacoum has limited solubility in water and is unlikely to reach concentrations of 100 ppb in the field after dissolution of pellets or continued introduction of brodifacoum attached to soil through runoff. Pure brodifacoum solutions were used to increase understanding of potential impacts of the rodenticide on corals in a controlled environment. The use of DMSO to dissolve brodifacoum to allow for manipulations of brodifacoum concentrations in a controlled setting followed the guidelines laid out by the Organization for Economic Cooperation and Development (2019) for testing of chemicals. Operational use of brodifacoum would not include increasing solubility by dilution with DMSO. Brodifacoum is lipophilic and could bioaccumulate in marine organisms. Despite a lack of detection of brodifacoum in water samples, it does bioaccumulate in marine organisms after rodent eradication events at low levels of 0.273 ± 0.056 µg/g (mean ± SE) in hermit crabs and 0.143 ± 0.027 µg/g (mean ± SE) in black spot surgeon fish (Pitt et al., 2015). Corals have not been sampled after eradication events, so there is no documentation of bioaccumulation of brodifacoum in corals. Brodifacoum is not highly mobile in soil with a tendency to adsorb to soil and suspended particles in water (K_oc = 1.4 × 105). Brodifacoum has an estimated bioaccumulation factor of 570, indicating high potential for bioaccumulation in aquatic organisms. (National Center for Biotechnology Information, 2022). Bioaccumulation of brodifacoum in corals is possible because corals accumulate hormones and toxicants from the surrounding environment and through feeding (Armoza-Zvuloni et al., 2012; Yang et al., 2019). It is not known whether bioaccumulation of brodifacoum would result in health effects in adult corals. The concentrations used in this study were likely higher than those that would be expected to occur in nearshore environments, hence, it is believed that brodifacoum may not affect coral health at levels that nearshore colonies would encounter.

Fertilization success and larval survival were significantly reduced after exposure to 0.4 and 4 g per L solutions of both brodifacoum treated and inert cereal bait pellets. Larval survival was only significantly decreased after prolonged exposure to high concentrations of either inert or brodifacoum pellets. Larvae are made of mostly lipid, which may have contributed to the tendency of larvae to stick to pellet pieces floating on the surface or on the bottom of the glass bowl (Richmond, 1987). Brodifacoum is lipophilic, and may be easily absorbed by larvae upon contact with pellet particles. The pellet formulations were identical, other than the addition of brodifacoum to the brodifacoum pellets. In the absence of brodifacoum, we saw similar results among treatments, demonstrating that the reactivity is the result of the pellets themselves, and likely not due to brodifacoum exposure. Although fertilization was not significantly reduced in the low concentration of either pellet, fertilization success was decreased and more variable in the low treatments (S.D. > 22% compared to ~10% in controls). Variable fertilization success is regularly observed in broadcast spawning species between spawning events, and may be enhanced by pellet addition (Padilla-Gamiño & Gates, 2012; Hédouin & Gates, 2013).

The lowest pellet concentration used in this study was 40 mg of pellet per L, or approximately one pellet per 44 L. Pellet densities around islands after aerial application are variable (Engeman et al., 2013), but the concentration of pellets used in this study might still be higher than expected from accidental drift of pellets during aerial applications. The medium and high concentrations of pellet that led to reductions in fertilization are likely only representative of extreme accidents where large amounts of pellets enter an area with high residence time, or an accidental spill leads to pellets being dropped directly into the ocean.

Non-active ingredients of the pellets were not disclosed, but likely include oats and sugars based on available information on other pellet types for residential and commercial use. We hypothesize that the introduction of organic material from the pellets leads to bacteria proliferation that results in reduced dissolved oxygen. Hypoxia decreases fertilization and gamete formation in polychaetes (Shin et al., 2014), causes behavioral changes in coral larvae (Jorissen & Nugues, 2021), and decreases survival of adult corals in hypoxic zones (Altieri et al., 2017). The 2013 Honolulu Harbor Molasses Spill resulted in the deaths of thousands of fish and all corals within the Honolulu Harbor due to a steep drop in dissolved oxygen after 1,400 tons of molasses was spill in the harbor. Like most of the ingredients in the cereal bait pellets, molasses is nontoxic, yet still negatively impacted marine organisms at the high concentration that resulted from the spill (Hawai’i State Department of Health, 2013; Han & Singh, 2022).

Additionally, increased turbidity and decreased light availability due to suspended pellet particles could potentially impact coral survival. Cereal pellets did not dissolve fully in water, leading to cloudy appearance in high concentrations, and particles floating and sinking in test containers. Increased suspended and deposited sediments decrease health and survival of corals (Gilmour, 1999; Erftemeijer et al., 2012; Tuttle & Donahue, 2020). Large amounts of pellet entering the coastal waters may smother benthic organisms, physically sheer adult corals, or prevent larvae from detecting suitable settlement substrate.

Management implications

Our results suggest that only large amounts of cereal bait pellets entering the nearshore waters after eradication efforts, if coinciding with coral spawning events, may affect the reproductive success of corals by reducing fertilization success. The chances of an extreme event taking place during the planned eradication efforts on Midway Atoll are low, but efforts to avoid peaks in coral reproductive periods will farther reduce potential risk to coral reproduction. In an abundance of caution, we recommend managers consider coral reproductive periods when planning the drops of pellets to allow time for dissipation of pellet material before a spawning event. Coral spawning events take place only a few times a year, so a missed spawning event could mean failed reproduction for a whole year. Broadcast spawning is the most frequent mode of reproduction in corals, and takes place predictably a couple times a year based on the season and phases of the moon (Richmond & Hunter, 1990). Risk of spills of large amounts of pellet to the marine environment could be reduced by using hand applications when possible. Aerial applications are necessary to cover all habitable space on most islands to ensure successful eradication.

Extreme events leading to accidental drops of large amounts of pellets into coastal zones with high residence time will likely inhibit survival of early life stages of corals, but these events are rare and unlikely. Rodent eradication efforts are carried about by trained professionals using best practices to avoid negative impacts on nontarget organisms. Successful eradication events have positive impacts on biodiversity and ecosystem health. Recovery of native species and bird populations have been observed after successful eradications (Jones et al., 2021). Additionally, recovery of terrestrial ecosystems leads to long term benefits to marine ecosystems (Kurle et al., 2021). Eradication of invasive species is a conservation priority to restore ecosystem functioning of both terrestrial and marine ecosystems (Graham et al., 2018).

Conclusions

This was the first laboratory study testing the effects of brodifacoum on corals. Exposure to pure brodifacoum solutions did not decrease fertilization success or larval survival in M. capitata at ecologically relevant concentrations. Since brodifacoum does not readily dissolve in water, it is unlikely that the concentration of brodifacoum in the water column would reach or exceed the concentrations used in this study. Cereal bait pellets used in rodent eradications efforts did decrease fertilization success and larval survival at high concentrations representative of extreme and rare accidents. These results suggest that brodifacoum alone is not a threat to fertilization success or larval survival in corals at environmentally relevant concentrations.

Best management practices would consider well documented coral reproductive events when planning rodent eradication efforts to avoid negative impacts on fertilization success. More research should be done on the effects of the brodifacoum treated cereal bait pellets used in this study on adult corals. Additionally, more research should be done on the effects of brodifacoum treated cereal bait pellets used for rodent eradication efforts on other marine invertebrates, especially those that consume sediment and may ingest pellet particles that fall into the ocean.

Supplemental Information