Introduction

The Deepwater Horizon oil spill occurred from April 20 to September 19, 2010, and released approximately 5.0 million barrels of oil into the Gulf of Mexico (McNutt and others, 2012), which prompted a Natural Resources Damage Assessment to evaluate damage from the oil spill (Deepwater Horizon Natural Resource Damage Assessment Trustees, 2016). The common loon (Gavia immer) was one of many bird species injured from the spill with the estimated number of common loons injured ranging from 561 to 910 (Deepwater Horizon Natural Resource Damage Assessment Trustees, 2016). In the context of a Natural Resources Damage Assessment, injury is a “measurable adverse change, either long- or short-term, in the chemical or physical quality or the viability of a natural resource” (43 CFR part 11.14(v)). Thus, natural resource injury can include mortality or lost reproduction (43 CFR part 11.62(f)). Deepwater Horizon Open Ocean Trustee Implementation Group funded the “Restoration of Common Loon in Minnesota” project to restore common loons lost from the Deepwater Horizon oil spill. The project has three objectives (Open Ocean Trustee Implementation Group, 2019):

In this report, we describe activities under the “Restoration of Common Loon in Minnesota” project for calendar year 2024. Specifically, we report progress on monitoring benchmarks defined in the Monitoring and Adaptive Management Plan (Open Ocean Trustee Implementation Group, 2019), which are (1) baseline status of lakes (objective 2); (2) length of linear shoreline acquired (objective 1); (3) number of artificial nesting platforms (ANPs) deployed (objective 2); (4) number of ANPs occupied (objective 2); (5) number of lake associations recruited to participate in a loon-friendly lake program (objective 2); (6) number of intervention activities to promote environmentally friendly fishing gear (objective 3); and (7) common loon presence/absence, territory occupancy, and nest productivity (objective 2). 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), 2022 (Beatty and others, 2023), and 2023 (Beatty and others, 2024). The study design resembled a before-after control-impact design. Pretreatment (that is, “before”) data were collected in 2021 and 2022 (Beatty and others, 2022, 2023). In 2023, we identified 99 focal territories from the sample of territories monitored in 2021 and 2022 (Beatty and others, 2024). Focal territories were territories from which inferences will be made for the study. We assigned 54 of these focal territories as control with the remaining 45 identified as treatment (that is, “impact”) units (Beatty and others, 2024). Focal territories and the status of each focal territory (control or treatment) will remain constant throughout the study with limited exceptions (discussed later in this section). Treatment was deployment of an ANP into a focal territory, which occurred early in the breeding season in 2023 (Beatty and others, 2024). Control territories did not receive an ANP. Thus, 2023 represented the first year of data collection with ANPs (that is, “after”).

Breeding common loons were monitored on territories in north-central Minnesota in 2024 across 53 lakes (table 1, fig. 1), which was the second year of treatment data collection and fourth consecutive year overall. We monitored focal territories and nonfocal territories. Nonfocal territories were territories that did not met our criteria (Beatty and others, 2023) for focal territory but were on the same lake as a focal territory. We monitored nonfocal territories to track common loon dynamics on lakes with focal territories, which allowed us to identify the status of focal territories with more certainty. We worked with private citizens and lake associations to deploy ANPs to 44 treatment territories before an effective deployment date (discussed later in this section). ANPs were deployed in approximately the same location in the same treatment territories in 2023 and 2024. In late 2023, one treatment focal territory was recategorized as a nonfocal territory (Crow Wing County Island Lake–South) because the ANP was placed in an unsuitable area, which reduced the number of focal territories from 99 to 98 and the number of treatment territories from 45 to 44 (Beatty and others, 2024).

We identified an effective deployment date for ANPs of May 10, 2024, which was earlier than 2023. ANPs deployed before the date were categorized as effectively available to common loons as a nesting site, whereas ANPs deployed after the date were categorized as effectively not available to common loons as a nesting site. We selected an earlier effective deployment date in 2024 compared to 2023 because ice-out occurred early to mid-April in most surveyed lakes in 2024. In contrast, ice-out occurred in late April to early May in most surveyed lakes in 2023. Additionally, the earliest observed nesting attempt in 2024 occurred on May 3 and a preliminary analysis indicated that 20 percent of nests were initiated by May 10, 2024. Thus, May 10 was an acceptable effective deployment date because it balanced nesting common loon phenology and ANP deployment logistics (Beatty and others, 2024). Furthermore, the effective deployment date in 2024 also aligns with general common loon nesting phenology, which typically starts nest construction in mid-May (Paruk and others, 2021).

We surveyed 53 lakes across seven counties in north-central Minnesota, which was 3 fewer lakes than surveyed in 2023. We did not survey Unnamed Lake in Becker County (not shown) in 2024 because it was identified as a multilake territory and categorized as a nonfocal territory in 2023. A multilake territory is a territory in which a breeding loon pair use multiple lakes during the breeding season. A parcel on Wabedo Lake (not shown) was under consideration for acquisition for this project in 2023. Thus, we surveyed Wabedo Lake in 2023, but we did not survey this lake in 2024 because the parcel was dropped from consideration for acquisition. Finally, we did not survey Island Lake in Crow Wing County (not shown) because the ANP was placed in an unsuitable area in 2023, and the lake was entirely removed from the study.

We surveyed each territory by way of motorboat, kayak, canoe, or shoreline observation (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 (Beatty and others, 2022). We defined a nest attempt as an adult loon observed incubating on a nest with or without visual confirmation of egg(s) or observation of a nest bowl with eggs from the current year. All data from surveys were recorded in an ArcGIS Survey123 version 3.19 application (Environmental Systems Research Institute, Inc., Redlands, California, United States). We visited loon territories every 3–13 days (mean=6.9 days) from April 29 to July 12, 2024, to collect information on territory occupancy, nesting, and chick survival (Beatty and others, 2022, 2023). Each territory was surveyed twice from July 29 to August 9, 2024, to document chick presence to estimate survival.

Results

We monitored 126 territories in 2021 (Beatty and others, 2022), 132 territories in 2022 (Beatty and others, 2023), and 141 territories in 2023 (Beatty and others, 2024). In 2024, we monitored 135 territories, including 54 focal control territories and 44 focal treatment territories (Beatty and others, 2025). In addition, we monitored 19 nonfocal territories that had at least one nontreatment ANP in 2024 or a previous year, as well as 8 territories discovered in 2022 or 2023. ANPs in nonfocal territories were placed by private citizens and lake associations outside the scope of the before-after control-impact study design. We also monitored three nonfocal territories that were multilake territories adjacent or near focal territories. Finally, we monitored seven territories that were near parcels under consideration for acquisition.

In the sample of 98 focal territories, we observed at least one nest attempt in 41 of 54 control territories and 33 of 44 treatment territories. A second nest attempt after a failed initial attempt was observed in 9 control territories and 7 treatment territories. A third nest attempt after a failed second attempt was observed in 2 control territories and 1 treatment territory. In treatment territories, 8 nests were on an ANP; the remaining nest locations were on natural substrates.

Chicks or other evidence of hatching were observed in 26 of 54 control territories and 21 of 44 treatment territories, with 7 of those successful treatment nests occurring on an ANP. In 2023, one nest attempt, which was successful, was on an ANP (Beatty and others, 2024). This report includes no formal analysis, but we plan to analyze data after collection of all field data in subsequent years.

Benchmarks to Evaluate Project Progress

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