Introduction

The Deepwater Horizon mobile drilling unit exploded on April 20, 2010, resulting in release of approximately 5.0 million barrels of oil into the Gulf of Mexico (McNutt and others, 2012). In accordance with applicable Federal laws and regulations, a Natural Resources Damage Assessment was completed to evaluate natural resource damage from the oil spill (Deepwater Horizon Natural Resource Damage Assessment Trustees, 2016). The common loon (Gavia immer) was one of more than 100 bird species injured from the spill (Deepwater Horizon Natural Resource Damage Assessment Trustees, 2016). The estimated number of common loons injured from the spill was 561 to 910, which included direct mortalities and lost reproduction owing to mortality of breeding adults (Deepwater Horizon Natural Resource Damage Assessment Trustees, 2016). To restore common loons lost from the spill, the Deepwater Horizon Open Ocean Trustee Implementation Group funded the “Restoration of Common Loons in Minnesota” project. The project has three objectives (Open Ocean Trustee Implementation Group, 2019):

In this report, we describe activities under the “Restoration of Common Loons in Minnesota” project for calendar year 2023. Specifically, we report progress on monitoring benchmarks defined in the Monitoring and Adaptive Management Plan (Open Ocean Trustee Implementation Group, 2019). Monitoring benchmarks are (1) baseline status of lakes; (2) length of linear shoreline acquired; (3) number of artificial nesting platforms (ANPs) deployed; (4) number of ANPs occupied; (5) number of lake associations recruited to participate in a loon friendly lake program; (6) number of intervention activities to promote environmentally friendly fishing gear; and (7) common loon presence/absence, territory occupancy, and nest productivity. Furthermore, summary statistics are provided, but no formal analysis is included because this is an ongoing project.

Methods

Breeding common loons were monitored on territories in north-central Minnesota in 2021 (Beatty and others, 2022) and 2022 (Beatty and others, 2023b). This study design closely resembled a before-after control-impact design, where pretreatment (that is, “before”) data were collected in 2021 and 2022. In 2022, 99 focal territories were identified from which inferences would be made (Beatty and others, 2023b). We identified 45 of 99 focal territories as treatment (that is, impact) units and categorized the remaining 54 territories as control units. Treatment was deployment of an ANP into respective focal territories. The abovementioned 99 territories are hereafter referred to as “focal territories” to distinguish them from other territories that were monitored.

We identified an effective deployment date to characterize ANPs as either available or not available in 2023. ANPs deployed on or before this date would be categorized as effectively available to common loons as a nesting site and ANPs deployed after the date would be characterized as effectively not available to common loons as a nesting site. The effective deployment date was identified as May 19, 2023, because it was approximately 2 weeks after ice-out, which provided partners sufficient time to deploy ANPs. Consequently, ANPs were considered available to breeding common loons as a nesting platform if they were deployed on or before May 19, 2023. Observers documented presence/absence of ANPs during the first week of surveys (May 8, 2023) and communicated with partners about deployment of ANPs that were absent the first week. In addition, the earliest observed nesting attempt in 2022 was May 16, and a preliminary analysis indicated that 25 percent of nests were initiated by May 24, 2022. Thus, May 19 was an acceptable effective deployment date because it considered nesting common loon phenology and the logistics of ANP deployment.

We worked with private citizens and lake associations to effectively deploy 42 ANPs to 45 treatment territories. In one territory, the platform was placed after the effective deployment date and in an unsuitable location (Crow Wing, Island Lake – South). After several attempts to facilitate a relocation of the ANP failed, we reassigned this territory from the focal treatment category to the nonfocal category, reducing the number of focal treatment territories to 44 and the total number of focal territories to 98. Although 42 ANPs were effectively deployed to 44 treatment territories in 2023, the remaining 2 ANPs were deployed after the effective deployment date.

Common loon monitoring was completed for a third consecutive year, and 2023 represented the first year of treatment data (that is, the after in before-after control-impact). We surveyed 56 priority lakes across seven counties in north-central Minnesota (Beatty and others, 2024), including four lakes that were not surveyed in 2021 or 2022 (Big Sugar Bush Lake [not shown], Big Trout Lake [not shown], Wabedo Lake [not shown], and Woman Lake [not shown]) (fig. 1, table 1). Thus, 52 lakes contained at least one focal territory. In addition, we did not monitor three lakes in 2023 that were monitored in 2022 (Norway Lake [not shown], Spider Lake [not shown], and Buck Lake [not shown]) because territories on these lakes were identified as multilake territories and categorized as nonfocal territories. These multilake territories were removed from the study because of uncertainty associated with nesting location. In 2024, we anticipate we will not monitor Island Lake (Crow Wing County) as well because it has no remaining focal territories after moving the Crow Wing, Island Lake – South territory from focal to nonfocal status in 2023, as described earlier in the section.

Each territory was surveyed by motorboat, kayak, canoe, or shoreline observation as described in Beatty and others (2022). We circumnavigated larger lakes and frequently stopped to scan for common loons. Information was collected on common loons and environmental variables that could affect loon or nest detection (refer to Beatty and others [2022] for a complete description), and all data from surveys were recorded in an ArcGIS Survey123 application. We visited loon territories every 3–15 days (mean=7.7 days) from May 8, 2023, to July 14, 2023, to collect information on territory occupancy, nesting, and chick survival (Beatty and others, 2022, 2023b). Each territory was surveyed twice from July 31, 2023, to August 11, 2023, to document chick presence to estimate survival.

Results

We monitored 126 territories in 2021 (Beatty and others, 2022) and 132 territories in 2022 (Beatty and others, 2023a). In 2023, 141 total territories, including 54 focal control territories and 44 focal treatment territories, were monitored, and 12 new territories were identified. In addition, we monitored 22 nonfocal territories that had at least one nontreatment ANP in 2023 or a previous year, 16 nonfocal territories that were not identified in 2021, and 5 nonfocal territories that spanned multiple lakes (that is, multilake territories). In the field, observers often noted that some ANPs did not have sufficient nesting substrate.

In the sample of 98 focal territories, we observed at least one nest attempt in 31 of 44 treatment territories, and a second nest attempt after a failed initial attempt was observed in 6 treatment territories. However, only one nest was on an ANP in a treatment territory; the remaining nests were natural. We observed at least one nest attempt in 37 of 54 control territories, and a second nest attempt after a failed initial attempt was observed in 5 control territories. Chicks or other evidence of hatching was observed in 17 of 54 control territories and 17 of 44 treatment territories, with 1 of those successful treatment nests occurring on an ANP. In 2023, the first nest was documented on May 15, 2023, and 25 percent were initiated by May 22, 2023. Thus, the effective deployment date of May 19 was considered appropriate.

Benchmarks to Evaluate Project Progress

The Monitoring and Adaptative Management Plan (Open Ocean Trustee Implementation Group, 2019) provides parameters to assess progress and we report on all parameters in the following sections.