Introduction

The Deepwater Horizon (DWH; fig. 1) oil spill caused extensive injury to natural resources in the Gulf of Mexico in 2010 (Deepwater Horizon Natural Resource Damage Assessment Trustees, 2016), and Gavia immer (common loon) was one of the species substantially affected by the oil spill (Beatty and others, 2022). The DWH Open Ocean Trustee Implementation Group funded the project “Restoration of Common Loons in Minnesota” to restore common loons lost from the spill. The project has three objectives that are detailed below (Open Ocean Trustee Implementation Group, 2019):

This report describes activities under the project “Restoration of Common Loons in Minnesota” for calendar year 2022. The Monitoring and Adaptive Management Plan (Open Ocean Trustee Implementation Group, 2019) identified benchmarks to evaluate project progress and completion. Thus, we report progress on monitoring benchmarks defined in the Monitoring and Adaptive Management Plan. Furthermore, we include no formal data analysis but report summary statistics based on data collected in 2021 and 2022 for this project (Beatty and others, 2023).

Methods

In 2022, we conducted common loon monitoring for a second consecutive year to collect additional pretreatment data before artificial nest platform (ANP) deployment in 2023. We surveyed priority lakes from May 9 to August 12, 2022 (fig. 1; table 1). We provided a detailed description of the methods used to identify priority lakes in Beatty and others (2022). Briefly, we identified an eight-county study area to focus project activities (Beatty and others, 2022). We identified priority lakes from Minnesota Loon Watcher citizen science data (K. Larson, Minnesota Department of Natural Resources, written commun., 2020) to focus activities on lakes with low common loon reproductive rates. All priority lakes had less than or equal to (≤) 0.85 chicks hatched per pair or ≤0.7 fledged chicks per pair in August based on Minnesota Loon Watcher data (Beatty and others, 2022). Although we anticipated surveys would begin on May 2, 2022, many survey lakes had ice present the first week of May. Consequently, we surveyed all territories the week of May 9, 2022.

We provided a detailed description of the methods used to monitor loon territories in Beatty and others (2022). Briefly, we surveyed each lake by motorboat, kayak, canoe, or shoreline observation. We circumnavigated larger lakes with a motorboat and frequently stopped to scan for common loons with binoculars or the naked eye. We thoroughly searched small inlets, bays, coves, and areas around islands for common loons. We also collected information on environmental variables that could affect detection. We recorded all data from surveys in an ArcGIS Survey123 application. We visited loon territories every 3–18 days (mean=8.6 days, median=7 days) from May 9 to July 15 to document territory occupancy, monitor nests, and document chicks. We visited one territory (Daisy Lake) two days in a row because the first visit was conducted at dusk. We conducted chick survival surveys from August 1 to August 12, 2022; we visited each territory twice during this period.

Results

In 2021, we monitored 126 territories on 55 lakes (fig. 1; table 1) throughout the study area, 16 of which contained an ANP, otherwise known as our intended future treatment, in 2021 (Beatty and others, 2022). Thus, Beatty and others (2022) reported monitoring 110 territories in 2021 that did not include an ANP because the goal of common loon monitoring in 2021 and 2022 was to collect pretreatment data from territories without ANPs. We conducted further revisions to our sample of focal territories in early 2022 before the 2022 field season commenced. We identified eight territories as multilake territories and re-categorized these as nonfocal territories because of uncertainty associated with the nesting location. Consequently, we identified 126–16–8=102 territories as focal territories for the study before field surveys commenced in 2022, and randomly assigned 45 of these territories to the treatment category with the remaining 57 territories assigned to the control category. Treatment territories represented sampling units that are planned to receive an ANP in 2023, whereas control territories would not receive an ANP for the duration of the study.

We observed a new ANP in three focal territories in 2022, and we re-categorized these territories to nonfocal territories (Crooked Lake, West Crooked East Bay territory; Crooked Lake, West Crooked West Bay territory; Thunder Lake, East Bay Island territory). We did not identify the responsible party for the new ANPs, but ANPs are typically deployed by landowners who have property on lakes. We identified six new territories in 2022, which were territories that were not detected in 2021. We identified new territories on Deer Lake (3), Crooked Lake (1), Goodrich Lake (1), and Stump Lake (1). We categorized these six new territories as nonfocal territories. Thus, we monitored 126+6=132 total territories in 2022, and we had a sample of 132–16–8–3–6=99 focal territories in 2022. We continued to monitor nonfocal territories to document loon dynamics on study lakes to better inform states of focal territories on the same lake.

In our sample of 99 focal territories, we observed an initial nest attempt in 68 territories and a second nest attempt in 17 territories. All second nest attempts were the result of a failed initial nest attempt. As in 2021, we considered a successful nest attempt as a territory where at least one chick was observed, or where other evidence of a successful nest was observed at the nest site (Beatty and others, 2022). In 2022, we observed a successful nest attempt in 33 of 99 focal territories.

Benchmarks to Evaluate Project Progress

The Monitoring and Adaptative Management Plan (Open Ocean Trustee Implementation Group, 2019) provides parameters for project progress, and we report on all parameters in the plan below. Data collected for this study in 2021 and 2022 are presented in Beatty and others (2023).